Top-down control by calanoid copepods in marine plankton communities

Production is a function of growth rate and extant biomass. Resource availability mostly controls the former and grazing the latter. The relative importance of such bottom-up and top-down control mechanisms in marine plankton communities is currently under debate. In this dissertation I examined how community structure and nutrient imbalance between predator and prey mediate top-down effects. Specifically, I examined how calanoid copepods controlled growth of lower food web components in ecosystems of contrasting productivity, including a temperate estuary, the oligotrophic open ocean, and a tropical river plume. Furthermore, I examined the effects of prey size and predator-prey nutrient imbalance in the expression of top-down effects with model experimental food webs, consisting of a copepod, an aloricate ciliate and two centric diatom species, and under field conditions. ^ Employing manipulative experiments and path analysis, I show that top-down control was pervasive, but quite variable and subtle in natural marine plankton communities. Trophic interactions as a whole explained approximately 50% of the variance in autotrophic and bacterial growth, and copepods exerted mainly indirect effects (through a variety of paths) on these groups. Copepods exerted strong direct grazing of phytoplankton only in coastal communities dominated by large autotrophic cells. Otherwise, copepods exerted mostly indirect effects. ^ In model food webs, both copepods and ciliates practiced compensatory grazing when offered a nitrogen-limited phytoplankton monoculture; relative to the nitrogen-replete culture, carbon ingestion rates increased by 25% for copepods and 160% for ciliates. However, when a mixed diet of small and large phytoplankton was offered to ciliates, grazing on the smaller phytoplankton compensated for nitrogen deficiencies of the larger phytoplankton. Copepods could not take advantage of the smaller, high quality phytoplankton, nor could they sufficiently supplement their diet with ciliates when available. Consequently, the effects of omnivory on trophic interactions were stronger than those of elemental imbalance. The results of field experiments also supported this conclusion.