importance of surface-subsurface exchange in stream ecosystems: the hyporheic zone

Importance of Surface-Subsurface Exchange in Stream Ecosystems: The Hyporheic ZoneAuthor(s): Stuart FindlaySource: Limnology and Oceanography, Vol. 40, No. 1 (Jan., 1995), pp. 159-164Published by: American Society of Limnology and OceanographyStable URL: http://www.jstor.org/stable/2838258 .

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Limnol. Oceanogr., 40(1), 1995, 159-164 ? 1995, by the American Society of Limnology and Oceanography, Inc.

Importance of surface-subsurface exchange in stream ecosystems: The hyporheic zone

Stuart Findlay Institute of Ecosystem Studies, Cary Arboretum, Box AB, Millbrook, New York 12545

Abstract In many streams, significant amounts of water are exchanged between saturated sediments surrounding

the open channel (the hyporheic zone) and the channel itself. Such exchanges with the hyporheic zone have the potential to cause large changes in streamwater chemistry because the rates of biogeochemical processes and the actual types of processes (e.g. anaerobic vs. aerobic metabolism) may be fundamentally different. I propose an organizational scheme for grouping stream systems into clusters of minimal, moderate, and maximal contribution of hyporheic metabolism to the overall ecosystem. Lack of information on quantitative hydrology for different stream systems prohibits actual testing of this framework. An alternative scheme is presented that organizes several well-studied streams as a function of subsurface residence time and differences in oxygen content. This second scheme is intended to stimulate discussion of conceptual frameworks for cross-system comparisons of the importance of hyporheic sediments in stream ecosystems.

It has been known for some time that water is exchanged laterally or beneath the stream channel with saturated sediments (Fig. 1). In this review, I define the hyporheic zone as those sediments hydrologically linked to the open stream channel. Definitions of subsurface stream habitats based on presence of stream organisms vs. true groundwater animals have also been used widely (e.g. Stanford and Ward 1988; Williams 1989). Stream- water chemistry, material budgets, etc. require knowledge of whether subsurface exchange has occurred and of the rates and types of processes occurring in the hyporheic zone.

Differences in rates of processes or presence-absence of particular processes may lead to large changes in the composition and quantity of dissolved materials carried by streamwater. Subsurface exchanges can affect the type and increase the rate of material transformation as water moves downstream. For example, the time-of-travel estimated for water in the stream channel may be too short to permit significant mineralization of organic nutrients. However, if hyporheic exchange is an important process, residence time within a reach and contact with subsurface sediments may result in dramatic alterations in material transported from the catchment to the receiving body of water. Moreover, the potential for streamwater contact with mineral surfaces, and anaerobic zones or entrain- ment of groundwater can lead to changes in composition that would not be predicted if one assumed that all water movement was within the open stream channel.

The significance of surface-subsurface exchange among fundamentally different environments is widely recognized in other fields. For instance, it is obvious that benthic nutrient regeneration is important in shallow marine

Acknowledgments Carolyn Fuss, Len Smock, Chris Hakenkamp, and Margaret

Palmer provided unpublished data. Discussions with David Strayer and Bill Sobczak and reviews by Cliff Dahm and an anonymous reviewer led to improvements in the manuscript.

systems and that processes such as wave-pumping and irrigation of animal burrows act to increase the connections between deep sediments and the overlying water column (e.g. Aller 1982). Similarly, processes that increase exchange between lake sediments and overlying water or regional groundwater have the potential to cause large changes in lake-water chemistry (Kratz et al. 1991).

The importance of surface-subsurface exchange in stream ecosystems has been known for many years, but only recently have a sufficient number of systems been examined to justify an attempt at review and synthesis. My goals here are to provide an introduction to this field of study for those in other disciplines and to synthesize and uncover generalities from the growing number of case studies in the literature.

