Zooplankton Ecology


Zooplankton Feeding

    A. Ingestion
        1. ml of water cleared per animal per time (F) versus density of food (D)
Zooplankton filtering rate versus phytoplankton cell density
                F = filtering rate or clearance rate
                Decreases at high cell density because filtering apparatus clogs
                Increased filtering rate for larger zooplankton (especially Daphnia)

        2. ingestion I = F * D
                D = cell density

Zooplankton ingestion versus phytoplankton density
                Ingestion increases as cells get more concentrated
                Curve levels off due to saturation/clogging

    B. Selection during feeding
        1. specialization of feeding types
            a. raptorial (copepods)
            b. filter feeding (cladocera)
            c. ciliary mucus (rotifers)
        2. size of food particles that can be taken
            a. can depend on size of filtering combs
            b. experiments
                i. beads of different size
                ii. coulter/particle counter
            c. varies with the size of the zooplankton
        3. polysaccharide coatings and spines
            a. Shape of algae
            b. Spines increase the effective length of the algal cells
            c. Algae with polysaccharide/mucilaginous coatings may be resistant to digestion even when they are consumed

        4. species-specific feeding rates on different algae
            a. Some mechanical effects – spines
            b. Some behavioral (selection) – taste
        5. differences in selectivity between different zooplankton species
            a. copepods more selective than cladocera
            b. herbivorous calanoid copepods do better at low food quantities and low food qualities

Differences between (Diaptomus) calanoid copeopod and Daphnia growth in low versus high food quality
Competition between Daphnia and Diaptomus based on food quantity and food quality

    C. Utilization of food
        1. some algae not digested
        2. nutritional effects – vitamins, etc.
            a. energy (total amount of food)
            b. 'food quality' -- amino acids, lipids (fatty acids, 'fish oils'), vitamins, phosphorus

Sterner and Schulz, 1998, Food Quantity/Quality Definition Figure
            c. often get best growth and reproduction with a mixed diet
         3. Use for energy budgets to help determine zooplankton production
             a. ingestion
             b. assimilation
             c. growth
             d. reproduction
             e. excretion – dissolved form
             f. egestion – bulk material in fecal pellets
             g. respiration
             h. molts
         4. assimilation
            a. the part of the ingested food that an organism can actually uses
            b. key to how well organisms can grow and reproduce
            c. not 100% -- generally is ~30-80%
            d. measure with radioactively labeled food
         5. use of detritus
            a. some organisms can grow on detritus
            b. often most of the nutrition comes from grazing on the decomposers on detrital surface (bacteria and fungi)
               
        6. antibiosis examples

        7. regeneration of nutrients
            a. nutrients  ---> algae --->  zoop  ---> fish
            b. the supply of nutrients is external to the lake or stream, BUT, zooplankton excrete nutrients back into the lake – recycle nutrients
            c. complex feedbacks
           d. remineralization of nutrients – primary excretory products
                i. PO43- (phosphate)
                ii. NH4+ (ammonium)
                iii. Also  DOP and DON
        8. Source of zooplankton food is sometimes not algae, but terrestrial
            a. Autochthonous production – C fixed within the lake by phytoplankton, macrophytes, benthic algae
            b. Allochthonous production – C fixed outside of the system (in this case terrestrial)
           
 

Zooplankton Population Dynamics

II. Zooplankton Population Dynamics – Secondary Production – how do you estimate the amount of zooplankton
            produced in a lake? (this is the food available for many fish!)
    A. Intrinsic rate of increase – r = b-d
        1. Background -- you need to know what is happening to each zooplankton population
        2. Standard exponential population growth model
              dN/dt=rN
              Nt = No er t
                   and r = b - d
              where,
                   Nt - the population size at time t
                   N0 - the initial population size
                   r - the intrinsic rate of growth
                   b - the instantaneous birth rate
                   d - the instantaneous death rate

        3. Logistic growth model
            •dN/dt = rN (K-N)
                                    K
            •K = carrying capacity of the environment
            •Growth slows as you near the carrying capacity

        4. Key to both is r (intrinsic rate of increase)
        5. Birth rate
            i. Temperature
            ii. Food availability – increasing food often increases the birth rates (not all food equally high in nutritional quality)
            iii. Body size
        6. Death rate - population can decrease even if food is not limiting if predation is high
            i. Predators
            ii. Parasites and disease
            iii. ‘Natural death’

        7. Generally, the standard exponential population growth model is used.
                If we have sampled the zooplankton population on two dates, then we can estimate r with the following equation:
                         Nt = N0 e rt
                         lnNt2=lnNt1 + rDt
                        Solution of the discrete exponential equation for r
                          where,
                               (t2-t1) is the time between the two sampling dates in days
                          should be a short time between sampling (assumes constant b, d, development time between dates)

    B. Cohort or Instar analysis –
        1. an instar is the life stage between each molt.
        2. a cohort is a group of individuals born at the same time
        3. For some taxa, like copepods, each of the separate instars can be easily distinguished
        4. For other taxa, such as cladocera, the instars can not be recognized easily.
        5. Can measure the number of individuals in each instar over time to calculate r
    C. Can we break this r down into b and d?
        Egg ratio method – using the number of eggs a female is carrying to estimate birth rates.(W.T. Edmondson)
        1. Because many zooplankton species carry their eggs, an indirect method of estimating population birth rates
            from female fecundity can be used.
        2. Population growth rates also can be estimated from sequential measurements of population size
        3. Death rates can then be estimated by difference.
        4. Specifics
            i. The number of eggs present in the population at one time represents the number of new individuals that will
            hatch between that time and the development time (D) of the eggs. The population would increase by this number if there
            were no mortality.
            ii. Assume a uniform egg age distribution and a steady hatching rate over the development time
            iii. Under these conditions we can estimate a finite population birth rate
                    (B = #eggs hatching per female per day) as follows:

                  Egg ratio = number of eggs per female = new females/old female
                        (easy for parthenogenic organisms)
                   B = E/D
                        where,
                        E = eggs/female
                        D = egg development time (function of temperature)
        5. Given these assumptions, the fraction of eggs hatched per day will be 1/D and the population will grow
                by E/D (= B) per female per day. Thus, if the population size on day 1 is 1, the population size the next day
                should be 1 + B, if there is no mortality. So, b, the instantaneous birth rate, can be determined as follows
                (Edmondson 1968):
                        b = (ln (B + 1) - ln (1))/1 = ln (B + 1) = ln (E/D + 1)
        6. Paloheimo (1974) pointed out that eggs are subject to the same mortality as adults (because they are being
            carried by the adults).  Egg age distribution is better approximated by an exponential function. His correction
            of the above formula for b has been shown to be more accurate and biologically meaningful:
                    because E/D = (ebD- 1)/D
                                  b = [ln (E + 1)]/D
        7.With b and r (estimated from population data) you can estimate d: r=b-d
        8. Examples

 

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