Food limitation and benthic regulation of populations of the copepod Acartia hudsonica Pinhey in nutrient‐limited and nutrient‐enriched systems

Abstract

Evidence that population size of copepods in coastal marine systems can be both resource and predator limited was obtained from experimental enclosures with and without the sediment community in place. The novel finding was that meroplankton and attached benthos were apparently effective competitors and predators of planktonic copepods in both nutrient‐limited and nutrient‐enriched systems. During the spring period of increase Acartia hudsonica, as well as other abundant species of copepods, reached considerably higher numbers in four mesocosms without sediments than in four other mesocosms with the normal sediment community in place regardless of rate of nutrient input to the mesocosms. Measurements of population parameters of A. hudsonica indicated equivalent rates of reproduction, growth, and recruitment in nutrient‐enriched treatments with and without sediments. Suppression of population growth in the presence of sediments was therefore not due to food limitation but, by inference, to higher rates of mortality associated with high abundances of potential predators of benthic origin. In unenriched control mesocosms, resource competition with the benthos may also have contributed to low population densities in tanks with sediments (reproductive rates were more food limited in enclosures with sediment that in those without sediment). These results demonstrate important interactions between benthic and pelagic fauna that have often been overlooked but nevertheless may be important in regulating dynamics of pelagic copepod populations in shallow, well‐mixed waters.