follows:
Secondary productivity is the rate of change in the biomass of grazers, ΔΛ 2 /Δt, plusthe portion of increase of biomass that is balanced by predation, C2−3. Some would
add the non-predatory death and molting amounts. This rate is appropriate for a
system in steady state, or it is a suitable representation of the production rate averaged
over a seasonal cycle. The problem is to measure this rate for all grazers or even for
one abundant species. Distinct approaches are sometimes termed growth methods and
physiological methods.
I Growth Methods
(^) The basic notion of a production measurement for a defined component of an
ecosystem, say one growth stage of one species, is to determine its growth rate per
unit biomass (called a specific growth rate, g, i.e. {mass added} mass−1 time−1, say g
C (g C)−1 d−1) and to multiply that by the standing stock or biomass (B): P = gB. With
such measures for all stages of that species, we could determine the production rate
for its whole population, using the Boyson (1919) equation:
(^) where 2oP is the secondary production rate of the population.
(^) One simple (if labor-intensive) approach is to let this summation happen in an
incubation, much like a primary-production estimate with isotopes, the tracer being
simply change in biomass measured as dry weight or organic carbon (it is possible to
use isotope tracers, but added mass itself will serve). In principle, one removes the
predators from an enclosed sample of the habitat, and then observes over a suitable
incubation period the immediate increase in herbivore biomass that results. This direct
technique has been applied, for example, by Kimmerer (1983), working with the tiny
tropical copepod Acrocalanus inermis abundant in Kaneohe Bay (Hawaii). He
collected water, animals, and their particulate food by dipping with buckets; filtered
the water through 333 μm mesh to remove larger animals; picked all stages of A.
inermis from replicate sets of both initial samples and samples incubated for 20–40
hours; and measured their carbon content with a combustion analyzer. He argued that