Nutrient cycling and phytoplankton dynamics in relation to mussel grazing in a mesocosm experiment

Abstract

Abstract An experiment was carried out with four 3 m3 land-based mesocosms in May/June 1993. The mesocosms were supplied with a high nutrient loading, and 4 different amounts (20, 40, 80 and 160) of 17-19 mm blue mussels (Mytilus edulis). Phytoplankton development, concentrations of nutrients, primary production, bacterial production, and mussel growth were followed during four weeks. Phytoplankton biomass was significantly reduced in the mesocosms with the highest mussel bio-mass. The phytoplankton in the mesocosm with the highest mussel biomass had a higher proportion of diatoms than the other mesocosms. Phytoplankton growth rates were highest in the mesocosms with high mussel biomass, which was explained as the result of a shift towards faster growing algae (diatoms) and increased nutrient availability. The reduction in phytoplankton biomass by grazing was higher than the increase of phytoplankton growth rates. As a consequence, total primary production was lowest in the mesocosm with high mussel biomass. Due to intraspecific food competition, mussel growth rate was reduced in the mesocosm with the highest mussel biomass. Mussel growth rate in the mesocosm with the lowest mussel biomass was reduced as well. This could not be explained from phytoplankton biomass or production, and it is suggested that reduced food quality was the cause.