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A Procedure to Estimate theResponse of Aquatic Systemsto Changes in Phosphorus andNitrogen Inputs

United StatesDepartment ofAgriculture

NaturalResourcesConservationService

National Water and Climate Center

(Response of Aquatic Systems to Changes in P and N Inputs, October 1999)

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Cover photo: Heavy growth of filamentous algae in a small stream

(courtesy of Eugene B. Welch, Professor Emeritous, Environmen-tal Engineering and Science, University of Washington, Seattle.)

Issued October 1999

(Response of Aquatic Systems to Changes in P and N Inputs, October 1999) i

The primary authors of this document are Bruce J. Newton, NationalWater and Climate Center, USDA Natural Resources Conservation Service,Portland, Oregon; and Wesley M. Jarrell, formerly at the Oregon GraduateInstitute of Science and Technology, now with Dynambio, LLC, Madison,WI. Substantial assistance in the development, writing, and critical reviewof this document was provided by the following individuals:

Nicholas Aumen, South Florida Water Management District

Robert Baumgartener, Oregon Department of Environmental Quality,Portland, Oregon

David Correll, Smithsonian Environmental Research Center, Edgewater,Maryland

Charles Lander, USDA Natural Resources Conservation Service, Washing-ton, DC

Jerry Lemunyon, USDA Natural Resources Conservation Service, FortWorth, Texas

Roberta Parry, United States Environmental Protection Agency, Washing-ton, DC

Val Smith, University of Kansas, Lawrence, Kansas

Richard Wedepohl, Wisconsin Department of Natural Resources, Madi-son, Wisconsin

Eugene Welch, University of Washington, Seattle, Washington

A workshop held October 19-21, 1997, at the Smithsonian EnvironmentalResearch Center, Edgewater, Maryland, provided the framework and manyof the ideas in the document. The initial concept for this project was devel-oped through the Southern Regional Extension and Research ActivityInformation Exchange Group 17 (SERA-IEG17) sponsored by the USDACooperative Research, Education, and Extension Service.

Financial support for this project was provided by the USDA Natural Re-sources Conservation Service and the U.S. Environmental ProtectionAgency.

Acknowledgments

ii (Response of Aquatic Systems to Changes in P and N Inputs, October 1999)

(Response of Aquatic Systems to Changes in P and N Inputs, October 1999) iii

Contents Introduction 1

A primer on eutrophication 1

Factors affecting the response of aquatic systems to nutrients 3

Dominant plant forms........................................................................................................... 4

Water residence time ............................................................................................................ 6

Temperature .......................................................................................................................... 6

Light ........................................................................................................................................ 6

Depth ...................................................................................................................................... 7

Stratification .......................................................................................................................... 7

Ground water ......................................................................................................................... 7

Forms of P and N Inputs ...................................................................................................... 9

Bottom sediment ................................................................................................................... 9

Nutrient availability .............................................................................................................. 9

Dissolved oxygen (DO) ...................................................................................................... 10

Estuaries ............................................................................................................................... 10

Evaluation procedure 11

Classification key ................................................................................................................ 13

Instructions for selected couplets .................................................................................... 14

Aquatic system descriptions 16

Next steps 25

Gaining more knowledge or professional assistance ..................................................... 25

Obtaining more information .............................................................................................. 26

Computer-based planning tools ........................................................................................ 26

Appendix A—Phosphorus cycle in aquatic ecosystems 31

Appendix B—Nitrogen cycle in aquatic ecosystems 32

Literature cited 33

Glossary 35

iv (Response of Aquatic Systems to Changes in P and N Inputs, October 1999)

Tables Table 1 Trophic state and associated characteristics in lakes 2

Table 2 Trophic state and associated characteristics in 2

periphyton-dominated streams that have low water velocity

Table 3 Trophic state and associated characteristics in estuaries 2

Table 4 Comparisons of model types for P and N nonpoint source 27

pollution

Figures Figure 1 Potential responses of aquatic systems to changing 3

nutrient loading

Figure 2 Aquatic plant categories 4

Figure 3 Temperature and dissolved oxygen profiles during 8

stratification and turnover in a lake

Figure 4 Classification key structure 12

Figure 5 Assignment of stream orders to a stream network 14

(Response of Aquatic Systems to Changes in P and N Inputs, October 1999) 1

Introduction

This document provides a simple assessment proce-dure to estimate the responsiveness of a waterbody tochanges in the supply (loading) of phosphorus (P) andnitrogen (N). Lakes, rivers, streams, and estuaries maybe assessed with this procedure. Few data about thewaterbody are required. The procedure consists of adichotomous key that leads to classifying the water-body in question according to key characteristics thatinfluence the responsiveness of the waterbody tochanges in nutrient loading.

The assessment procedure results in the followinginformation:

• An estimate of the responsiveness of thewaterbody to increases in P or N loading

• An estimate of the responsiveness of thewaterbody to decreases in P or N loading

• A description of key processes for the waterbodyand suggestions for management

• Suggestions for further analysis

This procedure provides a first level screening analy-sis. It may be used to identify sensitive waterbodies aspart of applying a Phosphorus Index-type planningtool. This procedure may be used in a watershedplanning context to evaluate waterbodies as part ofsetting priorities. Finally, this document will be usefulto landowners seeking a better understanding of hownutrients affect surface water.

The relationship between water quality and nutrientloading is complex and the results of this analysis willhave a high degree of uncertainty. Nonetheless, itprovides the best tool available for conservationists inthe field to use in situations where little data exist.This screening procedure does not determine thenutrient loading rates necessary to protect awaterbody from degradation or to restore a waterbodyto specific water quality objectives. To more accu-rately estimate responsiveness and to determineprotective loading rates, the user should consult aprofessional limnologist.

A primer on eutrophication

Nutrients are necessary for aquatic ecosystems tosupport plant growth and the rest of the food web.However, excessive availability of nutrients is detri-mental. The principle adverse impact of nutrientenrichment is to change the trophic state of a water-body. Trophic state refers to the overall level of nutri-ents and related algae and plant growth within thesystem (primary productivity and biomass) and therelationship of primary productivity to animal growth(secondary production). A human-induced increase introphic state is called cultural eutrophication oreutrophication for short. Eutrophication has manyadverse impacts.

Excessive algae and plant growth can lead to depletionof the oxygen that is dissolved in the water. Water canhold only a limited supply of dissolved oxygen (DO)and it comes from only two sources—diffusion fromthe atmosphere and as a byproduct of photosynthesis.Excessive growth leads to depletion of DO because ofnighttime respiration by living algae and plants andbecause of the bacterial decomposition of dead algae/plant material. Depletion of DO adversely affects manyanimal populations and can cause fish kills.

In addition to low DO problems, excessive plantgrowth can increase the pH of the water becauseplants and algae remove dissolved carbon dioxidefrom the water during photosynthesis thus altering thecarbonic acid - carbonate balance. Because plants andalgae provide food and habitat to animals, the relativeabundance of species affects the composition of theanimal community. Eutrophication can impart tasteand odor problems to drinking water supplies andincrease the costs of treating drinking water. Wastewa-ter dischargers to waterbodies with eutrophicationproblems must often install higher levels of wastewa-ter treatment at dramatically increased cost.

Finally, eutrophication interferes with recreation andaesthetic enjoyment of water resources by causingreduced water clarity, unpleasant swimming condi-tions, objectionable odors, blooms of toxic and non-toxic organisms, interference with boating, and "pol-luted appearances." The economic implications ofeutrophication are significant for many communities.

A Procedure to Estimate the Response of AquaticSystems to Changes in Phosphorus and Nitrogen Inputs

2 (Response of Aquatic Systems to Changes in P and N Inputs, October 1999)

Waterbodies are categorized by trophic state as fol-lows:Autotrophic—Systems in which primary productionequals or exceeds secondary production and respira-tion. These systems are generally divided into thefollowing four classes:

• Oligotrophic—Systems that have low suppliesof nutrients. The term means poorly nourished.Oligotrophic lakes are typically clear. Bog lakesthat receive little nutrient input because of alimited catchment are oligotrophic although theymay have a large standing crop of plant biomassand poor water clarity.

• Mesotrophic—Systems that have intermediatenutrient supplies between oligotrophic andeutrophic.

• Eutrophic—Systems that have a large supply ofnutrients. The term means well nourished.

• Hypereutrophic—Systems that have very largesupplies of nutrients.

Heterotrophic—Systems in which secondary produc-tion and respiration exceeds primary production.

These systems must have an outside source of chemi-cal energy. Forest streams in which leaf litter is themain energy source are heterotrophic.

In the mid-1800s, the German agricultural chemistJustus von Liebig published a series of books on therelationship between nutrients and plant production.In essence, Liebig proposed that the yield of a givenplant species should be limited by the nutrient that ispresent in the least amount relative to its demands forgrowth. This concept became known as Leibig's Lawof the Minimum. In aquatic systems, the nutrient thatis generally limiting is P or N. In most freshwater, P isthe limiting nutrient. Estuaries, hypereutrophic fresh-water, and certain rare freshwater systems are oftenN-limited.

Tables 1 through 3 provide descriptions for variouswaterbody types of the different trophic states andassociated water column nutrient concentrations andother characteristics. The nutrient concentrations canbe viewed as water concentrations at which the nutri-

Table 1 Trophic state and associated characteristics in lakes

Trophic state Total nitrogen Total phosphorus Chlorophyll-a Secchi depth(ug/L) 1/ (ug/L) 1/ (ug/L) 1/ (meters)

Oligotrophic <350 <10 <3.5 >4

Mesotrophic 350 – 650 10 – 30 3.5 – 9 2 – 4

Eutrophic 650 – 1,200 30 – 100 9 – 25 1 – 2

Hypereutrophic >1,200 >100 >25 <1

1/ Micrograms per liter or parts per billion.

Table 2 Trophic state and associated characteristics inperiphyton-dominated streams that have lowwater velocity 1/ (Dodds, Smith, & Zander, 1997)

Trophic state Total Total Chlorophyll-anitrogen phosphorus(ug/L) 2/ (ug/L) 2/ (mg/m2)

Eutrophic >300 >20 >150

1/ Higher water velocity (>15 cm/s) enhances nutrient uptake, andthe thresholds for such systems would be lower (around 10 ug/Ltotal P or 3-4 ug/L soluble reactive P).


Table 3 Trophic state and associated characteristics inestuaries

Trophic state Total Total Secchi depthnitrogen phosphorus(ug/L) 1/ (ug/L) 1/ (meters)

Mesotrophic <500 <10 6

Eutrophic 500 – 1,000 10 – 50 2

Hypereutrophic >1,000 >50 <1



ent would be limiting the further growth of algae.These concentrations can be taken only as roughguidelines because, as we will see, many factors influ-ence the relationship between nutrient concentrationsand trophic state. For example, systems dominated byvascular plants rooted in bottom substrata are lessinfluenced by the concentration of nutrients in thewater than are systems not dominated by vascularplants. (Vascular plants have their own internal systemfor moving nutrients around, somewhat like our ownbloodstream. Algae, by and large, do not have thisinternal transport system.)

