Grazing by mesozooplankton from Kiel Bight, Baltic Sea, on different sized algae and natural seston size fractions

Grazing experiments were conducted with natural mesozooplankton from Kiel Eight, Germany, using radioactive labelled phytoplankton cultures and seston size fractions. The results of experiments using phytoplankton cultures indicated that bivalve veligers performed highest clearance of particles within a size range of 4.7 to 6.3 mu m, whereas optimum particle size for copepods was 15 mu m. The results of experiments using labelled natural seston size fractions identified bivalve veligers and appendicularians as those responsible for the removal of particles within the smallest size class (<2 mu m). Seston size fractions larger than 5 mu m were mainly cleared by copepods and nauplii. As particle size increased, the contribution of copepod clearance to total zooplankton clearance within size classes increased from 57 % (<5 mu m size class) to more than 81% (30 to 100 mu m size class). When the nauplii clearance rates were included, the total copepod clearance accounted for 90 to 97.6% of the total volume cleared of particles bigger than 10 mu m Despite low abundances of bivalve veligers and appendicularians in Kiel Eight at the time of the experiment, we calculated that approximately 10 and 8.5%, respectively, of the carbon ingested by total mesozooplankton was due to veliger and appendicularian grazing. The importance of bivalve veligers might be seen in their grazing on seston particles that escape predation by copepods and on the amount of energy that is therefore directed from the water column to the benthos when larvae settle.