Perhaps the clearest and best-known example of the importance of connections between surficial and hyporheic sediments concerns the supply of oxygen to subsurface sediments and the consumption of oxygen within those sediments. Many fish deposit their eggs a few centimeters into the streambed; the eggs require well-oxy- genated water, and work has been done on small-scale factors governing 02 penetration into these sediments (e.g. Whitman and Clark 1982). Also, including hyporheic sediments in stream 02 budgets can drastically alter the autotrophic-heterotrophic balance of stream ecosystems (Grimm and Fisher 1984). Because hyporheic sediments must be heterotrophic components of stream ecosystems, any exchange of stream water with these sediments will tend to decrease the P/R ratio, so that an apparently autotrophic surface water system actually relies on al- lochthonous carbon inputs.

The fact that 02 uptake or anaerobic metabolism in hyporheic sediments can be appreciable implies a fairly rapid renewal of organic carbon in deeper sediments. Two general mechanisms of carbon supply seem likely: the episodic burial of particulate organic C (POC) following a disturbance (Meyer 1988; Metzler and Smock 1990) and the transport of POC or dissolved organic C (DOC) into hyporheic sediments by streamwater or groundwater

159



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intrusions (Ford and Naiman 1989; Rutherford and Hynes 1987; Vervier and Naiman 1992; Findlay et al. 1993). Rapid increases in discharge have been shown to rework sediments to a considerable depth (Meyer 1988), and organic material may be deposited as floodwaters recede. For highly permeable sediments such as cobbles, interstitial flow velocities may be adequate to transport fine particulate organic matter some distance into hyporheic sediments (Leichtfried 1988). However in most cases, flows will be too slow (velocities of a few centimeters per hour or less) to carry particulate matter more than a short distance into these sediments before settling, impaction, or adsorption removes the vast majority of POC.

In many marine sediments and to a lesser extent in lake sediments, bioturbation is recognized as a major avenue of organic matter transport (see Lopez and Lev- inton 1987), but stream invertebrates are seldom consid- ered significant agents of sediment turnover. Without in- vertebrate-induced turnover, POC inputs to hyporheic sediments will be episodic and associated with high flow events, and this material will be gradually depleted over relatively long time intervals (many weeks-years).

In contrast to POC, DOC may be supplied more reg- ularly, particularly if upwelling of groundwater or down- slope movement of soil water are predominant inputs (Kaplan and Newbold 1993). In this case, while there will be variability in DOC supply in response to variations in hydrology (Vervier and Naiman 1992; Hendricks 1992) or inputs of soil water from different soil horizons (East-

house et al. 1992), the supply of external DOC will, on average, be more predictable. The fact that few, if any, invertebrates can use DOC directly as a carbon source indicates the need for biotic or abiotic transformation into particulate form. Microbial uptake of DOC has been shown or inferred in several cases (Hendricks 1993; Find- lay et al. 1993), and adsorption of DOC to sediment particles or biofilms may be a significant process (Dahm 1981; McDowell 1985; Biirlocher and Murdoch 1989; Fiebig 1992). Variations in the quality of DOC may be important in controlling the trophic significance of DOC inputs, with the expectation that input of DOC derived from stream benthic algae may be readily assimilated by hyporheic microbes. Dissolved humic materials (largely fulvic acids, Thurman 1985) are commonly viewed as refractory to microbial decomposition or even inhibitory (Freeman and Lock 1992), but evidence from other systems suggests that these compounds can support bacterial growth at reasonable efficiencies (Moran and Hodson 1 990).

Our understanding of hyporheic food webs is rudi- mentary, but it seems likely that variations in the quantity and quality of POC and DOC will affect the types of organisms present and the overall secondary productivity of the hyporheic zone. POC may be directly used by a variety of invertebrates and so may represent a more direct pathway of carbon transfer than the "microbial loop" with its inherent respiratory losses. In contrast, hyporheic sediments that rely on DOC inputs may de-



Significance of hyporheic exchange 161

pend on biofilm processes to retain (biotically or abiot- ically) DOC in transport for subsequent attack by extra- cellular enzymes (Lock 1993). Thus, a POC-dominated system suggests a fauna with opportunistic, flexible strat- egies, while a DOC-dominated system may require a mi- crobially mediated transfer of DOC to larger consumers.