One of the challenges in evaluating trophic conditionis determining what the trophic state should be. Waterquality goals may conflict. For example, the aestheti-cally pleasing, clear, blue water of oligotrophic lakes isusually associated with low fish production. Further-more, the surrounding geology and basic hydrologytypically constrain the range of achievable trophicstates. The natural condition of many lakes (i.e., whatwould exist without any human influences) is a me-sotrophic or eutrophic state. It would be futile, forexample, to attempt to achieve oligotrophy in a regionwhere the natural condition is mesotrophy.

The potential responses of aquatic systems to chang-ing nutrient loading are shown in figure 1. As nutrientloading increases, the trophic level generally in-creases. At some point the system may become satu-rated. This occurs when the nutrient in question is nolonger limiting and another factor starts to limit thegrowth of algae and plants. Some systems have agreater capacity to tolerate increased nutrient loadingthan others do. Systems with an already high loadingdo not respond as dramatically as systems with a lowloading. Systems limited by shade or other non-nutri-ent factors respond less to increased nutrient loading.Lakes with long water residence times are more toler-ant to increased nutrient concentration in the inflowthan lakes with shorter residence times. Conversely,lakes with long residence times tend to respond moreslowly to decreases in nutrient loading.

As shown in figure 1, decreasing nutrient loading doesnot always result in an immediate lowering of thetrophic state. Nutrients may accumulate in bottomsediment, both in deposited clays and silts and depos-ited organic matter. In such cases nutrient releasefrom the sediment results in internal loading of nutri-ents. Systems that exhibit this sediment memory haveslower recovery rates after external nutrient loads arereduced than systems without sediment memory.

Aquatic systems can be classified according to thelimnological characteristics that influence how theyrespond to changes in nutrient loading. For example,

streams with extensive shading during the growingseason generally are light-limited and respond less tonutrient increase than streams without shading. Well-lighted streams with water velocity greater than 10centimeters per second (0.33 ft/s) and low nutrientsare generally characterized by diatoms instead offilamentous algae and are substrate-limited. Basicphysical characteristics can provide important insightsinto how an aquatic system functions with respect tonutrients. The evaluation procedure presented hereuses a waterbody classification system based onrelatively simple physical characteristics of thewaterbody. Predictions about how a waterbody willrespond to nutrients and advice for management arethen presented based on its classification.

Factors affecting the re-sponse of aquatic systemsto nutrients

The response of a waterbody to changes in nutrientloading depends on many factors. The essential factorsare those that affect algal and plant abundance andbiomass accumulation, such as the availability ofnutrients, light, substrate for attached plants, time forgrowth, temperature, grazing pressure, and physicalsuitability. Each essential factor may be controlled byadditional factors and processes. In this section wedescribe the factors important to an understanding ofhow an aquatic system functions and how it respondsto changes in nutrient loading. Many of these factorsare used directly in the evaluation procedure.

Figure 1 Potential responses of aquatic systems tochanging nutrient loading

Annualplant

growth

Curve 2

Curve 4Curve 1

Curve 3

Curve 3

Curve 1

Low

Curve 4

Curve 2

High

Time (yr)

Annualnutrientloading

Sensitivity to loading increase

Low

High

Sensitvityto loadingdecrease


As you read about these factors, you should consultthe diagrams in the appendixes. The diagrams depictthe transformations that can occur for P and N inaquatic systems. The transformations are depicted asnutrient cycles.

Dominant plant forms

Growth of aquatic vegetation is desirable in mostsystems to support the food chain. However, over-abundance of plants disrupts processes and leads todegradation of the system.

The three basic forms of aquatic plants are phyto-plankton, periphyton, and macrophytes (fig. 2). Eachform has different requirements for growth and differ-ent implications for management.

Phytoplankton—Microscopic algae suspended in thewater column. Their form may be single cell, fila-ments, or colonies of cells. Most have limited mobilityand, thus, are carried with the flow of water. Theymust take up nutrients directly from the water. Somecyanobacteria (also called blue-green algae) can usedissolved N2 gas as a source of N. Some cyanobacteriaalso have gas vacuoles that allow them to float to thesurface and promote the formation of a surface scum.

Periphyton—A community of organisms, often domi-nated by algae but including bacteria, fungi, protozoa,and other microbes, that grows attached to a surface.Periphyton may attach to any stable surface, such asrocks, woody debris, and vascular plants. The twomain types of periphyton–associated algae are filamen-tous and nonfilamentous. Filamentous periphytic algaeare composed of many cells linked together forminglong strands. Nonfilamentous periphytic algae grow assingle cells or colonies attached to bottom substrata.Nonfilamentous periphyton communities are generallydominated by diatoms.

Macrophytes—Technically, any plant large enough tobe visible without magnification. More commonly,macrophyte refers to vascular plants that have roots,stems, and leaves, although in some areas mosses andliverworts are important macrophytes. Macrophytesmay be rooted in the sediment or free-floating. Animportant characteristic of rooted macrophytes is thatthey obtain nutrients through their roots, translocatethe nutrients to other parts of the plant, and, therefore,often depend mostly on sediment for nutrients.

The physical and chemical environments of theaquatic system strongly influence which types ofplants dominate. For example, deep, turbid water doesnot support periphyton because light cannot penetrate

Figure 2 Aquatic plant categories (see also Terrell and Perfetti 1989)

Elodea, or waterweed (Anacharis) Pond weeds (Potamogeton)

Rooted macrophytes Floating macrophytes


Crustose Prostrate

Stalkedor short

filamentous Filamentous

Filamentouswith

epiphytes Prostrate

Anabaena

Euglena

Spirogyra

Chlamydomonas

Pinnularia

Cyclotella Rhodomonas

Cladophora

Phytoplankton and filamentous algae

Periphyton

Figure 2 Aquatic plant categories (see also Terrell and Perfetti 1989)—Continued


to the bottom, but could support phytoplankton,which would spend part of their time in the lightedzone. Rivers without sufficient residence time do notsupport phytoplankton because the organisms areswept away before their populations can expand.

Just as physical variables affect the kinds of plantsthat dominate the system, the kinds of plants thatdominate influence the system's biological and chemi-cal characteristics. For example, in favorable condi-tions phytoplankton populations can increase rapidlyand may cause an algal bloom.

An algal bloom is a rapid expansion of an algal popula-tion that occurs when conditions are favorable.Blooms may be seen as a green coloration of the wateror a surface scum if the algae are buoyant. If thebloom is large or when it dies, dissolved oxygen canbe rapidly depleted. Also, some algal blooms producetoxins that can kill livestock and fish. Macrophytescan move P out of the sediment and, through diebackor grazing, release P into the water column where it isavailable for phytoplankton. However, macrophytebeds also stabilize sediment and reduce resuspensionof sediment (and associated P) by wind-induced waterturbulence. In some systems phytoplankton createsufficient turbidity to prevent macrophytes fromestablishing because of light-limitation. In these sys-tems reducing water column nutrient concentrationssufficiently to control phytoplankton may lead toincreased macrophyte abundance, which may lead todifferent impairments.

Water residence time

The water residence time of the system is an impor-tant determinant of whether phytoplankton popula-tions can establish and accumulate. If the residencetime is too short (less than 7 days), phytoplankton areswept out of the system before their populations canincrease (i.e., population growth cannot overcomeloss). Phytoplankton populations in such waterbodiesmay remain low; however, nutrient enriched water anda building population of phytoplankton may cause aproblem downstream.

Residence time is measured by dividing the volume ofthe waterbody by the rate of flow out of the water-body. For example, if a lake has a volume of 200 acre-feet and the flow rate at the outlet is 30 cubic feet persecond, the water residence time is (200 ac-ft x 43,560ft3/ac-ft) / 30 ft3/s = 290,400 seconds, or 3.4 days. For astream, it is usually the travel time for a specific reachof the stream with some common characteristic, suchas dominant land use, gradient, or substrate composi-tion. Generally, the slower the water or the more

complex the stream (e.g., lots of woody debris,braided channels), the longer the residence time. Foran estuary, the residence time is the volume divided bythe net loss of water per day. Lakes are more efficienttraps of nutrients than are rivers, and P retention isproportional to residence time. Residence time canchange seasonally as flows or volumes change.

Temperature

Most algae grow more rapidly at higher temperatures.Within the range of 0 to 25 degrees Celsius (32 to 77°F), increasing temperature by 10 degrees Celsius (18°F) typically doubles the rate of growth. Therefore, theresponse of plants to nutrient inputs during wintershould be less pronounced than during summer. Blue-green algae tolerate higher temperatures than mostother forms of algae.

As temperature increases, the solubility of oxygen inwater decreases. For example, solubility in fresh waterat 20 degrees Celsius is 9.1 mg/L, but at 30 degreesCelsius the solubility decreases to 7.5 mg/L.

Controlling temperature in streams may be a feasiblealgal control strategy to reduce the adverse effects ofnutrients in limited situations. Water temperature risesmainly because of inputs of warmer water and directsunlight on the water. Shade from riparian vegetationand decreasing the width-to-depth ratio are the pri-mary strategies for lowering the temperature instreams. Shading is not a practical method of loweringthe temperature in lakes, reservoirs, and wide rivers.Eliminating inputs of warm water (e.g., surface irriga-tion return flows) is another method.

Light

Algae and macrophytes rely on sunlight for photosyn-thesis and growth. In most cases plant growth rate is afunction of light intensity up to some maximum,provided nutrient supplies and temperature are ad-equate. Light intensity is attenuated in several ways inaquatic systems.

Riparian vegetation—Riparian vegetation shadesnear-bank areas. Narrow streams surrounded bymature trees may be in full shade throughout the day.Well-shaded streams may have low amounts of per-iphyton because of light limitation, even with highnutrient concentrations.

Suspended sediment—Soil and organic particlessuspended in the water column scatter light and de-crease the depth of light penetration. As a result, plant


growth is limited to some maximum depth in the watercolumn.

Color—In some systems drained by wetlands andforests, dissolved organic material, such as tannins,affects the light regime.

Self-shading—As phytoplankton or macrophyteabundance increases, sunlight penetrates less deeplyinto the water. As a result, at some depth in the watercolumn not enough light is available to support plantgrowth and maintenance. Plants remaining below thatdepth too long die and decompose.

Depth

The depth of water in the system affects sensitivityto nutrient inputs. Particles in the water column scat-ter, reflect, and absorb light. The intensity of lightdecreases as depth increases below the water surface.The point at which the available light is too low tosupport plant growth is called the compensation point.The depth of the compensation point depends on theintensity of the light at the surface of the water and thenumber of particles in the water. Particles can includesuspended clays, organic detritus, and phytoplankton.Areas of a lake bottom or streambed that are belowthe compensation point do not support periphyton ormacrophytes.

In shallow (<6 ft) water, reducing suspended sedimentand phytoplankton can greatly increase the transpar-ency of the water and consequently increase the lightintensity at the sediment surface. If the bottom sedi-ment is high in nutrients, macrophytes and sometimesperiphyton may establish and grow although theoverlying water is low in nutrients.

Depth also is important because it is a major factor instratification.

Stratification

Less dense water floats on top of water with greaterdensity. In any waterbody, water layers can formaccording to their density. This layering is termedstratification. The layers do not mix with each otherand chemical concentrations in one layer can be verydifferent from concentrations in another layer. Infreshwater, water temperature is the most importantdeterminant of density. Water has the unusual prop-erty of being most dense at 4 degrees Celsius (39 °F).Because water is also a poor conductor of heat, layersof water with different temperatures (and thus, differ-ent densities) can become established and persist. In

summer, warmer layers float on cooler layers andsunlight warming the top layer tends to strengthen thislayering. In winter, a reversed pattern may form withwater at 4 degrees Celsius (39 °F) at the bottom andcolder (and, thus, less dense) water and ice floating ontop.

