NUTRIENT CYCLING AND PHYTOPLANKTON DYNAMICS IN RELATION TO MUSSEL GRAZING IN A MESOCOSM EXPERIMENT

An experiment was carried out with four 3 m(3) land-based mesocosms in May/June 1993. The mesocosms were supplied with a high nutrient loadi ng, and 4 different amounts (20, 40, 80 and 160) of 17-19 mm blue muss els (Mytilus edulis). Phytoplankton development, concentrations of nut rients, primary production, bacterial production, and mussel growth we re followed during four weeks. Phytoplankton biomass was significantly reduced in the mesocosms with the highest mussel biomass. The phytopl ankton in the mesocosm with the highest mussel biomass had a higher pr oportion of diatoms than the other mesocosms. Phytoplankton growth rat es were highest in the mesocosms with high mussel biomass, which was e xplained as the result of a shift towards faster growing algae (diatom s) and increased nutrient availability. The reduction in phytoplankton biomass by grazing was higher than the increase of phytoplankton grow th rates. As a consequence, total primary production was lowest in the mesocosm with high mussel biomass. Due to intraspecific food competit ion, mussel growth rate was reduced in the mesocosm with the highest m ussel biomass. Mussel growth rate in the mesocosm with the lowest muss el biomass was reduced as well. This could not be explained from phyto plankton biomass or production, and it is suggested that reduced food quality was the cause.