EXERCISE 16
Zooplankton Production
Most zooplankton, and some benthic animals,
reproduce continuously. As a population changes
by addition and growth over a given time interval,
a demographic turnover occurs [see reviews of
Edmondson (1974) and Rigler and Downing
(1984)]. Few, if any, of the individuals present
at the peak of an exponentially developing
population were alive at the beginning of the
exponential phase. An individual also undergoes
a biochemical turnover during its lifetime so
that, upon completing a mean lifespan, it will
have assimilated several times its final mass.
Production, in the context of a population,
then, is growth and is only one factor in the
material or energy budget for the whole
population (Edmondson, 1974).
p = C - F - U - R - G r
where P = production; C = consumption or
feeding; F = egestion (feces and regurgitation);
U = excretion; R = respiration; and G r = gametes.
Assimilation (A) is the difference between
ingestion and egestion (A = C - F).
With continuous reproduction, the cohorts of
the population overlap, so that it is either
difficult or impossible to observe changes in
abundance over time. To analyze the production
of populations with continuous reproduction, it
is necessary to use methods that do not require
complete evaluation of cohort differences.
A number of models of zooplankton production have been developed, but they fall into two
general classes: (1) direct models based on timedependent parameters of the zooplankton
species [e.g., Edmondson and Winberg (1971)
and Rigler and Downing (1984)], and (2) indirect
models based on inferred rates of zooplankton
filtering, assimilation, and consumption by fish
and other predators [e.g., Winberg (1971)]. Both
discrete time-interval and instantaneous models
are used to estimate production.
The change in numbers (N) in a population
over a time (t) interval is given by:
I1N = birth + growth - mortality
I1t
The population at time t, then, is
Nt = No + birth + growth - mortality
Mortality includes both natural death and losses
by predation.
Since it is necessary to measure recruitment
or birth rate, and with continuous birth and
death one cannot identify distinct cohorts,
production (P) can be calculated from either the
rate of growth (i.e., the duration) of different size
classes or of life history stages (Edmondson,
1974):
Nll1Wl N211w2 N311w3
N nl1wn
P=---+--+--+ ... - - -
Tl
T2
T3
Tn
where the subscripts 1,2,3 .... n = stages or size
classes; W = the weight of the class; T = the
duration of the stage, in days; and I1w = the
weight increment of the class (birth + growth -
mortality).
This relationship implies that, when a stage
lasts two days, one-half of the individuals will
pass from that stage on one day and the other
235
Zooplankton Production
Most zooplankton, and some benthic animals,
reproduce continuously. As a population changes
by addition and growth over a given time interval,
a demographic turnover occurs [see reviews of
Edmondson (1974) and Rigler and Downing
(1984)]. Few, if any, of the individuals present
at the peak of an exponentially developing
population were alive at the beginning of the
exponential phase. An individual also undergoes
a biochemical turnover during its lifetime so
that, upon completing a mean lifespan, it will
have assimilated several times its final mass.
Production, in the context of a population,
then, is growth and is only one factor in the
material or energy budget for the whole
population (Edmondson, 1974).
p = C - F - U - R - G r
where P = production; C = consumption or
feeding; F = egestion (feces and regurgitation);
U = excretion; R = respiration; and G r = gametes.
Assimilation (A) is the difference between
ingestion and egestion (A = C - F).
With continuous reproduction, the cohorts of
the population overlap, so that it is either
difficult or impossible to observe changes in
abundance over time. To analyze the production
of populations with continuous reproduction, it
is necessary to use methods that do not require
complete evaluation of cohort differences.
A number of models of zooplankton production have been developed, but they fall into two
general classes: (1) direct models based on timedependent parameters of the zooplankton
species [e.g., Edmondson and Winberg (1971)
and Rigler and Downing (1984)], and (2) indirect
models based on inferred rates of zooplankton
filtering, assimilation, and consumption by fish
and other predators [e.g., Winberg (1971)]. Both
discrete time-interval and instantaneous models
are used to estimate production.
The change in numbers (N) in a population
over a time (t) interval is given by:
I1N = birth + growth - mortality
I1t
The population at time t, then, is
Nt = No + birth + growth - mortality
Mortality includes both natural death and losses
by predation.
Since it is necessary to measure recruitment
or birth rate, and with continuous birth and
death one cannot identify distinct cohorts,
production (P) can be calculated from either the
rate of growth (i.e., the duration) of different size
classes or of life history stages (Edmondson,
1974):
Nll1Wl N211w2 N311w3
N nl1wn
P=---+--+--+ ... - - -
Tl
T2
T3
Tn
where the subscripts 1,2,3 .... n = stages or size
classes; W = the weight of the class; T = the
duration of the stage, in days; and I1w = the
weight increment of the class (birth + growth -
mortality).
This relationship implies that, when a stage
lasts two days, one-half of the individuals will
pass from that stage on one day and the other
235