The Open Ocean Environment:

Plankton, Productivity and

Food Webs of the Sea

Chapters 7, 9, and 10

Plankton: Definitions

• Plankton: organisms living in the water column, too small to be able to swim counter to typical ocean currents. This term refers to both animals and plants living in the water column.

Plankton: Definitions 4• Size classes

Ultraplankton < 2 m

Nannoplankton 2-20 m

Microplankton 20-200 m

Macroplankton 200-2000 m

Megaplankton > 2000 m

Phytoplankton (only plants)

• Occur singly or form chains

• Size range of nanno to microplankton

• Encased in silica shell consisting of two valves (pillbox)

• Usually radially symmetrical

• Reproduce asexually by binary fission

• Also sexual reproduction

• Doubling once or twice per day usually

• Dominate the seas over the world.

Diatoms

Phytoplankton

• Secrete organic test and have two flagellae• Size range of nanno and microplankton• Asexual and sexual reproduction• Often many life history stages• Many species are heterotrophic• Often abundant in tropics, mid-latitudes in

summer• A few species are the cause of red tides

Dinoflagellates

Red Tides

Red tides are caused by an increase in nutrients in seawater that causes an increase in dinoflagellate populations. Species that cause red tides produce a neurotoxic substance, saxitoxin.

This substance is dangerous to people. Shellfish (filter feeders) ingest the dinoflagellates and the toxin. Their tissues also become toxic.

Diatoms Dinoflagellates

Coccolithophore Flagellate Isochrysis

ZooplanktonCopepods

Females of different species with eggs

ZooplanktonCrustaceans - Euphausids (Krill)

ZooplanktonGelatinous Zooplankton - Cnidaria

Note muscular bell and tentacles

ZooplanktonGelatinous Zooplankton - Cnidaria

Siphonophores

By-the-wind-sailorVellela

Porpita (ca. 10 cm wide) Physophora (50 mm high)

ZooplanktonGelatinous Zooplankton - Ctenophores

Patchiness of the Plankton

• Plankton rarely distributed homogeneously in the water column

• Plankton occur in spatially discontinuous patches, sometimes distinct aggregations

• WHY?

Diurnal Vertical Migration of Zooplankton

• Zooplankton rise to shallow water at night, sink to deeper water during the day

• Found in many different groups of zooplankton• Zooplankters usually start to sink before dawn,

and start to rise before dusk• Cycle is probably an internal biological clock that

must be reinforced by day-night light changes

0 1 2 3

Night

Day

Twilight

0

100

200

300

400

Distance (km)

Dep

th (

m)

Vertical migration of planktonic shrimp Sergia lucens

Review Questions:

1)Why does diurnal vertical migration occur?

2) Do plankton have high or low Reynolds numbers?

Explain what a Reynolds number is.

3) How does plankton’s Reynolds number influence feeding in copepods?

4) Define the term drag in marine hydrodynamics. What adaptations do plankton have to decrease drag?

5) Why might it be important for plankton to stay in surface waters and not sink to great depths?

6) In some marine environments, copepods have strong diurnal migration and it others it is very weak. Why??

Critical Factors in Plankton Abundance

Chapter 9

Spring Phytoplankton Increase(or Spring Diatom Increase)

In midlatitudes, phytoplankton increase inthe spring, decline in summer, and may increase to a lesser extent in fall.

Question: What factors may cause these seasonal increases and declines?

Nutrientsat surface

SpringDiatomIncrease

Zooplankton

Availablesunlight

Winter Spring Summer Fall Winter

Variations on the Spring Phytoplankton Increase

The spring phytoplankton peak and the later zooplankton peak are shortest and sharpest in high latitudes, becomingindistinct in the tropics

Arctic

Temperate

Tropical

Phytoplankton

Phytoplankton

Herbivorezooplankton

PhytoplanktonHerbivorezooplankton

J F M A M J J A S O N D

Month

Productivity and Food Webs in the Sea

Chapter 10

©Jeffrey S. Levinton 2001

Productivity vs biomass

Biomass the mass of living materialpresent at any time, expressed as gramsper unit area or volume

Productivity is the rate of production of living material per unit time per unit areaor volume

Productivity

Primary productivity - productivity due toPhotosynthesis.Secondary productivity - productivity due toconsumers of primary producers.

Food Chain

Food chain - linear sequence showingwhich organisms consume which otherorganisms, making a series of trophic levels

Food web - more complex diagram showingfeeding relationships among organisms, notrestricted to a linear hierarchy

Food Chain Abstraction

Adultherring

Phytoplankton

Barnaclelarvae

Mollusklarvae Small

copepodseuphausid tunicate

cladocerans

amphipodsand eel

Young herring

arrowworm

Largercopepod

Phytoplankton

Copepod

Herring

Food chain Food Web

Transfer Between Trophic Levels

Transfer from one trophic level to thenext is not complete:

1. Some material not eaten2. Not all eaten is converted with 100% efficiency.3. 2% transferred from sunlight to primary producers; 10% transferred from primary producers, herbivores and up.

Oceanic Food Webs

Food webs in the oceans vary systematically in food chain efficiency,number of trophic levels, primary production

Oceanic Food WebsFood Chain

TypePrimary

Productivity

gCm-2y-1

Trophic Levels

Food Chain

Efficiency

Potential

Fish Production

mgCm-2y-1

Oceanic 50 5 10 0.5

Shelf 100 3 15 340

Upwelling 300 1.2 20 36,000

Oceanic Food Webs

Note: Great potential of upwelling areasdue to combination of high primary production,higher food chain efficiency, lower numberof trophic levels

Why does lower number of trophic levels increase potential of production?

Oceanic Food Webs

Stable, low nutrient Turbulent, high nutrient

Few trophic levels

Manytrophiclevels

Open ocean,gyre centers

Shelf, upwelling

Primary production(mg C/m2/day) <100 100-150 150-250 >250

Asia

NorthAmerica

SouthAmerica

NorthAmerica

Antarctica

IndianOcean

NorthAtlantic

SouthAtlantic

NorthPacific

SouthPacific

Africa

Satellite Images and marine sciences

Measuring Primary Productivity

Gross primary productivity - total carbon fixedduring photosynthesis

Net primary productivity - total carbon fixed during photosynthesis minus that part whichis respired.

Measuring Primary Productivity Satellite Approaches:

Satellites can use photometers specific towavelength to measure chlorophyll, Seawater temperature

Need ground truthing to get relationshipBetween chlorophyll concentration and primary production; varies with region

sun

Satellite

Color scanner

IrradianceRadiance

Phytoplankton

Satellite image of world productivity, from SeaWiFS satellite

What’s in this image?

How do sensors collectsatellite images?

What do the colors represent?

How do we get these colors?

Images are always collected in black and white first. A computer then adds color.

Gulf stream image with pixels

A satellite image of this Red tide event helped researchers determine the concentrations of diatoms and toxins in the water.