Given the high rates of metabolism in most hyporheic sediments, it is not surprising that there is good evidence for nutrient regeneration and return of mineral nutrients to the open stream channel (Hendricks and White 1991; Valett et al. 1990). Water returning to the stream channel may have elevated levels of inorganic N and P such that localized algal blooms occur at these discharge areas (Grimm et al. 1991; Valett et al. 1994). Moreover, hyporheic retention and subsequent remineralization can delay the loss of nutrients from a stream reach, thereby potentially increasing overall primary productivity and allowing rapid recovery from disturbance (Grimm et al. 1991). In contrast to these "beneficial" (to stream productivity) effects of hyporheic processes, many hyporheic systems are at least periodically anoxic, and this has led to considerable work on denitrification in these sediments (e.g. Triska et al. 1989; Duff and Triska 1990). In some cases, denitrification has been shown to be a major component of stream nitrogen budgets despite the apparent oxic nature of the system. Denitrification allows hyporheic sediments to serve a nitrogen removal function (anal- ogous to riparian buffers) that may ameliorate the downstream effects of high N loads to stream systems (Triska etal. 1993).

The interplay between organic matter supply, availability of oxygen, and the various biogeochemical trans- formations leads to various scenarios whereby hyporheic processes can influence stream nutrient budgets. Under- standing where, when, and why hyporheic sediments are significant components of stream systems is a major chal- lenge for stream ecosystem ecologists.

Despite the advances made in understanding particular stream-hyporheic systems and the increasing number of studies of such systems, no attempts seem to have been made to uncover generalities across systems or to provide an organizing framework to simplify intersystem comparisons. The balance of this review describes two pos- sible schemes for ranking hyporheic systems according to their system-level effects. With information on rates of biological activity (02 consumption for simplicity) and estimates of hyporheic water exchange (percent of total discharge passing through hyporheic sediments) we can organize streams into broad classes of large, moderate, and small ecosystem-level consequences of subsurface exchange (Fig. 2). For instance, if a large proportion of discharge passes through very active sediments (the upper right region of Fig. 2), the system-level contribution of hyporheic 02 uptake will be large. The opposite case is represented by small exchange volumes with inactive sediments (Fig. 2, lower left). An organizational scheme would allow us to predict a priori when hyporheic metabolism might be important in a particular stream and to organize these systems in a common conceptual framework to facilitate the search for generalities.

Significance to ecosystem budget

High

Moderate

P: Low

Proportion of discharge through hyporheic system

Fig. 2. Hypothetical scheme for organizing hyporheic systems into regions varying in contribution to overall system metabolism. For systems mapping in the upper right, hyporheic metabolism is a major fraction of total stream ecosystem metabolism.

However, there have been relatively few comparative quantitative hydrologic studies of these exchanges, which makes completion of this exercise difficult. Recent advances in hydrodynamic modeling and novel tracer approaches offer hope for expanding our knowledge of surface-subsurface exchanges (e.g. Castro and Hornberger 1991; Harvey and Bencala 1993; D'Angelo et al. 1993). Unfortunately, only a handful of such studies now exist, making it impossible to objectively delineate these regions. The present inability to quantitatively test predictions derived from this ideal scheme is a fairly serious weak- ness. The presentation of this idea is intended to stir debate about what the proper axes for organization might be or clever solutions to the methodological constraints.

In my alternate scheme, I pose the question of what characteristics separate systems in which hyporheic water is very similar (in oxygen content) to stream channel water from those systems in which there are marked differences in water chemistry. Because more data are available for 02 than other variables (nutrients etc.), I have used 02 in my example, but the scheme should also be applicable to DOC, nutrients, or other solutes. The response variable (change in 02 concentration) and the independent variables have been measured in a sufficient number of cases to enable me to place a few well-studied systems in a two- dimensional space for comparison.

