Copepods, with their fixed sequence of stages and their reliable presence in samples
from all marine waters, offer advantages for studies of population dynamics. So, much
of the work focuses on them.
(^) Most mid- and high-latitude plankton have a population cycle involving a resting
stage or period of quiescence. During this phase of the life cycle, all individuals stop
growing in one or a few adjacent life stages. In the copepod genus Calanus, it is the
fifth copepodite (C5) stage, and to lesser extents C4- and C6-adults, which
accumulate copious lipids and then swim down out of the near-surface growth habitat
and rest for months, a phenomenon called diapause. When unfavorable conditions
approach, estuarine and nearshore copepods like Acartia produce diapause eggs that
accumulate in sediments. The eggs hatch in a burst when good conditions return with
the cycle of the seasons. Recurrent stopping of population processes with all
individuals in just one life stage creates persistent cycling of stage and age
composition. Zooplankton population dynamics must be studied in the face of that
cycling, but can also take advantage of it. It involves specialized estimation
procedures and lately modeling of reproduction, development, and mortality.
(^) Mortality rates are the most difficult parameters to estimate, and with exceptions
they become a row of free dials on numerical models, one dial for each distinctive
stage. We simply turn them until the model output looks like our time series of stage-
abundance data. Possibly, this will prove to be the most effective scheme for obtaining
mortality estimates. Development timing is obtained from rearing experiments, or by
following cohorts as they develop in the field. Those have been adequately covered
under secondary production (Chapter 7). The experiments are flawed because we
cannot exactly mimic the nutritional situation in the field. Cohort following is flawed
by sampling statistics with staggering variance; by advection moving variations in
population timing past a sampling site (thus, development can sometimes appear to
back up, the age distribution shifts toward youth); and by the fact that the progress of
cohort age- or stage-structure is confounded in field data with effects of mortality.
Reproduction, however, is readily measured, and it has become very popular to
measure it.
Reproductive Biology and Fecundity
(^) For most kinds of zooplankton, even the various hermaphroditic forms, reproduction
begins with mate finding and copulation. In some groups, females advertise their
readiness for mating with chemical signals (pheromones) dispersed in the water.
Males follow the signals, find the source, and initiate mating. Among copepods,
Calanus (Fig. 8.1), Temora, Pseudocalanus, Centropages, and Oithona attract and
find mates in that fashion (Weissburg et al. 1998; Kiørboe & Bagøien 2005). Many