Figure 3 illustrates temperature and dissolved oxygenprofiles during summer stratification and turnover in alake. The region of warm water near the surface istermed the epilimnion, the cold deep water is thehypolimnion, and the intermediate layer is themetalimnion.

Stratification is broken down (a process termed turn-over) either by turbulence or by changes in the tem-perature of the surface layer. Turbulence can resultfrom waterflow (in a river) or from wind. In lakes,wind-induced turbulence can be sufficient to break upstratification if the wind is strong, there is a long fetch(i.e., length of lake surface on which the wind acts),and the lake is shallow. In temperate parts of theworld, seasonal air temperature changes cause thesurface layer to either cool (in the fall) or warm (in thespring) to the same temperature as the lower layer.This results in a breakdown of stratification and aturnover of the lake (fig. 3). In many areas a turnoveroccurs in the spring and again in the fall although notall lakes follow this pattern.

Stratification has important implications for nutrientdynamics. Because the hypolimnion is isolated fromthe atmosphere, dissolved oxygen in the hypolimnionmay be entirely depleted by microbes decomposingorganic matter in the water and in the sediment. Theresulting anaerobic conditions release P bound withferric iron in the sediment. When the stratificationbreaks down, the layers mix and P becomes availablewithin the surface water where light conditions aresuitable for algal growth. If conditions are right, sus-tained nutrient enrichment and rapid algal growth canresult. However, oxygenation of the formerly anaero-bic water oxidizes the ferrous iron to ferric iron,resulting in the binding and sedimentation of P. Acycle of internal release, circulation, binding, and sedi-mentation exists in many lakes.

Ground water

Although most people think of a stream as beingcontained in its channel, a significant exchange occursbetween surface water and ground water. Some sec-tions of a stream have significant inflows of groundwater (called gaining reaches), and some sectionshave significant outflows of surface water to groundwater (called losing reaches). Even where the net gain


Figure 3 Temperature and dissolved oxygen profiles during summer stratification and turnover in a lake (epilimnion isshown by a; metalimnion by b; and hypolimnion by c)

°C 0

a

b

c

2 3 4 5 6 7 8 9

10 15 20

4 ft

8 ft

12 ft

16 ft

20 ft

24 ft

Temperature

Oxygen

Midsummer

O2 in mg/liter

°C 2

2 4 6 8 10 12 14 16

4 6 8

4 ft

8 ft

12 ft

16 ft

20 ft

24 ft

Temperature

Oxygen

Spring and fall overturn

O2 in mg/liter


or loss of flow is not significant, a high degree ofexchange between the surface water and ground watercan occur. The zone below and adjacent to the streamchannel in which a mixture of surface water andground water can be found is called the hyporeic zone.

Unpolluted ground water generally contains P and Nconcentrations less than 100 ug/L or ppb. Althoughlow for N, such concentrations could represent asignificant source of P. In some areas of the world,natural geological processes result in even higherconcentrations of P in ground water. This naturalenrichment can also occur with N, but is rare.

The temperature of deeper ground water is generallynear the mean annual air temperature of a given site.As a result, ground water is usually cooler than surfacewater in the summer. Increasing the quantity ofground water feeding a stream, or even a small lake orreservoir, generally lowers its temperature in summer.

Forms of P and N Inputs

Phosphorus and N are carried into water in dissolvedand particulate forms. In addition, N2 gas dissolvesinto water from the atmosphere and can be used bycyanobacteria (blue-green algae).

Chemical analysis of water samples frequently in-volves measuring the dissolved and total concentra-tions of nutrients. Total concentrations are measuredafter a strong acid treatment of the sample breaksdown all particles and compounds into simple ions.

Dissolved forms

The inorganic dissolved form of P is orthophosphate.Because the analytical method for measuring ortho-phosphate is not perfect and measures a small fractionof other forms, analytical results are often reported assoluble reactive phosphorus (SRP). Dissolved N formsare predominantly nitrate or ammonium, rarely nitrite.Both N and P can be in small dissolved organic ions aswell. These nutrients can be taken up directly in somecases, while in others, the molecules or ions must befurther broken down before uptake can occur.

Dissolved inorganic P and N forms are bioavailable.That is, algae and plants can absorb and use thesenutrient forms immediately.

Particulate forms

The solid forms can be divided into several classes:• Organic material—The P and N in organic

particles (from plant residue or manure) are heldin strong bonds that are generally broken downby enzymatic processes only.

• Low solubility inorganic particles—Phos-phorus can occur in particles as compounds ofcalcium, iron, or aluminum. Under some condi-tions, such as low dissolved oxygen concentra-tions or high pH, bioavailable P may be releasedfrom these particles into the water. Nitrogenrarely occurs in low solubility inorganic par-ticles.

• Adsorbed or exchangeable forms—Some Pand ammonium-N may be held on the surface ofsolid particles (especially clays) in forms thatcan enter solution readily if dissolved concentra-tions are lowered. These particles can be eithersources of P or sinks depending on the concen-tration of soluble P in the water. When particlescontaining adsorbed P or N come into contactwith rainwater (which is acidic with very low Pand N), much of the adsorbed P and N is immedi-ately released into the water.

Bottom sediment

Bottom sediment is characterized as either rock orsoft.

Rock refers to bedrock, boulder, cobble, or gravelbottoms that provide a stable attachment surface forperiphyton. These substrata are generally chemicallystable and do not by themselves remove or add P or Nto the water.

Soft sediment refers to sand, clay, organic matter, or amixture. Sand is not very reactive chemically, anddoes not consolidate (stick together). Clay is a termfor very fine particles (<2 microns in diameter) thatgenerally are active in affecting the chemistry of thewater. The fine clay material may remove P from thewater or release P to the water, depending upon pHand orthophosphate concentrations in the water. Anyiron phosphates, especially clay-sized particles, areliable to dissolve if oxygen concentrations becomelow.

Organic matter that accumulates in the bottom sedi-ment is partly decomposed by microbes and releasespart or all of the P and N it contains into the overlyingwater. Rates of decomposition are most likely toincrease at higher temperatures and if dissolved oxy-gen is present.

Nutrient availability

Phosphorus and nitrogen are termed essential nutri-ents because without them organisms cannot grow orcomplete their life cycle. Phosphorus is required for


membrane stability, ATP, and nucleic acids (DNA andRNA). Nitrogen is important in proteins, includingenzymes, and nucleic acids; plants require significantamounts for chlorophyll. At low nutrient concentra-tions, growth is inhibited.

In many systems the "desired condition" requires lowconcentrations of either P or N. In most freshwatersystems, P is the most likely limiting element.

Algae and plants can obtain nutrients from sediment,the water column, and from ground water dischargesinto the waterbody. Additionally, atmospheric N canbe converted to bioavailable N by some cyanobacteria(blue-green algae). Water column concentrations ofnutrients are often useful to measure, but in somecases (noted in the system descriptions describedlater) water column concentrations may not be useful.The criteria for nutrient concentrations in water fordifferent trophic states are provided in tables 1, 2, and3.

Dissolved oxygen (DO)

Oxygen gas, dissolved in water, supports the metabo-lism of all aquatic plants, animals, and most micro-organisms. At 20 degrees Celsius (68 °F), freshwater inequilibrium with air (saturated with air) contains 9.09mg O2/L (9.09 ppm dissolved oxygen or DO). Thisconcentration is 22,000 times lower than the concen-tration of oxygen in the atmosphere. Solubility, andthus the amount of oxygen in the water at saturation,decreases as the water temperature increases.

Micro-organisms, plants, and animals take up and useDO from the water. Many organisms begin to be af-fected adversely at DO concentrations below 6 mg/L.Microbes can eventually remove all DO from the waterif the oxygen is not replenished rapidly from the air(through reaeration) or from photosynthesis (oxygenis released from algae and plants during the day).

In aquatic systems where the sediment becomesanoxic (containing no DO), processes occur that canrelease P into the water. In particular where sedimentcontains iron phosphates, iron is chemically reducedby micro-organisms from Fe3+ to Fe2+. The Fe2+ form ismuch more soluble and, when it dissolves, the associ-ated P enters solution as well. If O2 enters this wateragain, the iron may convert back to the insoluble Fe3+

form and precipitate, taking some of the dissolved Pwith it. However, enough dissolved P may remain insolution to support relatively rapid algal growth.

Estuaries

An estuary is a body of water where river water mixeswith and measurably dilutes seawater (Ketchum,1951). Circulation patterns and mixing within estuariesare complex and are the result of salinity differences,streamflow energy, tidal effects, wind, and the geo-morphology of the estuary itself. Estuaries serveseveral, often conflicting, societal and ecologicalfunctions. For example, estuaries support commercialand recreational fisheries. From 60 to 90 percent of theAtlantic Ocean species spend some portion of their lifein estuaries (Seaman, 1988). However, estuaries arealso important waterways for commercial transportand often serve as the direct recipient of processedindustrial and municipal waste. Being at the down-stream end of river systems, estuaries tend to receivehigh loads of pollutants from point and nonpointsources.

Most observational and experimental studies havesupported the general conclusion that nitrogen is themost likely nutrient to limit algal production in coastalmarine environments (Boynton, et al., 1982; Smith1998) and that estuarine phytoplankton biomass andproductivity are correlated with external N inputs(Howarth 1993). However, Hecky and Kilham (1988)concluded that the evidence for N-limitation in saltwater was much weaker than that for P-limitation in


fresh water. Smith (1998) concluded that the degree ofN-limitation versus P-limitation of algal productivity inany given coastal marine ecosystem depends on the N-to-P supply ratio in its water. This, in turn, is a func-tion of regional land use, population density, andhydrodynamic and atmospheric conditions. Most ofthe research devoted to assessing nutrient limitation inestuaries has been water-column based, and morestudies are needed to assess whether the benthic plantcommunities respond in a similar fashion (Fong, et al.,1993).

Evaluation procedure

The evaluation procedure used in this document isbased on classifying aquatic systems using a dichoto-mous key. At each numbered step of the key, the useris asked a question about the waterbody and given ananswer couplet. Brief instructions for how to answerthe question are provided — if more detailed instruc-tions are available, that is noted. The choice that isselected to answer the question directs the user to thenext numbered question in the key.

Figure 4 provides a flowchart illustrating the structureof this classification key.

The aquatic system classification is in the form of aletter designation. Once the system is identified, turnto System Descriptions starting on page 15 for infor-mation about responsiveness, important processes,management advice, and further analysis recommen-dations for that system.

This evaluation should be conducted in the field be-cause you need to make physical observations. Apreliminary run through the key in the office helps todetermine what equipment to bring and if any informa-tion is required that is not readily available in the field.For example, a preliminary run through the key mayshow that you most likely will need to know the hy-draulic residence time. That characteristic wouldbetter be estimated in the office than in the field.

The first question to address before conducting anevaluation is what are the waterbodies that could beevaluated. Small lakes and ponds are straight-forward— each is a waterbody. Larger reservoirs can be morecomplex. Large reservoirs can have several arms thatare somewhat isolated from each other and may needto be evaluated separately. River networks are themost complex for deciding what to evaluate. Youshould divide a river network into segments based oncommon characteristics of flow, shading, and depth.Each segment may need to be evaluated as a separatewaterbody.