In this scheme (Fig. 3), the vertical axis shows the dif-



162 Findlay

QD

Q) 100 C)

Os L 80 O

U)

O 60-

OR SYC N 40

O 40 STG . oOR MAP Q) 20 * LLM

EB o BB > 0- I I I -1 0 1 2 3

Log10 Contact Time (h) Fig. 3. Plot of hyporheic oxygen (expressed as a percentage

of surface water 02) against length of contact with hyporheic sediment. Key: OS-Oberer Seebach, Bretschko 1991; SYC- Sycamore Creek, Valett et al. 1990; MAP-Maple River, Hen- dricks and White 1991; LLM-Little Lost Man, Duff and Triska 1990; STG-Stillaguamish River, Vervier and Naiman 1992; EB-East Branch, Findlay et al. 1993. For the two points labeled with horizontal arrows (OR-Ogeechee River, Meyer 1988; BB- Buzzards Branch, Metzler and Smock 1990) samples were col- lected beneath the channel with no information on flowpaths. Therefore, I assumed a vertical flowpath, which undoubtedly underestimates contact time since there must be a horizontal component. These points were not included in the regression, %02 = 51.9-1 1.7 x contact time (h); P = 0.17, r2 = 42%.

ference in oxygen content between hyporheic water and the open channel. A value of 100% suggests little alter- ation of streamwater as it moves through hyporheic sediments. Values near 0% indicate almost complete removal of 02. For systems with slow flow velocities over a long path, even low to moderate rates of a process have the potential to lead to marked changes in water chemistry. Conversely, very rapid flow via short pathways through hyporheic systems would make it difficult for even quite rapid rates of, for instance, respiration to alter water chemistry.

Contact time has two components: interstitial flow velocities and length of flowpath. Estimates of interstitial velocities were taken directly from individual studies in four of the six cases (see legend for Fig. 3). When nec- essary, I estimated interstitial velocities from hydraulic head and conductivity (e.g. Vervier and Naiman 1992; Findlay et al. 1993). Distance traveled along a hyporheic flowpath from the open channel to the point of sampling was given in some of the papers (e.g. Duff and Triska 1990). In other cases, the distance was estimated from contour maps of concentrations (e.g. Hendricks and White 1991; Valett et al. 1990). With these estimates of velocity and distance, contact time is simply calculated as distance divided by velocity. These estimates were usually for only one or a few locations in a stream, so it is uncertain

whether they are representative. For certain stream systems (e.g. Duff and Triska 1990; Valett et al. 1990), data are available for several locations within a reach, but I only used data from a single location to avoid overre- presenting these streams in the analysis.

Interstitial flow velocities are controlled by permeability of the sediments (hydraulic conductivity) and hydraulic head. Permeability is controlled mainly by grain size, although the degree of particle sorting may also influence velocities because well-sorted sediments are more conductive than poorly sorted sediments with a similar median particle size (Freeze and Cherry 1979). The hydraulic head will vary with stream gradient; steeper streams generally offer greater opportunities for rapid interstitial flow. Breaks in streambed gradient (i.e. riffle- pool transitions), meanders, and constrictions in the channel also provide locations for rapid changes in streamwater elevations, thus generating heads to provide impetus for interstitial flow (Harvey and Bencala 1993; White 1993; Boulton 1993).

Even with a small number of observations, a weak pattern emerges: the greatest conitact times result in the greatest oxygen depletion. It is worth noting that this pattern among systems has a physical analog in that if one examined 02 along a flowpath within a system, one would expect the oxygen content of a water parcel to decline with increasing time after contact with the surface. The emergence of this pattern despite large differences among many other characteristics of the streams (e.g. temperature) suggests that overriding importance of hydrology as a control on stream-hyporheic zone relationships (Hynes 1983; Hakenkamp et al. 1993).

