Population regulation and role of mesozooplankton in shaping marine pelagic food webs

Copepods constitute the majority of the mesozooplankton in the oceans. By eating and being eaten copepods have implications for the flow of matter and energy in the pelagic environment. I first consider population regulation mechanisms in copepods by briefly reviewing estimates of growth and mortality rates and evidence of predation and resource limitation. The effects of variations in fecundity and mortality rates for the demography of copepod populations are then examined by a simple model, which demonstrates that population growth rates are much more sensitive to variations in mortality than to variations in fecundity. This is consistent with the observed tremendous variation in copepod fecundity rates, relatively low and constant mortality rates and with morphological and behavioral characteristics of pelagic copepods (e.g., predator perception and escape capability, vertical migration), which can all be considered adaptations to predator avoidance. The prey populations of copepods, mainly protozoa (ciliates) and phytoplankton, may be influenced by copepod predation to varying degrees. The highly variable morphology and the population dynamics (e.g., bloom formation) of the most important phytoplankton prey populations (diatoms, dinoflagellates) suggest that predation plays a secondary role in controlling their dynamics; availability of light and nutrients as well as coagulation and sedimentation appear generally to be more important. The limited morphological variation of planktonic ciliates, the well developed predator perception and escape capability of some species, and the often resource-unlimited in situ growth rates of ciliates, on the other hand, suggest that copepod predation is important for the dynamics of their populations. I finally examine the implications of mesozooplankton activity for plankton food webs, particularly their role in retarding vertical fluxes and, thus, the loss of material from the euphotic zone.