MEPS prepress abstract  -  DOI:

Density dependent grazing rates in a natural microzooplankton community

Danilo Calliari*, Laura Rodríguez-Graña, Peter Tiselius


ABSTRACT: 1. Density dependence is a common phenomenon that affects individual performance in the widest range of organisms. Negative density dependence involves diminished individual rates, e.g., feeding, growth, under high organismal concentration. Microzooplankton (µZ) are key consumers in marine ecosystems and their grazing is frequently estimated by the dilution technique, which involves the experimental manipulation of population concentrations of both grazer and prey. However, the potential interference of density dependent processes on grazing estimates has not been evaluated in the general context of µZ ecology, nor in the specific context of the dilution technique. Density dependent effects on µZ grazing rates were evaluated for the natural community of grazers in the Gullmar Fjord (Skagerrak, Sweden) for a wide but realistic range of µZ densities and under controlled algal prey concentration. Net algal growth rates (k), grazing rate of the µZ community (G), and per capita grazing rates (SG) by the components of the µZ were estimated based on algal cell counts and chlorophyll-a (as metrics for prey concentration) and µZ counts (as measure of predator concentration). The three responses (k, G and SG) showed clear evidence of negative density dependence under moderate and high levels of µZ concentrations. Results imply that negative density dependent effects on µZ grazing rates may actually occur in marine ecosystems.