their function for long periods (Stoecker et al. 2009).
(^) Some of the importance of microheterotrophy arises because of the rapid potential
growth of protists with division rates similar to those of phytoplankton. Thus, the
overall rate of grazing can more or less keep pace with phytoplankton stocks.
Estimates of division rates for the ciliates Strombilidium and Strombidium (Montagnes
1996) are mostly in the range 0.6 to1.0 d−1 (recall that one doubling each day equals
an exponential rate of 0.69 d−1), about the same as typical phytoplankton growth
rates, and can reach 2.2 d−1 with saturating food at the highest habitat temperatures.
Strom and Morello (1998) found that coastal ciliates were growing at 0.77 to 1.01 d−1.
Heterotrophic dinoflagellates increased more slowly, at 0.41 to 0.48 d−1, which is still
fast enough to make them significant contributors to grazing. These are maximum
rates from laboratory batch cultures, not gross rates in the field and not net rates after
predation.
(^) Biological oceanographers are still assimilating the importance of protistan grazing.
It is clear that well over half of the metabolic processing of pelagic primary
production occurs in the microbial food web. That is true both in oligotrophic regimes
far out to sea and in richer coastal zones. Mesozooplankton like appendicularians,
copepods, euphausiids, and pteropods take a large fraction of their nutrition from
microheterotrophs, acting as “giant” carnivores, and planktivorous fish have stepped
out perhaps two trophic levels compared to the place they were believed 40 years ago
to occupy.
Evaluation of Mesozooplankton Production
(^) Next, zooplankton must be fitted into the scheme of production analysis that we
started on with phytoplankton. Zooplankton are “secondary” producers, so we want to
determine the rates of their production, the amount of new zooplankton tissue
elaborated each day or year. However, at least mesozooplankton are not finely
dispersed particles like phytoplankton, and they do not take up a well-defined and
easily labeled substrate (CO 2 ), such that they can simply be filtered and their
productivity calculated from the content of an incorporated tracer, although that has
been tried and may be quite appropriate to protistan zooplankton. Estimation is
difficult, and we have no complete sets of values for any defined portion of the
community. However, work on the problem generates insights about zooplankton
themselves. The hope has been offered at times that, if we knew the rates of secondary
production in pelagic ecosystems, we could estimate the ecological efficiency of the
lower trophic levels and develop an expectation for amounts of production (of fish,
squid, shrimp, ...) that might be harvested from higher levels.
(^) The place of particle grazers in the trophic–dynamic scheme can be presented as