Zooplankton Feeding

    A. Ingestion
        1. ml of water cleared per animal per time (F) versus density of food (D)

                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

                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

    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

        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:
                         N_t = N₀ e ^rt
                         lnN_t2=lnN_t1 + rDt
                        
                          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 = (e^bD- 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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