Seasonal Dynamics of Sublittoral Meiobenthos in Relation to Phytoplankton Sedimentation in the Baltic Sea

Abstract

Meiobenthic metazoans (40–500)μm were sampled monthly at a 37m deep 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 shell-free dry weight biomass was lowest in winter (0·9mg10cm−2in January), and increased rapidly following the spring bloom, to high values in May–July (peak 1·7mg10cm−2in July). As an annual average, ostracods contributed most to biomass, 38%, while nematodes and harpacticoids made up 24 and 15%, respectively. Only nematodes were common below 2cm depth in the sediment, and few nematodes penetrated below 4cm. Of Wieser’s morphologically based nematode feeding groups, epistrate feeders dominated the surface sediment, and non-selective deposit feeders dominated the deeper layer in May. Total nematode abundance was significantly different among dates, with lowest numbers in winter and spring (October–April), and almost doubled within about 2 months after the spring phytoplankton bloom in March. There was a significant increase in selective deposit feeders and epistrate feeders after the spring bloom. Harpacticoid copepods were almost all of two species,Pseudobradyasp. andMicroarthridion littorale, both of which differed significantly in abundance among months, and displayed continuous reproduction throughout the year, with a peak in pairs in precopula in winter forPseudobradyasp. and in ovigerous females inM. littoraleafter the spring bloom.Pseudobradyawas significantly more numerous in winter than in other seasons.Microarthridion littoralehad its highest abundance from July to October. Three species of ostracods were common throughout the year and all differed significantly in numbers among months. Turbellaria, Kinorhyncha were found in lowest numbers during winter and peaked in summer. The peak of newly settledMacoma balthicaspat in June disappeared rapidly, as predicted from laboratory experiments showing they are eaten by amphipods. The results support the hypothesis that meiobenthic animals react in two ways to phytoplankton sedimentation, with surface feeders directly assimilating sedimented phytoplankton, and increasing markedly following the spring bloom, while subsurface feeders experience a more stable food supply, and rely only indirectly on sedimented phytoplankton.