The second question to address is which waterbodiesshould be evaluated. The segment or waterbody that isof initial interest may not be the most criticalwaterbody in terms of potential impacts of nutrientload changes. Any single waterbody is likely part of alarger system, and nutrient loading changes to thewaterbody being evaluated may affect a downstreamwaterbody. As part of the evaluation procedure, down-stream waterbodies must be identified and it must bedetermined if they are potentially critical waterbodiesfor the analysis. If downstream waterbodies are likelyto be highly influenced by the loading changes beingexamined, then the analysis procedure should beapplied to the downstream waterbody in addition tothe waterbody that is of initial interest.


Figure 4 Classification key structure

2. Stream order

3. Canopy and shading

4. Light penetration

≤3

>80% <80%

Cannot seebottom

Seebottom

6. Inorganic turbidity>20 NTU <20 NTU

7. Velocity>10 cm/s <10 cm/s

8. Residence time<7 days >7 days

10. Bottom type

Mud Rock/hard

9. Bottom typeMud Rock/

hard

5. Bottom typeMud Rock/

hard

≥4

1. Streams and rivers

A

B

E

F

G H I J

C D

Heterotrophiclight-limited

Heterotrophic

Periphytondominated

Macrophytedominated

Periphytondominated

Shifts betweenphytoplanktonand macrophytes

Shifts betweenphytoplanktonand periphyton

Phytoplanktondominated

Macrophytedominated

Periphytondominated

11. Stratification

12. Hypolimnetic oxygen

13. waterbody

zmax>6m 3m<zmax<6m

Oxic AnoxicPolymictic

Estuary Lake orreservoir

15. Residences time>7 days <7 days

16. Macrophyte>50% surfacecoverage

<50%surface

17. Light penetrationSee bottom/macrophytes

Cannot seebottom

18. Bottom typeMud/soft Rock/

hard

14. Hypolimnetic P<6 >6

zmax<3m

1. Ponds lakes, reservoirs, estuaries

K

LQ

G H

R

M N

Oligotrophicor mesotrophic

Phytoplanktonpotential

P

O

Macrophytedominated

Macrophytedominated

Periphytondominated

Light-limitedheterotrophic

Estuary

Deep lakew/hypolimnetic

transfer

Deeplake

transfer during growing season-Osgood index


Finally, the user should bear in mind that nutrientloading is only one component of ecological condition.Water temperature, habitat, sedimentation, organicenrichment, and hydrology are a few of the otherfactors that affect water quality and ecological condi-tion. In addition to this procedure, it may be useful toconduct a general assessment using the NRCS StreamVisual Assessment Protocol (USDA, 1998) or the WaterQuality Indicators Guide (Terrell, 1989).

Classification key

1. Is the system a stream/river or not?

Streams and rivers are above the head of tide anddo not include run-of-the-river reservoirs. How-ever, rivers with low head dams that do notreduce the water velocity below what wouldexist in a natural pool should be treated as rivers.

Stream or river ......................................... go to 2Not a stream or river ............................. go to 11

2. What is the stream order?

See next section for instructions on how todetermine stream order.

First, second, or third order ................... go to 3Greater than third order .......................... go to 6

3. What is the percent canopy shading?

Percent canopy shading is measured from thecenter of the stream. Looking up, what percentof the sky is blocked by canopy? This should beestimated if trees are not in full leaf out.

Greater than 80% .................................. System ALess than 80% ........................................... go to 4

4. How deep does light penetrate the water?

See bottom ............................................... Go to 5Cannot see bottom............................... System B

5. What is the bottom type?

Mud/soft ................................................ System CRock/hard .............................................System D

6. How much inorganic turbidity is in the

system?

See next section for instructions on how todetermine turbidity.

Turbidity >20 NTU ............................... System ETurbidity <20 NTU .................................. Go to 7

7. What is the velocity of the stream?

See next section for instructions on how todetermine velocity.

Velocity >10 cm/s ................................. System FVelocity <10 cm/s .................................... Go to 8

8. What is the residence time in the system?

<7 days ...................................................... go to 9>7 days .................................................... go to 10


Mud/soft ................................................System GRock/hard .............................................System H


Mud/soft ............................................... System IRock/hard ............................................ System J

11. What is the maximum depth?

Maximum depth >6 m ......................... go to 12Maximum depth 3 – 6 m ...................System OMaximum depth <3 m ......................... go to 15

12. Dissolved oxygen in the hypolimnion?

See next section for instructions for thiscouplet.

Oxic ..................................................... System KAnoxic ................................................... go to 13

13. Is the waterbody an estuary or a lake/

reservoir?

Estuary ................................................ System LLake/reservoir ...................................... go to 14

14. What is the Osgood index for the system?

Osgood Index <6 .............................. System MOsgood Index >6 ................................System N

15. What is the mean residence time?

>7 days .................................................. go to 16<7 days .................................................. go to 17

16. How much of the surface of the lake/reser-

voir is covered with macrophytes during

maximum biomass accumulation?

>50% .................................................... System P<50% ....................................................System Q

17. How deep into the water does light pen-

etrate?

See bottom ........................................... go to 18Cannot see bottom ............................ System R


Mud/soft ..............................................System GRock/hard ...........................................System H


Figure 5 Assignment of stream orders to a streamnetwork

12

2

3

3

33

3

33

44

4

44

5

5

55

2

2

2

2 22

2

1

1

1

1

11

11

11

Instructions for selected couplets

This section provides instructions for answeringselected couplets in the key.

Couplet 2 – Stream order

Stream orders are counted from the headwaterstreams to the point where the river enters the ocean.A stream at the top of the watershed is first order, thesecond level down is second order, and so forth. Ifthere is a confluence of streams of different orders(e.g., order 2 joins order 3), the new stream assumesthe same order as that of the larger of the two joiningstreams (it is still order 3). It only increases in order(e.g., to an order 4) if the two joining streams are ofthe same order (e.g., both order 3). Figure 5 illustrateshow stream orders are named.

The definition of stream should be seriously consid-ered. It may start where a water channel containsflowing water for more than 6 months of the averageyear, for example. Generally, the higher the order, thewider and slower the stream. However, this relation-ship can vary greatly depending upon the nature of thelandscape.

Couplet 6 – Water turbidity

This couplet asks if inorganic turbidity is greater than20 NTU (nephelometric turbidity units). It is importantto distinguish inorganic turbidity from organic turbid-ity. Inorganic turbidity is caused by clays suspended inthe water and is yellow to light brown. Organic turbid-ity is caused by phytoplankton and organic materialand is green or dark brown. Use a clear drinking glassor beaker to observe the color of a water sample.

Turbidity can be measured in a number of ways. Atransparency tube is available from Lawrence Enter-prises (about $35; www.acadia.net/h2oequip or207-276-5746) and from Ben Meadows(www.bmeadows.com). It is filled with a water sampleand, while sighting through the sample, water is drawnoff until a pattern in the base of the tube becomesvisible. Transparency is then read using the appropri-ate scale on the side of the tube. A transparency valueof 30 cm roughly corresponds to 20 NTU. The bestmethod is to use a turbidity meter. You may be able toborrow a meter or have samples tested at a sewagetreatment plant, agency, college, or commercial labo-ratory. Volunteer monitoring groups may also have aturbidity meter. A secchi disk can also be used. Asecchi disk is a weighted 20 cm disk painted with awhite and black pattern (two quadrants black and twoquadrants white). It is lowered into the water until the

pattern disappears. The depth is noted, and the disk israised until the pattern reappears. Secchi depth is theaverage of the two readings. A secchi depth of roughly0.3 meters (12 inches) corresponds to 20 NTU.

Couplet 7 – Water velocity

In this couplet you must determine whether the streamvelocity exceeds 10 cm/s (0.33 ft/s). Water velocity canbe measured using an orange or a stick of wood as afloat. An orange is a good object because it floats justbelow the surface where the maximum velocity typi-cally exists. Using a stop watch, time how long it takesfor the orange to float a measured distance, such as 2meters (6.5 feet). If the orange takes time less than [10x length in m] seconds, the velocity is greater than 10cm/s. For example, if the travel length is 2 meters, atravel time of less than 20 seconds would indicate avelocity greater than 10 cm/s. Velocity measurementsshould be taken during the summer when plant growthis expected to occur. It should be done in at least 5 to10 localized sections, and the average velocity calcu-lated.

Couplets 8 and 15 – Residence time

Residence time is measured by dividing the volume ofthe waterbody by the rate of flow at the outlet. For alake or reservoir, it is the volume divided by the flowrate of the stream draining it (see example on page 6).For a stream, residence time is generally the traveltime for a specific reach of the stream with somecommon characteristic. (A reach is a section of a00stream or river.) Residence time can change season-


ally as flows or volumes change. Residence timeshould be estimated for the critical period for waterquality. In most cases that is mid-summer when flowsare low and temperature and light are high.

Couplet 12 – Hypolimnetic oxygen

This couplet asks whether the hypolimnion is oxic(aerobic) or anoxic (anaerobic). This is a criticalfactor for deep lakes. Unfortunately, it is not easilydetermined. A lake association, volunteer monitoringgroup, state agency, or local college may have moni-toring data on DO concentrations. If data are notavailable, either conduct monitoring or continue usingboth responses for couplet 12. You will end up withtwo potential system descriptions, but the informationmay still be useful for your purpose.

If monitoring, the easiest method is to monitor thebottom 1 meter of the hypolimnion. If dissolved oxy-gen remains above 2 mg/L, the hypolimnion, and thusthe surface of the sediments, is assumed to be oxic. Ifit falls below 1 mg/L, assume it is anoxic. Water

samples should be taken during the critical period forthe lake (late summer) and can be tested using aninexpensive test kit. Samples should be taken early inthe morning when the lowest DO values are typicallyfound, before photosynthesis increases concentrations.

Couplet 14 – Osgood index

The Osgood Index is defined as the mean depth (z) ofa waterbody in meters divided by the square root ofthe surface area (A) in km2, or z/A0.5. It reflects thedegree to which a lake or reservoir will mix because offorces of wind. Low numbers indicate a shallow, largelake that is readily mixed by wind, although during aperiod of calm days, it may become temporarily strati-fied.

The mean depth, z, is best determined by dividing lakevolume by lake surface area (volume/area). However,unless a bathymetric map is available, volume may beunknown. Alternatively, z can be estimated by measur-ing depth along a transect across the pond or lake andaveraging the values. Depth can be measured using aweighted survey tape or a depth meter (fish finder).


Aquatic system descriptions

This section provides information for each of theaquatic system types identified through the key. Thisinformation includes the sensitivity of the system tochanges in nutrient loading, important aspects of thelimnology of the system, management considerations,and analysis suggestions.

For all stream conditions where periphyton are impor-tant, the system responds more to changes in concentra-tion than load. Although loads are referred to throughout,

if increased or decreased loads do not result in concomi-tant increases or decreases in the predominant concen-tration during the growing season, then concentrationshould be considered the primary driving parameterfrom the nutrient supply standpoint. An increase in loadwithout an increase in the predominant concentrationcould occur if the load increase is associated only with,for example, storm runoff. In this example the periphy-ton would experience no concentration change or ahigher concentration for only a short period.

