(^) Vidal combined his results into a complex predictive equation for growth
(secondary production) as a function of body size, temperature, and food availability.
In principle, the equation could be used, given measures of T, food concentration, size
distribution and abundance of Calanus pacificus, as well as phytoplankton food
availability, to determine secondary productivity of this copepod in Puget Sound.
However, an unsolved problem is how to estimate food availability in the field in the
same way that the copepods see it. We cannot do that. As a result, secondary-
production studies, no matter how rich in careful laboratory rearing results, often end
without reaching a satisfying field result.
II Physiological Methods
(^) The secondary-production diagram is an input–output relation, so one approach is to
estimate all of the input and output rates and calculate from them the secondary
productivity. It is also necessary to measure the population biomass of the animal or
animals of interest, a measure usually afflicted with variability on the order of half to
double or greater. This is called the physiological method. It turns out to be rather
theoretical, since practically all applications to the field lack one or more variables
and some ad hoc fix is applied. For a given species, we need estimates of ingestion
rate (I), defecation rate (D) and thus, absorption efficiency (A = [I − D]/I), respiration
(R), molting (E, for ecdysis), and mortality (M). With estimates in hand, production,
2 oP, would be calculated by:
(^) Units can be dry weight or mass of carbon added (increase of Λ
2 ) per day or per
year. As for direct estimates of growth, production estimates are often expressed as a
ratio of increase to standing stock, say, gC gC−1 d−1, known as the P : B ratio. At
steady state or on average,
(^) All of these rate variables will change with animal size and age, so we cannot take a
sample of zooplankton, weigh it, and get secondary production from relationships of
I, A, R, E, and M to habitat variables like food availability and temperature, although
recent attempts to make that work are reviewed below. All processes must be
considered in detail with respect to size and species composition. The most difficult
process rate to estimate is mortality, M, especially M as distinct from C2–3. However,
in principle, if we could get rates for enough components (species, stages, times of
year) then we could add everything up and estimate secondary productivity.
(^) Probably many zooplankton workers will disagree, but there seems to be no
physiological estimate of secondary production that is good enough to present as a