MEPS 622:83-92 (2019)  -  DOI: https://doi.org/10.3354/meps13003

ABSTRACT: Density dependence is a common phenomenon that affects individual performance in a wide range of organisms. Negative density dependence involves diminished individual rates, e.g. feeding and 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 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 a natural community of grazers in the Gullmar Fjord (Skagerrak, Sweden) across a wide but realistic range of µZ densities and under controlled algal prey concentrations. Net algal growth rates (k), grazing rate of the µZ community (G), and per capita grazing rates (SG) by the components of the µZ community were estimated based on algal cell counts and chlorophyll a (as metrics for prey concentration) and µZ counts (as a measure of predator concentration). The 3 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.

KEY WORDS: Ciliates · Dinoflagellates · Dilution technique