Plankton Ecology and Pelagic Ecosystems

I. Plankton flotation mechanisms
    A. Problem of sinking -- gravity
 
    B. Stoke's law – (you don't need to memorize the equation!)
        Density of water will change with temperature and salinity and affect sinking rates
        Objects of different shapes but similar mass will fall at different rates – more surface area leads to more resistance and slower sinking
        For the same shape, the smaller the organism, the greater the SA/volume; sphere: SA = 4 P r2 , volume = 4/3P r3

Stoke's Law
             SR = sinking rate
             W1 = density of organism
             W2 = density of water
             R = surface of resistance
             Vw = viscosity of water
    C. Mechanisms for reducing sinking
        1. reduce density
            a. heavier chemical ions may be replaced with lighter ones
            b. pockets of gas
            c. accumulation of fats and oils

        2. change in resistance (drag) -- increased surface area to volume
            a. body size is small
            b. flattened body shapes
            c. spines and body projections
        3. actively swim up and down in the water column
    D. Currents can keep in suspension --
        1. convection cells – heating and cooling during day and night changes water density
        2. Langmuir circulation – wind > 3m/sec

            a. convergence zones - windrows; accumulate floating organisms and debris (trash, tar balls, oil, etc.)
            b. divergence zones – heavier organisms (e.g., plankton) are brought toward the surface and concentrated

II. Primary production
    A. Photosynthesis

photosynthesis equation
    B. Production
        1. Primary productivityrate of formation of organic compounds from inorganic materials;
                carbon fixed per unit area (or volume) per time
        2. Gross production -- total amount of fixed carbon
        3. Net production -- gross production minus respiration by the phytoplankton themselves -- amount of fixed
            carbon available to other trophic levels
        4. Standing crop – total amount of biomass present at a given time (per area or volume)

    C. Methods – measure rate of disappearance of CO2 or appearance of O2
        1. light and dark bottle oxygen techniques
            a. clear bottle – photosynthesis and respiration
            b. dark bottle – only respiration
            c. measure oxygen before and after incubation
            d. net community photosynthesis = light bottle – initial
                new production
            e. respiration = dark bottle – initial
            f. gross photosynthesis = NPP + respiration (light – dark)
 
        2. 14C method –
            a. add radiolabelled inorganic carbon; measure amount taken up by algae
            b. during incubation some 14C will be taken up and then respired – so this is closer to net PP than gross PP

        3. Problems with these methods
               a.  Bottle effects: bacterial growth, abnormal phytoplankton behavior
               b.  Contamination effects (e.g., increase trace nutrients)
        4. Other methods
            a. Count number of cells over time
            b. Measure the physiological condition of the phytoplankton
     
III. Factors affecting primary production
    A. Physical and chemical factors
        1. temperature

        2. light
            a. varies with
                    amount reaching the water surface (weather, latitude, season)
                    surface conditions
                    absorption of light by water, dissolved and suspended materials
            b. can get inhibition of photosynthesis at the surface

            c. different phytoplankton types have different requirements
            d. compensation depth
                    i. Fixed depth where light is such that PS=R – no net production
                    ii. PS decreases as get deeper – less light
                    iii. Respiration is more constant with depth
                    iv. Comp. depth varies with the amount of light reaching the water column and the clarity of the water
                    v. Self-shading
                    vi. Depth at which is ~ 1% of incident light
            e. critical mixing depth
                    i. Depth of mixed layer (warm surface layer) at which total gross production of phytoplankton in the water column
                        equals the total respiration (no net community production)

                    ii. critical depth always greater than the compensation depth

        2. nutrients
            a. much more dilute than on land
            b. rapidly used up in the surface waters when there is a bloom

            c. critical nutrients
                N -- NO32- (nitrate), NO22- (nitrite), NH4+ (ammonium)
                P -- PO43- phosphate
                SiO2 – (silicate) -- diatoms and silicoflagellate
                iron (Fe)
            d. deep waters below the photic zone have more nutrients

                (1) wind and mixing
                (2) tropics -- stratified year round – upper water very nutrient poor

    B. Biological Factors -- Bottom up versus top down
        1. bottom up -- resource limitation (e.g., nutrient limitation and light) -- limitation by factors lower in the food web
        2. top down -- grazing or predation -- limitation by factors higher up in the food web

