PHYTOPLANKTON ECOLOGY II

IV. Nutrient requirements
    A. Macro and micronutrients
        1. Macronutrients:
            a. C,H,O
            b. Ca,Mg,Na,K,S,Cl (usually abundant)
            c. N,P,Si (diatoms) – often limiting
        2. Micronutrients: Fe, Mn, Cu, Zn, B, Mo (N fixation), V, Co (B12)
    B. Minimum levels and toxicity
    C. Redfield ratio
        1. Algal composition – elemental ratios in marine environments
        2. Element:     C        H        O        N        P        S        Fe
            # atoms:     106     263     110    16       1        0.7     0.01
        3. Variability in the ratios of these elements actually found in algae is often used as an indicator of nutrient status/nutrient limitation
            a. N:P = 16  growing well, not P limited
            b. N:P >30 not enough P in cell; may be an indication of P limitation
    D. Growth curves – general
General phytoplankton growth curve -- lag, exponential growth, stationary phase, decline
       1. r = growth of population = m - d
        2. m = specific growth rate (division rate of one cell)
        3. d = death rate
        4. Lag phase
        5. Exponential growth: r>0, m >d
        6. Stationary phase: m =d
        7. Decline (death): r<0, m <d
    E. Growth rates -- Nutrient uptake velocity
        1. Growth and nutrient uptake curves generally follow the same pattern
        2. Generally follows a Monod Growth Curve/ Michaelis-Menton uptake curve
Monod Growth Curve and Michaelis-Menton nutrient uptake curve
 

        3.equation for growth or uptake kinetics
            a. m = growth rate (or nutrient uptake rate)
            b. S = substrate concentration
            c. Ks = substrate concentration where growth rate is half of maximum

    F. Factors affecting uptake rate
        1. Cell size – amount of surface area relative to volume; surface area/volume gets lower as cell gets bigger
            (4Pr2 = area of a sphere; 4/3Pr3 = volume; so A/V = 3/r)
        2. Nutritional state of cell
            a. Luxury uptake – cells take up more than they need
            b. Inhibition by internal stores
        3. Transport limitation

           a. sinking speed or swimming speed
            b. turbulence
        4. Inducible enzyme systems affect Ks
        5. Toxicity effects (if nutrient abundance too high)
    G. Determining the limiting nutrient – How do we determine the limiting nutrient?
        1. Liebig’s law of the minimum – only 1 thing limits growth at any one time (something else may be close)
                    nutrient in shortest supply relative to needs
        2. Stoichiometries – again, deviations from the expected Redfield ratio.
        3. Bioassay techniques 
        4. Co-limitation
            
    H. Other nutrient factors
        1. Organic nutrients and mixotrophy
            a. Happens most often under several conditions:
                1) DOM at high concentrations
                2) Under the ice when light is low
                3) If the algae can produce specific enzymes to assist taking up organic nutrients – e.g., alkaline phosphatase & DOP
            b. Problems: bacteria have a lower Ks than do algae
        2. Vitamins – B12 is essential for cyanobacteria, diatoms, greens and dinoflagellates
        3. Organic compounds – antibiosis – chemical warfare between algae or between some macrophytes and algae;

V. Phytoplankton competition for nutrients
              Tilman’s resource competition models
    A. Nutrient use patterns by phytoplankton species
Differences in growth responses to nutrient concentration for two different phytoplankton species
                        -species B has a lower maximum growth rate, but higher growth rate at a low nutrient conc.
                        -species A can win at high nutrient because grows faster (as long as species B doesn’t deplete all of the nutrient first)
                        -species B will win at low nutrient because can use it more efficiently.

    B. Incorporation of loss/death rates
        But you also have death rates (natural loss, sinking, grazing, viruses) – if algae are growing below the death rate level,
                then the populations will not persist

Graphical depiction of R* values - nutrient values necessary for growth to balance the death rates

            N*B and N*A are the concentrations of nutrients needed to get growth rates equal to the death rates for the two species –
                    break-even nutrient points.
            If nutrient value is greater than the N*, then that algal species will increase.
            If not, then growth rate, m, will be less than the death rate

    C. What happens if you have two nutrients?
        1. Example with two diatom species

Growth of two diatoms (Asterionella and Cyclotella) on different Phosphorus concentrations

        Asterionella will change conditions (take up P) so that below P*C Cyclotella can not persist, but Asterionella can –
                for P, Asterionella will outcompete Cyclotella under all nutrient levels.
        How do Asterionella and Cyclotella ever co-exist?

        Now add a second nutrient:

        At low Si, Cyclotella will win.

Growth of two diatoms (Asterionella and Cyclotella) on different Silica concentrations

            At low Si, Cyclotella will win.

