Seasonal changes in the grazing of coastal picoplankton by the suspension-feeding polychaete Ditrupa arietina (O.F. Müller)

Experiments were carried out monthly between December 1997 and December 1998 to investigate seasonal changes in the grazing of picoplankton by the suspension-feeding polychaete Ditrupa arietina. Seawater collected near the bottom was incubated both in the absence and in the presence of worms. Grazing was then computed based on the observed decrease in picoplankton concentrations. On a yearly basis, picoplankton accounted for 16% of total planktonic chlorophyll a. It was most abundant during summer with a clear succession between picoeukaryotes, cyanobacteria and prochlorophytes. There was a significant grazing of D. arietina on picoplankton. On a yearly basis, picoplankton accounted for 15% of ingested Chl a. Grazing correlated positively with picoeukaryote and prochlorophyte concentrations. This correlation was less clear for cyanobacteria. Picoeukaryotes accounted for 95% of the grazed picoplanktonic carbon. Picoeukaryote availability thus appears to constitute one of the main factors controlling grazing on picoplankton. The ingested fraction of the picoplankton was significantly enriched in chlorophyll c relative to ambient seawater. These results suggest that D. arietina has a high retention efficiency for particles down to 2 μm in size.