System A—Heterotrophic small stream

Increased loading Decreased loading

Phosphorus Insensitive Insensitive

Nitrogen Insensitive Insensitive

System functioning: These streams are typicallylight limited because of heavy shading from trees. Thelack of direct sunlight prevents aquatic plants fromgrowing, and the aquatic biological community derivesenergy from outside food sources, such as leaf litter.These systems are not sensitive to changes in inor-ganic nutrient loading, but may be sensitive to otherpollutants, such as DO-depleting substances (manureand ammonia), acid mine drainage, sediment, oil andgrease, and toxic substances.

Management considerations: Maintenance of theriparian vegetation and the shading provided by theriparian vegetation is essential. Maintaining access ofthe stream to its flood plain is also important. Hydro-logic changes and excess sediment loads may causechannel widening that eventually displaces the ripar-ian vegetation and reduces the amount of shading ofthe stream.

Monitoring and further analysis: There is littleneed for monitoring beyond channel morphology andriparian condition.

System B—Phytoplankton-dominated small stream


Phosphorus Sensitive Sensitive

Nitrogen Potentially sensitive Potentially sensitive

System functioning: With little shading along thestreambanks, adequate light is available to supportsignificant phytoplankton growth if nutrient concen-trations are in the mesotrophic/eutrophic range andstream velocity is low. Although most streams are P-limited, in rare cases N may be the limiting factor (ifbackground P is naturally high and N concentrationsare low, such as is sometimes the case for undisturbedforest).

Management considerations: Nutrient load reduc-tion will reduce algal biomass and related water qual-ity problems if present. Note: If phytoplankton abun-dance is decreased in these systems, such that lightcan penetrate to the bottom, the system may shift to Cor D.

Monitoring and further analysis: Water columntotal P is a valid characteristic to monitor in thesesystems. Values should be compared to the lakethresholds in table 1.


System C—Macrophyte-dominated small stream




System functioning: These streams are controlled bysubmerged and emergent vascular plants and by somefloating macrophytes, such as Lemna (duckweed).They may resemble wetlands. In most cases suppliesof P and N from the sediment are sufficient to supportmacrophyte growth.

Management considerations: There may be situa-tions where N becomes deficient. Decreasing N loadsto the stream may result in gradual depletion of N inthe sediment. Macrophyte growth would then begradually curtailed. More riparian trees and shrubs canincrease shading and lower temperature and lightavailability in the stream water.

Monitoring and further analysis: Sediment input tothese streams is the most important factor to try toestimate and control. Secondarily, if there is evidenceof N limitation (low N concentrations in stream sedi-ment) it may be worthwhile to monitor N inputs toassess the feasibility of attempting N limitation.

System D—Periphyton-dominated small stream


Phosphorus Highly sensitive Highly sensitive


System functioning: This system can respondquickly to increases in P. Because there is little sedi-ment memory, these systems also respond quickly todecreases in P. In most cases N is not the limitingfactor unless ground water P is high and N input is lowas is sometimes the case for undisturbed forest.

Management considerations: Because periphytonextract all their nutrients from the water, lowering Pwater concentrations inhibits growth. Ground waterdischarge may be an important source of N and P.

Monitoring and further analysis: Because periphy-ton are capable of quickly extracting nutrients fromthe water column, monitoring growing season watercolumn concentrations is not recommended. Groundwater moving into the stream through the streambottom may also be an important source of nutrients.Ground water concentrations can be determined usingmonitoring wells, but wells will not provide dischargeinformation. Nutrient loading rates (product of con-centration times discharge) from ground water can beestimated using devices to measure ground waterdischarge rates and collecting ground water frombelow the streambed or from seeps along the banksfor concentration analysis. Concentrations can also beestimated by monitoring surface water concentrationsduring periods when algae are not growing. However,the sampling should be as representative as possible ofgrowing season conditions.


System E—Heterotrophic large stream




System functioning: These systems are character-ized by high inorganic turbidity. The water appearanceis light brown because of the suspended sediment. Thehigh turbidity prevents phytoplankton, periphyton, androoted plant growth because of light limitation. Al-though aesthetically unappealing, these systems donot experience water quality problems from eutrophi-cation. Other problems may result from the suspendedsediment, however. Fish and other organisms may beadversely affected.

Management considerations: Management goals forthese systems can include reducing turbidity. Asturbidity is reduced, the trophic state may shift tomesotrophic or eutrophic.

Monitoring and further analysis: Water column TPmeasurements help estimate the eutrophication poten-tial. Computer models are useful to estimate nutrientloading under various scenarios of suspended sedi-ment control.

System F—Periphyton-dominated, large stream




System functioning: In these streams and rivers,water velocity is sufficiently high to prevent macro-phytes from establishing throughout, although back-water areas may allow them to root. Two factors limitmacrophyte presence—soft bottom material is gener-ally swept away by the current, and the plants thathave a high cross-sectional area and shallow roots areuprooted and washed away.

Management considerations: Because periphytonextract all their nutrients from the water, loweringwater P concentrations reduces growth. However, ifground water is an important source of P, controllingnutrients in surface inputs may not be fully successful.

Monitoring and further analysis: Water column Pconcentration is not a useful characteristic to monitorexcept during periods when plants are not growingbecause periphyton remove P from the water columnas they grow. Ground water inputs of nutrients may beimportant.


System G—Macrophyte-dominated large stream or lake




System functioning: These systems are character-ized by macrophytes because residence time is notsufficient to permit phytoplankton populations tobuild. The muddy bottom provides good substrate formacrophytes.

Management considerations: Because macrophytesextract nutrients from the stream or lake bottomsubstrate, they generally are insensitive to changes inP or N inputs to the water column over the short term.Over the long term, sediment may either accumulateor release P and N until they are in equilibrium withthe water column. Exceptions can occur where sedi-ment inputs decrease while sediment losses remainconstant; gradual loss of the soft substratum couldresult in exposure of buried hard material. This wouldresult in a shift to system H.

Monitoring and further analysis: Water columnnutrient concentrations are less useful to monitor inthese systems because growth is supported by sedi-ment sources.

System H—Periphyton-dominated large stream or lake




System functioning: These systems do not havesufficient residence time to allow phytoplanktonpopulations to establish and are clear enough to sup-port periphyton growth. The substrate is hard and notoptimal for macrophytes. Periphyton, therefore, arethe dominant type of producer.

Management considerations: These systems arehighly susceptible to inputs that continually enter thesystem. Periphyton make efficient use of low concen-trations of soluble P or N in water. Sediment retentionof nutrients will be low so the system should quicklyrespond to reductions in loading.

Monitoring and further analysis: Input of nutrientsvia surface and ground water is the critical factor forthese systems. However, monitoring is complicated bythe removal of nutrients from the water column byperiphyton. Loads can be estimated by monitoringwater concentrations during periods when algae arenot growing. However, the sampling should be asrepresentative as possible of growing season condi-tions. Ground water concentrations and discharge maybe important.


System I—Large stream that shifts between phytoplanktonand macrophytes


Phosphorus Moderately sensitive Insensitive


System functioning: These systems have sufficientresidence time that phytoplankton can become estab-lished. If phytoplankton are established they mayshade out macrophytes. These systems can be com-plex and difficult to predict. Temperature can beimportant. If winter is sufficiently cold, substantialmacrophyte dieback could give phytoplankton anadvantage. Spring time P concentrations may be acritical factor.

Management considerations: Macrophyte domina-tion may be advantageous for sport fish production.

Monitoring and further analysis: Spring and sum-mer TP concentrations can be monitored along withmacrophyte abundance to determine the extent thatphytoplankton production might be controlled throughreduction of P loading.

System J—Large stream that shifts between phytoplanktonand periphyton




System functioning: These systems have sufficientresidence time that phytoplankton can become estab-lished. The substrate is not suitable for macrophytesso periphyton are expected. If phytoplankton areestablished, they may shade out periphyton.

Management considerations: If soft sediment is notprevalent, the system may have low internal loads(sediment memory) and respond quickly to nutrientload reductions.

Monitoring and further analysis: During periods ofphytoplankton domination (indicated by reducedwater clarity or greenish color), water column TP canbe monitored to estimate loading rate. Nongrowingseason monitoring of TP can also be used to estimategrowing season loading. However, the sampling shouldbe as representative as possible of growing seasonconditions. Ground water inputs may be significant.


System K—Oligotrophic or mesotrophic deep lake or reservoir


Phosphorus Highly sensitive Insensitive (if oligotrophic)Sensitive (if mesotrophic)


System functioning: These lakes are deep enough tostrongly stratify, yet the oxygen demand within thehypolimnion and sediment is not sufficiently large todeplete all of the DO within the hypolimnion. Becausethe hypolimnion remains oxic, P within the sedimentstays bound. Nutrient loading to these lakes is alreadylow to moderate, which supports a mesotrophic oroligotrophic state.

Management considerations: These systems areextremely sensitive to increases in P loading. A smallincrease in algal production may be sufficient todeplete hypolimnetic DO, which would release sedi-ment-bound P. This P would make its way into theepilimnion either through diffusion or during turnover.The additional internal loading combined with thecontinuing external loading would move the lake intoa eutrophic or hypereutrophic state.

Monitoring and further analysis: Total P andSecchi depth should be monitored regularly. Water-shed loading models can help identify loading sourcesand critical areas. Total P inputs of major streams canbe monitored.

System L—Estuary



Nitrogen Sensitive Sensitive

System functioning: Estuaries have sufficient Psupplied from marine water and from rapid cyclingthrough sediment in most cases, although exceptionshave been found. Denitrification (reduction of nitrateto gaseous forms that are lost to the atmosphere)occurs in the sediment so N is frequently the limitingnutrient for aquatic plant growth.

Management considerations: Estuaries respond toreduced N loading rates.

Monitoring and further analysis: Estuarine dynam-ics are complex. The sensitivity and responsiveness toN loading changes depend on circulation patterns,mixing patterns, and several other factors. MonitoringN loading from tributaries, atmospheric sources, andground water is important for refining managementplans.


System M—Stratified deep lake with hypolimnetic P transfer


Phosphorus Moderately sensitive Insensitive


System functioning: These lakes are mesotrophic,eutrophic, or hypereutrophic. They stratify, and Pconcentrations build up in the anoxic/anaerobic hy-polimnion. A fair amount of hypolimnetic P is trans-ferred to the epilimnion because of wind-inducedturbulence.

Management considerations: These systems areextremely difficult to restore. Aggressive measures,such as P inactivation by chemical treatments andhypolimnetic aeration, may be needed.

Monitoring and further analysis: Careful analysisby a professional limnologist is advised to determinewhat combination of load reductions and in-laketreatments will be successful.

System N—Stratified deep lake


Phosphorus Sensitive Insensitive


System functioning: These lakes can be oligotrophic,mesotrophic, eutrophic, or hypereutrophic. Thehypolimnetic P transfer is less than that in system Mlakes. Because of this, external loads from surfacewater inputs are relatively more important. Becausethe hypolimnion becomes anoxic during part of theyear, internal P loading occurs. This released P is notavailable to algae until it reaches the photic zone,often during turnover.

Management considerations: P loading needs to bereduced to prevent further eutrophication in theselakes. The response to load reductions depends on therelation between internal load and external load andthe history of sediment P loading.

Monitoring and further analysis: Monitor TPthrough the year, particularly during turnover events.Estimates of surface water P loading rates are useful.


