Probably as a consequence of eutrophication, both the length of the annual phytoplankton
blooms and the mean concentrations of flagellates (dominated by Phaeocystispouchetii) have increased
in the western part of the Wadden Sea during the last decades. Planktonic diatoms, on the other hand,
have hardly increased, but fluctuate heavily from year to year. Among the benthic tidal-flat fauna,
which has increased in total numbers and biomass during the last decades, the bivalve Macorna balthica
was studied in detail. The 15 yr (1974 to 1989) patterns of its annual growth and condition were
compared with abundance patterns in phytoplankton components (during the growing seasons of M.
balthica). Fluctuations in M. balthica were found to parallel fluctuations of planktonic diatoms but not
those of P. pouchetii. It is concluded that M. balthica responded by faster growth and higher condition to
better feeding conditions in years of high diatom abundance. Diatoms rather than P. pouchetii or other
flagellates will be the most important food source for M. balthica.