SEASONAL DYNAMICS OF SUBLITTORAL MEIOBENTHOS IN RELATION TO PHYTOPLANKTON SEDIMENTATION IN THE BALTIC SEA

Meiobenthic metazoans (40-500) mu m were sampled monthly at a 37 m dee p station in the north-western Baltic Sea proper. Nematodes dominated the meiofauna, ranging from 67% of total abundance in February to 91% in September. Harpacticoid copepods were the second most common group, ranging from 2% in September to 15% in February. Total meiofauna shel l-free dry weight biomass was lowest in winter (0.9 mg 10 cm(-2) in Ja nuary), and increased rapidly following the spring bloom, to high valu es in May-July (peak 1.7 mg 10 cm(-2) in July). As an annual average, ostracods contributed most to biomass, 38%, while nematodes and harpac ticoids made up 24 and 15%, respectively. Only nematodes were common b elow 2 cm depth in the sediment, and few nematodes penetrated below 4 cm. Of Wieser's morphologically based nematode feeding groups, epistra te feeders dominated the surface sediment, and non-selective deposit f eeders dominated the deeper layer in May. Total nematode abundance was significantly different among dates, with lowest numbers in winter an d spring (October-April), and almost doubled within about 2 months aft er the spring phytoplankton bloom in March. There was a significant in crease in selective deposit feeders and epistrate feeders after the sp ring bloom. Harpacticoid copepods were almost all of two species, Pseu dobradya sp. and Microarthridion littorale, both of which differed sig nificantly in abundance among months, and displayed continuous reprodu ction throughout the year, with a peak in pairs in precopula in winter for Pseudobradya sp. and in ovigerous females in M. littorale after t he spring bloom. Pseudobradya was significantly more numerous in winte r than in other seasons. Microarthridion littorale had its highest abu ndance from July to October. Three species of ostracods were common th roughout the year and all differed significantly in numbers among mont hs. Turbellaria, Kinorhyncha were found in lowest numbers during winte r and peaked in summer. The peak of newly settled Macoma balthica spat in June disappeared rapidly, as predicted from laboratory experiments showing they are eaten by amphipods. The results support the hypothes is that meiobenthic animals react in two ways to phytoplankton sedimen tation, with surface feeders directly assimilating sedimented phytopla nkton, and increasing markedly following the spring bloom, while subsu rface feeders experience a more stable food supply, and rely only indi rectly on sedimented phytoplankton. (C) 1997 Academic Press Limited.