System O—Polymictic lake


Phosphorus Sensitive if mesotrophic/oligotrophic Sensitive if mesotrophic/oligotrophicInsensitive if highly eutrophic Insensitive if highly eutrophic(TP >50 ug/L) (TP >50 ug/L)

Nitrogen Insensitive Insensitive(sensitive if hypereutrophic)

System functioning: Polymictic lakes experienceshort-term stratification. If they are currently at amesotrophic or oligotrophic state, they are sensitive toincreases in P loading. The risk is when the bottomwater goes anaerobic during the usually short periodsof stratification. When that happens, P is released fromthe sediment and the frequent mixing events deliver Pto the epilimnion. In cases where the bottom watergoes anaerobic, internal P loading rates can be veryhigh, producing a hypereutrophic condition in which Nmight be limiting. In such cases blooms of N2-fixingcyanobacteria may dominate the algal community.

Management considerations: The current conditionof the lake dictates sensitivity. If the lake is me-sotrophic or oligotrophic, it is extremely vulnerable toincreased P loadings. If the lake is eutrophic, it isdifficult to alter the trophic state because of the highinternal loading rate. If the lake is highly eutrophic (TP>50 ug/L) algae are most likely light limited and maynot respond to further increases in P loading.

Monitoring and further analysis: If the lake ishighly eutrophic, careful analysis by a professionallimnologist is advised to determine what combinationof load reductions and in-lake treatments will besuccessful.

System P—Shallow lake with high macrophyte coverage


Phosphorus Sensitive Insensitive

Nitrogen Insensitive Potentially sensitive

System functioning: These lakes are macrophytedominated (at least 50 percent of surface area). Shal-low lakes have two alternative stable states—phy-toplankton-dominated or macrophyte-dominated.Macrophytes are beneficial in these systems. Theykeep sediment from being suspended and, therefore,help keep nutrient levels in the water column low.Macrophytes keep the water clear and protect zoop-lankton that graze on phytoplankton. They providehabitat for sport fish, such as bass. If the macrophytesare removed or if P loading increases, the systemcould be shifted to phytoplankton-dominated. In thisstate, macrophytes cannot reestablish because ofshading by phytoplankton. The sediment (and associ-ated P) is vulnerable to resuspension from wind-induced turbulence or from bioturbation (bottom-feeding fish stirring up the sediment). Sport fishpopulations decline. These systems may be N-limited

because the macrophytes can get sufficient P fromsediment while denitrification is occurring within thesediment.

Management considerations: In these systems it ispractically impossible to get rid of the aquatic "weeds"and not have phytoplankton take over. Conditionsunder phytoplankton domination would be less suit-able for human use (there would be reduced fishing,tainted fish flesh, reduced water clarity, and a highertrophic state). The macrophytes should be accepted asbeneficial in these systems.

Monitoring and further analysis: Secchi depth orchlorophyll-a monitoring in the open part of the lakeprovides early warning if the lake begins to shift frommacrophyte domination to phytoplankton domination.


System Q—Shallow lake with low macrophyte coverage




System functioning: Refer to the description ofsystem P. System Q is the other alternate stable statefor this type of lake. High P concentrations result fromresuspension of sediment. External loading rates havelittle effect.

Management considerations: Converting this sys-tem to macrophyte domination would be beneficial.However, attempts by researchers and lake managersto accomplish this shift have had mixed results. Tem-porary elimination of bottom feeding fish may benecessary.

Monitoring and further analysis: An estimate ofexternal and internal loading (calculated from massbalance) is useful to assist in analysis by a profes-sional limnologist.

System R—Shallow lake with short residence time andhigh turbidity




System functioning: This system does not havesufficient residence time to permit phytoplankton toestablish and, because of high turbidity, is light lim-ited.

Management considerations: If the turbidity levelswere reduced, periphyton and/or macrophytes willestablish.

Monitoring and further analysis: Watershed load-ing models can be used to identify sources of turbidityand nutrients and to evaluate mitigation alternatives.


Next steps

The procedure described in this document is a screen-ing level analysis. In many cases the results of thisanalysis are sufficient for the management needs athand. In other situations a more thorough analysis ofthe waterbody and its watershed may be necessary oradvisable.

The next steps a conservationist might take are de-scribed in this section. This section is organized intothree subsections as follows:

• Gaining more knowledge or professional assis-tance

• Obtaining more information about the waterbodyand watershed

• Using computer-based analysis tools

Gaining more knowledge orprofessional assistance

Undertaking a more thorough analysis of the responseof a waterbody to changes in nutrient loading requiresa basic understanding of limnology and aquatic ecol-ogy. The interrelationship of chemical, physical, andbiological processes in aquatic ecosystems is a fasci-nating topic. Conservationists who are intrigued bywhat they have learned as a result of using the proce-dure in this document should consider further readingin limnology and aquatic ecology. The following booksand papers are recommended.

Texts and manuals:

United States Environmental Protection Agency. 1988.The lake and reservoir restoration guidancemanual. EPA 440/4-90-006. Available fromNCEPI, 11029 Kenwood Road Building 5, Cincin-nati, OH 45242, Fax 513-489-8695, and from theEPA Clean Lakes Web site at www.epa.gov/owowwtr1/lakes/quality.html

Moss, Brian. 1998. Ecology of fresh waters: man andmedium, past to future. Blackwell Science,Malden, MA, 557 p.

Welch, E.B. 1992. Ecological effects of wastewater:applied limnology and pollution effects, 2nd ed.Chapman and Hall.

Cooke, G.D., E.B. Welch, S.A. Peterson, and P.R.Newroth. 1993. Restoration and management oflakes and reservoirs. Lewis Publishers: BocaRaton, FL.

General eutrophication reviews:

Carpenter, S.R., N.F. Caraco, D.L. Correll, R.W.Howarth, A.N. Sharpley, and V.H. Smith. 1998.Nonpoint pollution of surface waters with phos-phorus and nitrogen. Ecological Applications8:559-568.

Correll, D.L. 1998. The role of phosphorus in theeutrophication of receiving waters: a review. J.Env. Qual. 27:261-266.

Smith, V.H. 1998. Cultural eutrophication of inland,estuarine, and coastal waters. In (M.L. Pace andP.M. Groffman, eds.) Successes, limitations, andfrontiers in ecosystem science. Springer-Verlag,New York, p. 7– 49.

Rivers and streams:

Dodds, W.K., V. H. Smith, and B. Zander. 1997. Devel-oping nutrient targets to control benthicchlorophyll levels in streams: a case study of theClark Fork River. Water Research 31:1738-1750.

Dodds, W.K., J.R. Jones, and E.B. Welch. 1998. Sug-gested classification of stream trophic state:distributions of temperate stream types by chlo-rophyll, total nitrogen, and phosphorus. WaterResearch 32:1455-1462.

If the resource management situation is particularlycomplex or involves costly management alternatives(and if funds are available), a professional limnologistshould be consulted. A professional limnologist is ableto efficiently conduct an assessment, develop a planfor monitoring if necessary, and develop managementalternatives and recommendations. The North Ameri-can Lake Management Society maintains a listing ofCertified Lake Managers and has local chapters inmost States that can provide assistance in locating aprofessional limnologist (visit their Web site atwww.nalms.org). Engineering firms in your state mayhave specialized expertise in limnological analyses.Review the credentials of a prospective consultant andreports from similar studies they have completed. Talkwith their previous clients and with others in a posi-tion to offer a recommendation, such as state agencyand university scientists.


Obtaining more information

To conduct a more thorough analysis, additionalinformation will most likely be needed. Informationgenerally falls into two categories—information aboutthe waterbody and information about the watershed.

Information about the waterbody encompasseswaterbody characteristics, such as physical dimen-sions and configuration; seasonal patterns of watertemperatures and stratification; spatial and temporalpatterns of oxygen, phosphorus, chlorophyll, andnitrogen concentrations; and areal extent of macro-phyte beds. Information about the watershed includesland use, hydrology, nutrient loads, and other informa-tion related to sources of nutrients.

The information to be collected should be guided bythe analyses that will be conducted. For example, toevaluate source contributions from different areas,data on land use are needed, and, if models are used,data on soils, slopes, and crops may be required. Toassess the current internal and external phosphorusloading rates, tributary phosphorus concentrations,lake TP concentrations, and hypolimnetic DO and TPconcentrations are needed.

Some information requires data collected through amonitoring system. It is critically important that amonitoring system be carefully designed. Key aspectsof monitoring system design follow:

Monitoring objective—A successful monitoringsystem requires a clear, concise objective statementand that the monitoring system be designed to addressthat objective with statistical rigor. A monitoringsystem designed to address a narrow, specific objec-tive is more likely to be successful than unfocusedmonitoring.

Statistical considerations—Part of the design of amonitoring system is identifying the statistical analy-ses that will be used to interpret the data and ensuringthat the design collects the correct types of data andenough data to support the analyses with a knownlevel of statistical confidence.

Quality assurance/quality control—Without carefulattention to quality assurance/quality control (QA/QC),the data may be unusable. QA/QC measures must beestablished, and they must be followed. Standardoperating procedures for both the field and the labora-tory, use of field blanks/replicates/spikes, samplechain of custody, analysis methods, and lab QA arejust a few of the aspects of QA/QC that should beconsidered.

Data management—Data entry is a common sourceof errors. Error checking within the data entry pro-gram and double data entry are recommended. Datashould be reviewed and analyzed on a regular basis,preferably quarterly, to spot any potential problems inthe monitoring system operation.

Several useful manuals are available to assist in thedesign and implementation of a monitoring system.They include:

United States Department of Agriculture, NaturalResources Conservation Service. 1996. Nationalhandbook of water quality monitoring. 450-vi-NHWQM, December 1996. National Water andClimate Center, Portland, Oregon.

United States Environmental Protection Agency. 1990.Monitoring lake and reservoir restoration. EPA440-4-90-007.

United States Environmental Protection Agency. 1991.Volunteer lake monitoring: a methods manual.EPA 440/4-91-002.

United States Environmental Protection Agency. 1993.Statistical methods for the analysis of lake waterquality trends. EPA 841-R-93-003.

United States Environmental Protection Agency. 1996.The volunteer monitor's guide to quality assur-ance project plans. EPA 841-B-96-003.

United States Environmental Protection Agency. 1997.Volunteer stream monitoring: a methods manual.EPA 841-B-97-003.

Computer-based planning tools

Computer-based planning tools are available to helpestimate loadings generated within a watershed orpredict responses within an aquatic system. Thesetools rely on models of physical, chemical, and biologi-cal processes. Models are mathematical simplifica-tions designed to describe the behavior of complexsystems. If a model can accurately capture the mostimportant processes occurring in a system, it can beused to both describe the system and predict its be-havior. Models are also useful as a means of assem-bling important information and as an aid to diagnosisand planning.

Models vary greatly in their complexity. A good over-view of models was provided by EPA (1992). Simplemodels require the least effort to set up and use, butthey frequently require data or empirical relationships


specific to a type of situation or local region. Simplemodels are compilations of expert judgment andempirical relationships that can often be applied byusing a spreadsheet program or handheld calculator.They rely, in general, on large-scale aggregation andneglect important features of small patches of land.They rely on generalized sources of information and,therefore, have low requirements for site-specific data.Predictive relationships are derived from empiricalrelationships that are based on regional or site-specificdata. Outputs are generally expressed as mean annualvalues.

