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 matterand energy in the pelagic environment. I first consider populationregulation mechanisms in copepods by briefly reviewing estimates of growthand mortality rates and evidence of predation and resource limitation. Theeffects of variations in fecundity and mortality rates for the demography ofcopepod populations are then examined by a simple model, which demonstratesthat population growth rates are much more sensitive to variations inmortality than to variations in fecundity. This is consistent with theobserved tremendous variation in copepod fecundity rates, relatively low andconstant mortality rates and with morphological and behavioralcharacteristics of pelagic copepods (e.g., predator perception and escapecapability, vertical migration), which can all be considered adaptations topredator avoidance. The prey populations of copepods, mainly protozoa(ciliates) and phytoplankton, may be influenced by copepod predation tovarying 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 incontrolling their dynamics; availability of light and nutrients as well ascoagulation and sedimentation appear generally to be more important. Thelimited morphological variation of planktonic ciliates, the well developedpredator perception and escape capability of some species, and the oftenresource-unlimited in situ growth rates of ciliates, on the other hand,suggest that copepod predation is important for the dynamics of theirpopulations. I finally examine the implications of mesozooplankton activityfor plankton food webs, particularly their role in retarding vertical fluxesand, thus, the loss of material from the euphotic zone. This revised version was published online in July 2006 with corrections to the Cover Date.