ARE MARINE PLANKTONIC INVERTEBRATES FOOD LIMITED - THE FUNCTIONAL-RESPONSE OF MYSIS-MIXTA (CRUSTACEA, MYSIDACEA) IN THE BALTIC SEA

The mysid shrimp Mysis mixta is an important zooplanktivore in the Bal tic Sea and has been suggested to contribute in structuring the zoopla nkton community. To describe this predation quantitatively, its functi onal response was determined in laboratory feeding experiments using a s prey a sample of the natural zooplankton assemblage from the Baltic Sea, Experiments were made in darkness in 271 black plastic bags, incu bated 24 h at 13 degrees C and with prey concentrations between 7 and 300 ind. 1(-1). M. mixta was a selective predator, consuming prey in t he following preference order: Pseudocalanus juveniles approximate to Acartia adults greater than or equal to Bosmina approximate to Acartia juveniles > Eurytemora adults approximate to Eurytemora juveniles. A Type II functional response model was fitted to consumption data, alth ough the saturation level (i.e. maximum consumption rate) was not reac hed in the experiments. Combining a bioenergetics model, the individua l in situ growth rate of M. mixta and our functional response model in dicates that ambient abundances of zooplankton (10 to 20 ind. 1(-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 n ecessary. The consumption rates also indicate that even when the zoopl ankton abundance is at its annual maximum, Baltic Sea M. mixta is food Limited.