    C. Geographical variations in productivity (temperate seas, tropical seas, polar seas)
        1. Temperate Seas
            a. Light varies seasonally
            b. Thermal structure of water column varies seasonally
            c. Mixing occurs when density of upper and lower layers is equal – fall, winter, spring
            d. Seasonal productivity changes
                (1) spring bloom
                (2) Summer – nutrients decline; production decreases
                (3) Fall – thermal stratification declines, nutrients re-supplied; small fall bloom declines as light declines
        2. Tropical Seas
            a. Less seasonal variation
            b. Constant thermal stratification
            c. Light optimal, but nutrients low
        3. Polar seas
            a. Productivity peak in summer when ice disappears (allows light penetration)
            b. Nutrients not limiting; no strong stratification
    D. Productivity in inshore and coastal waters
        1. inshore production is influenced considerably by runoff from the land
           
        2. water depth may be shallower than the critical depth
        3. persistent thermocline is not often present nearshore
        4. but sediment load is often higher

        5. human effects on productivity - eutrophication

    E. Comparison of marine productivity to other systems
        -ocean NPP = 48.5 petagrams (1015 grams)
        -terrestrial NPP = 56.4 petagrams

        -ocean -- 140 g C/m2
        -land -- 420 g C/m2

        -ocean photosynthesizers use 7% PAR
        -land photosynthesizers use 31% PAR

IV. Pelagic Ecosystems
    A. Classical Model
        1. interactions of the larger plankton
        2. grazing
            a. copepods can reduce the phytoplankton populations

            b. Differences between Atlantic and Pacific Ocean
 

        3. diel vertical migration (DVM) -- daily up and down movements of zooplankton (and other small organisms)
            a. generally organisms are deep during the day and come to the surface at night
            b. reverse migration -- daylight rise and midnight sinking
            c. may move 100-400 m up and down each day
            d. major ‘proximate’ stimulus - light (temperature; depth)
            e. ‘ultimate stimulus’
                (1) avoid predation by visual predators (e.g., fish, cephalopods and birds)
                    a. lots of support in freshwater and for some marine copepods [more than mentioned in text]
                    b. but
                        i. often enough light for visual predation where many zooplankton stop
                        ii. some zooplankton migrate below light level of visual predation (expending excess energy)
                        iii. some migrators are bioluminescent
                        iv. many predators migrate, too
                (2) light damage avoidance – no current experimental evidence

                (3) allows the zooplankton to change their horizontal position

                (4) it is energetically beneficial to migrate
                    i. proposes that phytoplankton production higher with discontinuous grazing
                    ii. zooplankton respire less in deep cold waters and feed more efficiently in warm surface waters
                    iii. fits the pattern of more migration in the tropics
                    iv. DISPROVEN in freshwaters
    B. A changing model—new view of pelagic ecosystems
        1. importance of picoplankton, nanoplankton, bacteria and viruses
        2. made possible with modern technology
        3. productivity
            a. nanoplankton (especially coccolithophores)  probably account for more of the primary productivity and biomass than was thought
            b. may be 80% of photosynthetic activity and 75% of the phytoplankton biomass in the ocean
            c. nano and picoplankton show less seasonal variation
                i. greater SA/V - outcompete larger phytoplankton for nutrients
                ii. sink slower

        4. respiration and grazing
            a. small phytoplankton are consumed by flagellates and cilitates
            b. these nano and microzooplankton consume most of the primary production

        5. bacteria, particulate and dissolved organic matter
            a. DOC or DOM is the largest reserve of organic C in the biosphere
            b. from material lost or leaked from cells
            c. up to 50% of the DOC is taken up by bacteria
            d. viruses
            e. microbial loop DOM taken up by bacteria that are eaten by microzooplankton; some of this energy may be
                passed up to the traditional ‘classical’ or ‘grazer chain’ food web
           
            f. marine snow – larger bacteria are associated with particles
            g. viruses may determine species composition; may aid in nutrient recycling
        6. spatial distribution of plankton – patchy; complicates study
        7. differences in productivity in different areas of the ocean
                oligotrophic
                eutrophic

      
 

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