The variability in contact time among streams (3.5 orders of magnitude) is indicative of the variabilities in streambed sediment composition, local geomorphology, and hydrology that influence the penetration of water into the hyporheic zone. For instance, in a stream with minimal change in oxygen (contact time of 1 h, Bretschko 1991), bed sediments have a mean particle size of >2 cm. Most streams with 02 depletion of 50% or more (contact times > 10 h) have bed sediments with particle sizes on the order of a few millimeters. Geomorphology is also significant in that the two cases with longest contact times (Vervier and Naiman 1992; Findlay et al. 1993) both represent lateral gravel bars but of very different scales. The sampling transect in the Stillaguamish (Ver- vier and Naiman 1992) was several hundred meters while the entire bar in the East Branch (Findlay et al. 1993) is only 30 m.

My initial expectation was that to reveal even a weak pattern, rates of respiration would have to be included in this analysis. For four of the streams in Fig. 3 (BB, SYC, EB, and OR) and one stream not shown (Goose Creek, see Palmer 1990 for general description), I obtained estimates of hyporheic oxygen uptake, and these showed a substantially smaller range [mean = 0.74 ug 02 (g DW)-1 h-1, SD = 0.42, range 0.2-1.25] than the 3.5 order-of- magnitude variation in contact time shown for these same systems. Admittedly, this data set is unsatisfactorily small and does not completely span the range of stream types,



Significance of hyporheic exchange 163

but it appears that physical factors influencing interstitial velocities and pathways will be the predominant control on the importance of hyporheic metabolism. This conclusion does not preclude biotic factors (respiration, temperature, carbon quality, etc.) playing an important role in the categorization of the large number of streams that fall in the midrange of physical variation.

The scheme presented above shows how systems vary in their capability to alter water chemistry (02 content) but does not directly address the issue of whether such alterations are a major or minor component of whole- system 02 budgets. This scheme cannot answer the bud- getary question partly because the points in Fig. 3 do not represent annual averages and because locations sampled were not random but were usually selected for a particular purpose. More important, the dependent variable-change in 02-does not directly translate into large or small system-level contributions of hyporheic metabolism. For example, if hyporheic exchange is minimal, interstitial 02 may be completely depleted, but only diffusive processes would connect those sediments to the stream as a whole. Conversely, 02 concentrations in deep sediments might only be reduced by 10-20% relative to water in the open channel, but if water exchange is very large, then even such a small reduction may well be a significant debit in the whole-system oxygen budget. Because ofthe imperfect match between observed differences in water chemistry and the whole-system significance, further advances in hydrodynamic modeling and conservative tracer techniques are required to allow reach-scale estimates of interstitial discharge. Such estimates, coupled with changes in water chemistry (oxygen, nutrients, DOC, etc.), would facilitate conclusions about the contribution of hyporheic zones to stream ecosystem function.

Two major conclusions arise from this literature review. First, variability among stream ecosystem in the importance of hyporheic processes will be largely a function of physical constraints on water exchange rather than of the rate of biotic processes. The tremendous range in sediment particle sizes and hydraulic forces causing water movement are capable of generating log-scale variability in hyporheic residence times. Variations in rates of respiration, nutrient cycling, etc. due to temperature, microbial abundance, or other factors are superimposed on the hydrologic template and will play a smaller part in accounting for the ecosystem-level importance of hyporheic exchange. The second major conclusion-the need for better understanding of hydrology-follows logically from the first. Not only must numerical simulations or tracer techniques continue to improve, but they must be amenable to interfacing with mechanistic or empirical models of biogeochemical processes. Integration of hydrology and biogeochemistry will happen only when hydrodynamic models become tools for understanding stream ecosystem behavior rather than ends in them- selves. This integration will require better training in hydrology for many stream ecologists and simpler guidance from hydrologists about the availability and applicability of models and approaches for studying stream-hyporheic exchanges.

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Submitted: 1 February 1994 Accepted: 15 June 1994 Amended: 12 July 1994



importance of surface-subsurface exchange in stream ecosystems: the hyporheic zone

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