            The outcome of competition between these diatoms depends on the relative abundance (ratio) of the two nutrients

            Easy way to view – isoclines of the two nutrients:

Graphical representation of phytoplankton growth on two nutrients showing zero net growth isoclines -- nutrient 'break even points'
            Zero net growth isocline – points at which the population breaks even

            P/Si increases, then Cyclotella will win
            P/Si decreases, then Asterionella will win

            Predictions from growth curves match the competition results in lab and in the field

      2. Diatoms versus blue-greens

           blue-greens have little N requirement (or Si requirement), but a high P requirement – low N:P ratios favor blue-greens

Growth of blue-greens versus diatoms on Silica versus Phosphorus
        The species change the nutrient concentrations – that’s how they are competing, but in nature there is also a resource supply rate.
        You can think of resource/nutrient supply rates as vectors going across the graph at the resource supply ratio
            (this may vary throughout the year).
        These supply rates can determine the outcome of phytoplankton competition -- which species will win.

Asterionella and Cyclotella competition outcomes for Si and P, showing several possible resource supply rates

VI. Interaction of Factors Affecting Growth Control (temperate lake examples)
    A. Temperature cycle

General temperature conditions over an annual cycle in a temperate lake
    B. Light availability

Incident and available light over an annual cycle in a temperate lake

C. Nutrient supply

Nutrient availability in surface waters over an annual cycle in a temperate lake



D. Integration of growth factors

    How can we integrate these three major growth factors?
             Best times for growth when these factors match
             Right after ST ends and before FT begins – temp ok, light high, nutrients ok
             Second best time – mid summer
             Worst at winter time – low light and temperature
             Sometimes there is a bloom right after the fall turnover due to increased nutrients

Overall pattern of phytoplankton growth over an annual cycle in a temperate lake

VII. Attrition control
    A. Sinking rate
        - Under the ice there is often dominance of phytoflagellates that can control their position in the water column

Importance of sinking rates for phytoplankton over the course of an annual cycle in a temperate lake
    B. Grazing pressure
        -In the spring there is a LAG in the increase of grazing zooplankton
        -An early summer 'clearwater' phase often is apparent due to the grazing of zooplankton
        -In the late summer, the predators on zooplankton have increased
        - In the winter there are not many grazers (not enough food)
General pattern of grazing rate pressure on algae over an annual cycle in a temperate lake
    C. Washout
        - Movement of cells out of the lake (in outflow)
        - Dilution during spring run off
        - Depends on the relative turnover time of water in the lake

    D. Parasitism
        a. Fungi – chytrids (can infect many cells – range <1-70% of cells)
        b. Viruses – can have large periodic effects; are often species specific

VIII. General seasonal succession
        Can now make generalizations about which algae will dominate when
            (These events also happen in tropical lakes, but are prompted by wet/dry seasons or turnover events – different periodicity)

            SPRING               SUMMER                                                LATE SUMMER
           Diatoms              Greens                                                    Blue-greens
            High nutrients        Good competitors at low nutrients               Lowest nutrients (N fix.)
            Low grazing          Moderate grazing                                        High grazing – resist by being unpalatable (sheath/toxin)
            Low sinking          High sinking rates (many flagellated)             Moderate sinking

            WINTER – small phytoflagellates; sometimes motile dinoflagellates

        -Each major group’s abundance curve is made up of individual species curves
        -Hundreds of species of algae live in any one lake over the course of a year
        -To predict each you need to know nutrient requirements, responses to temperature, light, grazing, sinking rates

PRIMARY PRODUCTION

I. Fate of Energy

Fate of solar energy in a lake; GPP, NPP, Respiration...

        NPP = GPP – E – R
        The whole process is 0.03-2% energy efficient

        Standing crop/biomass vs. production

II. Measurement
    A. General equation and units
        1. units of carbon produced or oxygen emitted (sometimes calories; 1 mg C~10 cal energy,
                depending on storage material – fat, starch…)
        2. per volume or surface area of lake
        3. per time
    B. oxygen change method
        1. light-dark bottles

            Measure initial and incubate the others for a period of time
            R=I-D
            NPP=L-I (assumes the same respiration in the light and dark)
            GPP=L-D

            Problems with this method:
                (1) Enclosure/bottle effects
                (2) Sensitivity

        2. whole environment
                measure oxygen change in a lake or stream over a day
                avoid enclosure effects
                must compensate for invasion and evasion of oxygen to the lake

    C. C change -- 14C method
        Add radioisotope of carbon (14C) as bicarbonate, H14CO3-, and it is converted to labeled carbon by the algae
        Incubate in light and dark bottles
        Measure of roughly NPP (how much 14C is incorporated into the algae)
        Is more sensitive than oxygen method

        Problems with this method
            (1) 14C and 12C don’t have the same reactivity
            (2) Doesn’t measure 14C that entered the cell and then left by excretion or respiration before the end of the experiment

    D. yield method
        Look at the change in algal biomass over time
        No bottle effects
        Only used with lots of growth so that there is no sensitivity problem
        What is the problem with this measure?  Doesn't account for attrition – gives an underestimate of production
        Also a problem with moving water masses – spatial heterogeneity – may be sampling different water masses

III. Patterns of productivity
    A. Productivity versus latitude

General pattern of primary productivity with latitude

     B. Productivity (or standing stock of chlorophyll a) verus total P (nutrients)
 


General pattern of primary productivity with total Phosphorus




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