Mid-range models attempt a compromise between theempiricism of simple models and the complexity ofdetailed mechanistic models. The advantage of mid-range models is that they evaluate pollution sourcesover broad geographic scales with greater resolutionand can be used to target conservation efforts. Severalmid-range models are designed to interface with ageographic information system (GIS), which greatlyfacilitates input parameter estimation. Greater use ofsite-specific input data compared to simple modelsgives these models relatively broad applicability indifferent regions. However, the use of simplifyingassumptions limits the accuracy of their predictions towithin about an order of magnitude and restricts theirusefulness to relative comparisons of scenarios.

Complex models best represent the current under-standing of watershed and water quality processes.Because they attempt to simulate the specific mecha-nisms that drive processes, they are called mechanisticor process-based models. If properly applied andcalibrated, complex models can provide relativelyaccurate predictions of variable flows, pollutant con-centrations, and water quality anywhere in the water-shed. The greater resolution and accuracy comes atthe expense of considerably more time and resourceexpenditure. These models must generally be appliedby highly skilled specialists. The input and output ofcomplex models have greater spatial and temporalresolution. Because of their focus on processes, theycan be used to predict the effects of different designconsiderations for practices.

Table 4 lists several commonly used watershed orwaterbody models in order of their complexity. It isimportant that any user understand the benefits andlimitations of a given model for his or her question. Abrief description of each model follows.

Table 4 Comparisons of model types for P and N nonpoint source pollution

User resources Simple Midrange Complex

Setup time Minimal Significant

Monitoring data Usually not required Required

Time step Typically annual Daily/hourly

Equations/algorithms Empirical Process-based

Input data required Few parameters Many parameters

Model name

Model type 1/ L, W L, W W L L L, W L, W L L L L, W L

1/ L = watershed loading model; W = water quality response model.

GW

LF

SWA

T

BA

SIN

S

AG

NP

S

SWM

M

AN

SWE

RS

HSP

F

WE

PP

WIL

MS

EU

TR

OM

OD

BA

TH

TU

B

WE

ND


Simple models:

WILMS (Wisconsin Lake Model Spreadsheet)A spreadsheet-based procedure developed usingregression equations developed from internationallake data sets. Watershed loads are estimated usingwatershed area in different land uses and exportcoefficients. Lake response is based on empiricallyderived response models. Ten lake response modelsare available to predict spring overturn and growingseason mean TP concentrations. The procedure pro-vides parameter range guidance to help the user selecta lake response model and also includes an uncer-tainty analysis, watershed load back-calculation, lakecondition description, and a phosphorus steady stateresponse time estimate (Panuska, et al., 1994).

Available at www.nalms.org.

EUTROMOD

A spreadsheet-based modeling procedure for eutrophi-cation evaluation developed at Duke University anddistributed by the North American Lake ManagementSociety. EUTROMOD is a watershed and lake modeldesigned to estimate nutrient loadings and the result-ing trophic state parameters. The parts of a watershedarea in different land use categories are the basis forestimating nutrient exports. Computation algorithmswere developed based on empirically derived statisti-cal relationships from lakes data from across theUnited States.

Available at www.nalms.org.

BATHTUB

One of a suite of three simplified models developed bythe Corps of Engineers. The suite focuses on lake andreservoir response and not on watershed loadingestimation. The interrelated programs (FLUX, PRO-FILE, and BATHTUB) simplify assessments ofeutrophication-related processes and effects. FLUXallows estimation of tributary mass discharges (load-ings) from sample concentration data and daily flowrecords. Five estimation methods are available, andpotential errors in estimates are quantified. PROFILEfacilitates the analysis of in-lake water quality data.Algorithms are included for calculation ofhypolimnetic oxygen depletion rates and estimation ofarea-weighted, surface-layer mean concentrations ofnutrients, and other eutrophication response variables.BATHTUB applies a series of empirical eutrophicationmodels to morphologically complex lakes and reser-voirs. The program performs steady-state water andnutrient balance calculations in a spatially segmentedhydraulic network that accounts for advective anddiffusive transport and nutrient sedimentation.

Eutrophication-related water quality conditions (totalphosphorus, total nitrogen, chlorophyll-a, transpar-ency, and hypolimnetic oxygen depletion) are pre-dicted using empirical relationships derived fromreservoir data.

Available from the Environmental Laboratory, U.S.Army Engineer Waterways Experiment Station, 3909Halls Ferry Road, Vicksburg, Mississippi 39180. Website at www.wes.army.mil/el/elmodels/emiinfo.html.

WEND (Watershed Ecosystem Nutrient Dynamic)A watershed-scale phosphorus budget model thatsimulates the generation of animal wastes, utilizationand management, distribution within the watershed,and transfer to watercourses. The model identifies andquantifies pathways of all significant P import, export,and internal cycling fluxes on an average annual basis.Evaluations can be conducted for a variety of manage-ment scenarios to simulate changes in watershed Pdynamics over long time periods. WEND emulates theinfrastructure through which P is stored and cycled ina watershed and must be tailored to reflect watersheddifferences. The model operates in STELLA, an objectoriented programming language. Planned enhance-ments include nitrogen fluxes and pathogen dynamics.

Available from Alan Cassell, School of Natural Re-sources, Aiken Hall, University of Vermont,Burlington, VT 05405-0088, [email protected] is required and can be purchased from:High Performance Systems, Inc., 45 Lyme Rd., Suite200, Hanover, NH 03755 (800) 332-1202. Web site atwww.hps-inc.com.

Mid-range models:

GWLF (Generalized Watershed Loading Function)A mid-range model developed at Cornell University toaddress P and N loading from large mixed land usewatersheds (Haith and Shoemaker, 1987; Dodd andTippett, 1994). Based on loading functions to estimatenutrient loads produced by a watershed. Loadingfunctions represent a middle ground between theempiricism of export coefficients and the complexityof chemical simulation models. GWLF was developedwith the expressed purpose of requiring no calibrationand makes extensive use of default parameters.

An enhanced version was developed by the ResearchTriangle Institute in FoxPro for Windows.TM ContactMichael McCarthy, Research Triangle Institute, P.O.Box 12194, Research Triangle Park, NC 27709, (919)541-6895.


SWAT (Soil and Water Assessment Tool)Designed to predict the effect of management deci-sions on water, sediment, nutrient, and pesticide yieldswith reasonable accuracy in large, ungaged riverbasins. Nutrients and pesticides are considered forpollutant transport. Sediment transport and depositionthrough ponds, reservoirs, and channels is modeled.Nutrient transformations are not evaluated.

Supported by the USDA Agriculture Research Service,Grassland Soil and Water Research Laboratory, 808 E.Blackland Rd., Temple, TX 76502, Phone: (254) 770-6500. Visit their Web site at www.brc.tamus.edu/swat/.

BASINS

A relatively user-friendly package developed by theEPA in which key data and analytical components arebrought together in one CD. Data include STORETwater quality data and other environmental data, pointsources information, and various GIS layers. Analysistools are grouped into five categories:

• national data bases• assessment tools (TARGET, ASSESS, and Data

Mining)• utilities including local data import, land-use and

DEM reclassification, watershed delineation andmanagement of watershed delineation, andmanagement of water quality observation data

• watershed and water quality models including anonpoint source screening model (based onHSPF), TOXIROUTE, and Qual2E

• post-processing output tools for interpretingmodel results

Data bases and assessment tools are directly inte-grated within an ArcViewTM GIS environment. Thesimulation models run in a WindowsTM environment,using data input files generated in ArcView.

Available free of charge from the USEPA on the Webat www.epa.gov/OST/BASINS. ArcView 3.x is required.

AGNPS98 - AGNPS

AGNPS was originally developed as a single eventmodel to predict the amount of runoff, sediment, andnutrients produced by farmland during a storm event.It has been used widely in the Midwest. A grid cellsystem is used to represent the spatial distribution ofwatershed properties. Model output includes runoff,sediment, nutrients, and pesticide loads although thereis no chemical transformation of P or N. AGNPS98 isan upgraded version running in a continuous simula-tion mode. Data input is time consuming although aGIS interface is planned.

Available from the USDA Agriculture Research Ser-vice, National Sedimentation Laboratory, Oxford,Mississippi. Web site at www.sedlab.olemiss.edu/AGNPS98.

Complex models:

SWMM (Storm Water Management Model)A detailed watershed model originally designed toaddress urban stormwater hydrology. A large, com-plex model capable of simulating the movement ofprecipitation and pollutants from the surface throughpipe and channel networks, storage/treatment units,and finally to receiving water. Both single-event andcontinuous simulation may be performed oncatchments having storm sewers and natural drainagefor prediction of flows, stages, and pollutant concen-trations. Can be used for both planning and design.The planning model is used for an overall assessmentof urban runoff and proposed abatement options. Adesign-level, event simulation also may be run using adetailed catchment schematization and shorter timesteps for precipitation input. Data intensive and re-quires calibration and validation. Main utility is forurban areas where the design of pollution controlstructures is part of the study.

Available from the USEPA, Environmental ResearchLaboratory, Athens, Georgia. Web site at http://www.epa.gov/CEAM.

ANSWERS (Areal Nonpoint Source Watershed Envi-ronmental Response Simulation Model)A comprehensive model to evaluate the effects of landuse management in both agricultural and urban water-sheds. Focuses on sediment erosion and transportduring storm events. A mainframe computer is re-quired, there are no chemical transformations of N orP, and building input files is complex and time con-suming.

Distributed by the Department of Agriculture Engi-neering, North Carolina State University, Raleigh,North Carolina, (919) 515-2694.

HSPF (Hydrological Simulation Program-FORTRAN)A comprehensive modeling package developed by theEPA to simulate hydrology and pollutant fate in com-plex watersheds. Extensive monitoring data areneeded for calibration. Requires highly trained staff.

Supported by USEPA, Environmental Research Labo-ratory, Athens, Georgia. Web site at http://www.epa.gov/CEAM.


WEPP (Water Erosion Prediction Project)Process-based, distributed parameter, continuoussimulation, erosion prediction model for use on per-sonal computers. Does not simulate nutrient transport.Current version (v98.4) available through the Internetis applicable to hillslope erosion processes (sheet andrill erosion) as well as simulation of the hydrologicand erosion processes on small watersheds.

Supported by USDA Agriculture Research Service,National Sedimentation and Erosion Laboratory, 1196Building SOIL, Purdue University, West Lafayette,Indiana 47907-1196, (765) 494-8673. Web site at http://topsoil.nserl.purdue.edu/weppmain/wepp.html.


Appendix A Phosphorus cycle in aquaticecosystems

AdsorbedPO4

AdsorbedPO4

(suspended)

Fe, Alcompounds

SolublePO4

Pore waterPO4

Organic P

Organic P

Algae/plants/cyanobacteria

Fe, Alcompounds

Decomposition

Dissolution inanaerobic conditions

Uptake

Die off

Terrestrial input

Organic P Adsorbed P Soluble P


Appendix B Nitrogen cycle in aquatic ecosystems

NitrateNO3

AmmoniumNH4

+/NH3

Organic nitrogen

Animals/bacteriaAlgae/plants/cyanobacteria

DisolvedN2

Uptake

Uptake

GaseousN2, N2O, NO

Consumption

Die off/excretionDie off

Mineralization

Nitrification(aerobic)

Denitrification(anaerobic)

Fixation bycyanobacteria

Terrestrial input

Organic N Ammonium N

Nitrate N


Literature cited

Boynton, W.R., W.M. Kemp, and C.W. Keefe. 1982. Acomparative analysis of nutrients and otherfactors influencing phytoplankton production. InV.S. Kennedy (ed.), Estuarine Comparisons,Academic Press, NY, pp 93-109.

