ABSTRACT: The mysid shrimp Mysis mixta is an important zooplanktivore in the Baltic Sea and has been suggested to contribute in structuring the zooplankton community. To describe this predation quantitatively, its functional response was determined in laboratory feeding experiments using as prey a sample of the natural zooplankton assemblage from the Baltic Sea. Experiments were made in darkness in 27 l black plastic bags, incubated 24 h at 13°C and with prey concentrations between 7 and 300 ind. l^-1. M. mixta was a selective predator, consuming prey in the following preference order: Pseudocalanus juveniles ~= Acartia adults >= Bosmina ~= Acartia juveniles >Eurytemora adults ~= Eurytemora juveniles. A Type II functional response model was fitted to consumption data, although the saturation level (i.e. maximum consumption rate) was not reached in the experiments. Combining a bioenergetics model, the individual in situ growth rate of M. mixta and our functional response model indicates that ambient abundances of zooplankton (10 to 20 ind. l^-1 in August, which is the annual maximum) are too low to maintain observed growth rates without taking prey patchiness into account. To explain the observed growth of M. mixta in the Baltic Sea, zooplankton patches with densities 3.5 to 6 times that of average ambient densities are necessary. The consumption rates also indicate that even when the zooplankton abundance is at its annual maximum, Baltic Sea M. mixta is food limited.

KEY WORDS: Mysismixta · Functional response· Zooplankton· Prey selectivity· Baltic Sea

Published in MEPS Vol. 150 (1997) on April 30
ISSN: 0171-8630. Copyright © Inter-Research, Oldendorf/Luhe, 1997