Carpenter, S.R., N.F. Caraco, D.L. Correll, R.W.Howarth, A.N. Sharpley, and V.H. Smith. 1998.Nonpoint pollution of surface waters with phos-phorus and nitrogen. Ecological Applications8:559-568.

Cooke, G.D., E.B. Welch, S.A. Peterson, and P.R.Newroth. 1993. Restoration and management oflakes and reservoirs. Lewis Publishers: BocaRaton, FL.

Correll, D.L. 1998. The role of phosphorus in theeutrophication of receiving waters: a review. J.Env. Qual. 27:261-266.

Dodd, R.C., and J.P. Tippett. 1994. Nutrient modelingand management in the Tar-Pamlico River Basin.Research Triangle Institute.

Dodds, W.K., and E.B. Welch. 1998. Trophic classifica-tion. In USEPA, Establishing nutrient criteria forrivers and streams (draft).

Dodds, W.K., J.R. Jones, and E.B. Welch. 1998. Sug-gested classification of stream trophic state:distributions of temperate stream types by chlo-rophyll, total nitrogen, and phosphorus. WaterResearch 32:1355-1462.

Dodds, W.K., V.H. Smith, and B. Zander. 1997. Develop-ing nutrient targets to control benthicchlorophyll levels in streams: a case study of theClark Fork River. Wat. Res. 31:1738-1750.

Fong, P.R., M. Donohoe, and J.B. Zedler. 1993. Compe-tition with macroalgae and benthiccyanobacterial mats limits phytoplankton abun-dance in experimental mesocosms. MarineEcology Progress Series 100: 97-102.

Haith, D.A., and L.L. Shoemaker. 1987. Generalizedwatershed loading functions for stream flownutrients. Water Resources Bulletin, Vol. 23, No.3, pp. 471-478.

Hecky, R.E., and P. Kilham. 1988. Nutrient limitation ofphytoplankton in freshwater and marine environ-ments: a review of recent evidence on the effectsof enrichment. Limnology and Oceanography 33:796-822.

Howarth, R.W. 1993. The role of nutrients in coastalareas. In Managing Wastewater in Coastal UrbanAreas, Report from the National Research Coun-cil Committee on Wastewater Management forCoastal Urban Areas, National Academy Press,Washington, DC, pp. 177-202.

Ketchum, B.H. 1951. The flushing of tidal estuaries.Sewage and Industrial Wastes 23:198-209.

Lathrop, Richard C., and Richard A. Lillie. 1980. Ther-mal stratification of Wisconsin lakes. Wisc. Acad.Sci, Arts, Let. 68:90-96.

Moss, Brian. 1998. Ecology of fresh waters: man andmedium, past to future. Blackwell Science,Malden, MA, 557 p.

Nixon, S.W. 1990. Marine eutrophication: a growinginternational problem. Ambio 19:101.

Osgood, Richard A. 1988. Lake mixis and internalphosphorus dynamics. Arch. Hydrobiol.113(4):629-638.

Panuska, John C., Nate Booth, and Anita D. Wilson.1994. Wisconsin lake model spreadsheet, version2.00 user's manual. Wisconsin Department ofNatural Resources, Madison, WI, 15 pp. with 3appendices.

Seaman, W. Jr. (ed.). 1988. Florida aquatic habitat andfishery resources. Florida Chapter, AmericanFisheries Society, Eustis, FL.

Smith, D.G., R.J. Davis-Colley, J. Knoef, and G.W. Slot.1997. Optical characteristics of New Zealandrivers in relation to flow. Journal of the AmericanWater Resources Association 33:301-312.

Smith, V.H. 1998. Cultural eutrophication of inland,estuarine, and coastal waters. In M.L. Pace andP.M Groffman (eds.), Successes, Limitations, andFrontiers in Ecosystem Ecology, Springer-Verlag,New York, pp. 7-49.

Terrell, Charles R., and Patricia Bytnar Perfetti. 1989.Water quality indicators guide: Surface waters.USDA-SCS, SCS-TP-161, 128 p.


United States Department of Agriculture, NaturalResources Conservation Service. 1996. Nationalhandbook of water quality monitoring.. NationalWater and Climate Center, Portland, OR, 450-vi-NHWQM, December 1996.

United States Environmental Protection Agency. 1988.The lake and reservoir restoration guidancemanual. EPA 440/4-90-006.

United States Environmental Protection Agency. 1990.Monitoring lake and reservoir restoration. EPA440-4-90-007.

United States Environmental Protection Agency. 1991.Volunteer lake monitoring: a methods manual.EPA 440/4-91-002.

United States Environmental Protection Agency. 1992.Compendium of watershed-scale models forTMDL development. EPA 841-R-92-002.

United States Environmental Protection Agency. 1993.Statistical methods for the analysis of lake waterquality trends. EPA 841-R-93-003.

United States Environmental Protection Agency. 1996.The volunteer monitor's guide to quality assur-ance project plans. EPA 841-B-96-003.

United States Environmental Protection Agency. 1997.Volunteer stream monitoring: a methods manual.EPA 841-B-97-003.

United States Department of Agriculture, NaturalResources Conservation Service. 1998. StreamVisual Assessment Protocol. NWCC TechnicalNote 99-1, National Water and Climate Center,Portland, Oregon.

Wisconsin Natural Resources Board. 1997. Technicaldevelopment of phosphorus water quality stan-dards: A report of the Phosphorus TechnicalWorkgroup. (draft dated May 1997.)


Glossary

Aerobic Describes life or processes that require the presence of molecular oxygen.

Algae Small aquatic plants that occur as single cells, colonies, or filaments.

Anaerobic Describes processes that occur in the absence of molecular oxygen.

Anoxia A condition of no oxygen in the water. Often occurs near the bottom offertile stratified lakes in the summer and under ice in late winter.

Autotrophic Ability to synthesize organic compounds from inorganic substrates, usinglight energy (photoautotrophs) or chemical energy (chemoautotrophs).

Bathymetric map A map showing the bottom contours and depth of a lake. Can be used tocalculate lake volume.

Biomass The mass of living organisms present in a waterbody at any one time; theresult of processes of growth and death in the system.

Chlorophyll-a A type of chlorophyll present in all types of algae, sometimes in directproportion to the biomass of algae.

Dissolved oxygen Gaseous oxygen in an aqueous solution. Expressed as milligrams O2 perliter of water. The saturation concentration decreases with increasingtemperature. At 20 degrees Celsius and 1 atmosphere, the saturation con-centration is 9.09 mg/L.

Epilimnion Uppermost, warmest, well-mixed layer of a lake during summertime ther-mal stratification. The epilimnion extends from the surface to the ther-mocline.

Eutrophic From Greek for "well nourished," describes a lake of high photosyntheticactivity and low transparency.

Eutrophication The process of physical, chemical, and biological changes associated withnutrient, organic matter, and silt enrichment and sedimentation of a lake orreservoir. If the process is accelerated by human influences, it is termedcultural eutrophication.

Fetch Length of lake or reservoir surface for wind to act upon; in general, thelonger the fetch, greater the possibility of wind effects on mixing.

Heterotrophic Ability to obtain energy through the use of organic compounds producedby other organisms.

Hypolimnion Lower, cooler layer of a lake during summertime thermal stratification.

Lentic Relating to standing water.

Limnology Scientific study of fresh water, especially the history, geology, biology,physics, and chemistry of lakes. Also termed freshwater ecology.

Littoral zone That part of a waterbody extending from the shoreline to the greatest depthoccupied by rooted plants.


Lotic Relating to running water.

Macrophytes Rooted and floating aquatic plants visible to the naked eye or larger than0.5 millimeters. These plants may flower and bear seed. Some forms arefree-floating without roots.

Mesotrophic Describes a waterbody of moderate plant productivity and transparency.

Metalimnion Layer of rapid temperature and density change in a thermally stratifiedlake. Resistance to mixing is high in the region.

Nitrogen An element essential to life. Occurs in water in various forms. The formsmost useful to organisms are organic nitrogen, nitrate (NO3

-–), and ammo-nia/ammonium (NH3/NH4

+). Aqueous concentration is expressed as mg N/L("nitrate-N," "ammonia-N"), which means expressed as the mass of N(atomic weight 14) and not the mass of nitrate or ammonia.

NTU Nephelometric turbidity unit. A measure of the scattering of light by par-ticles suspended within a water sample.

Nutrient An element or chemical essential to life.

Oligotrophic From the Greek for "poorly nourished," describes a lake of low plant pro-ductivity and high transparency.

Periphyton Complex assemblage of autotrophic and heterotrophic organisms attachedto submerged substrates and embedded in a polysaccharide matrix.

pH Refers to the power of hydrogen ion concentration in an aqueous solutionand is the negative log of the concentration of hydrogen ions. For example,pH = 7 means the concentration of H+ ions is 10–7 moles/liter. Values rangesfrom pH=1 (very acidic) to pH=14 (very basic). pH of 7 is neutral and mostsurface water ranges between pH values of 6 and 9.

Phosphorus An element essential for life. Occurs in water in various forms. The formsmost useful to organisms are organic P and orthophosphate. Concentrationof orthophosphate is often expressed as mg PO4–P/L; this means the formis PO4, but the reported mass is based on P (atomic weight 31).

Photic zone The lighted part of a lake where photosynthesis takes place. Extends downto a depth where photosynthesis and respiration are balanced by theamount of light available.

Phytoplankton Microscopic algae and microbes that float freely in open water.

Plankton Organisms that float freely in the water and are not attached to a substrate.

Primary productivity The rate at which algae and macrophytes fix or convert carbon dioxide tosugar in plant cells. Commonly measured as milligrams of carbon persquare meter per hour.

Residence time Commonly called the hydraulic residence time; the amount of time re-quired to completely replace the volume of water in a waterbody with anequal volume of "new" water.

Respiration Process by which organic matter is oxidized by organisms. The processreleases energy, carbon dioxide, and water.


Riparian The zone adjacent to a stream or any other waterbody (from the Latin wordripa, pertaining to the bank of a river, pond, or lake).

Secchi depth A measure of the transparency of water obtained by lowering a black andwhite disk of 20 cm diameter into water until it is no longer visible. Ex-pressed in units of depth.

Stratification Layering of water caused by differences in water density.

Thermal stratification Stratification caused by temperature-created differences in water density.

Thermocline A horizontal plane across a waterbody at the depth of the most rapid verti-cal change in temperature. See Metalimnion.

Trophic state The degree of nutrient enrichment of a waterbody.

Turnover The mixing of bottom and surface water following the breakup of stratifica-tion in lakes and reservoirs.

Water column Water in a waterbody between the interface with the atmosphere at thesurface and the interface with the sediment at the bottom. Concept derivesfrom vertical series of measurements used to characterize lake water.

Zooplankton Microscopic animals that float freely in lake water, graze on detritus par-ticles, bacteria, and algae, and may be consumed by fish.

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