No abstract

ABSTRACT:The diatom Skeletonema costatum is one of the dominant phytoplankton species during spring in the northern Baltic Sea. We followed the uptake of radio-labelled S costatum by all major meiofauna species in a laboratory experiment. The uptake of labelled diatom carbon varied greatly among major meiobenthic taxa and anlong species belonging to the same class or phylum. Both total uptake and uptake per unit biomass were by far highest in the ostracod Candona neglecta accounting for 4 6 % of the total meiofauna uptake. The total uptake of I4C was significantly different among all 3 common ostracod species, Candona neglecta taking 10 and 100 times more than Paracyprideis fennica and Heterocypndeis sorbyana respectively. Nematodes accounted for over 40% of the total uptake of 14C in the microcosms, of which 84 % was taken up by large nematode species such as Paracanthonchus spp. Nematodes with similar buccal cavities and of similar size showed surprisingly large differences in the uptake of the radio-labelled material. There seems to be quite strong selection both for and against the diatom among epistrate feeders as well as anlong so-called non-selective deposit feeders Only a small portion of the total meiofauna population was found below 1 cm in the sediment; this was composed almost solely of nematodes. These nematodes assimilated as much I4C per unit biomass as the surface ones did, which contradicts the hypothesis claiming that meiobenthic animals react in 2 ways to phytoplankton sedimentation, with surface feeders directly assimilating sedimented phytoplankton. while subsurface feeders experience a more stable food supply and rely only indirectly on sedirnented phytoplankton KEY WORDS: Pelagic-benthic coupling. Food-web . Meiofauna . 14C radio label

A laboratory experiment was performed to quantify the fate of diatom phytodetritus and how this is affected by the presence of benthic amphipods. A Baltic Sea spring bloom sedimentation event was simulated by adding 14 C-labeled diatoms (Skeletonema costatum) to microcosms with varying densities of the amphipods Monoporeia affinis and Pontoporeia femorata, as well as to microcosms without amphipods, where the sediment was disturbed mechanically. After 1 mo of incubation, 51 to 77% of the added diatom carbon was still in the sediment; 2 mo later 49 to 66% remained. The effect of amphipods on the fate of the phytodetritus differed between species. At near-field density, M. affinis incorporated 6 to 11% of the added 14 C, P. femorata only 1.2%. The results indicate that burrowing slows mineralization, presumably by mixing organic material to anoxic depths in the sediment. The effect of P. femorata on mineralization could not be distinguished from the effect of mechanical stirring. M. affinis feeding and respiration resulted in a significant increase in mineralization; at low densities this compensated for the mixing effect, at high densities M. affinis feeding resulted in enhanced net mineralization.

Three species of ostracods (Candona neglecta, Paracyprideis fennica and Heterocyprideis sorbyana) are common in the Baltic proper and often contribute more to the total meiobenthic biomass than any other taxon. An earlier experiment has shown that C. neglecta assimilates more labelled diatoms (Skeletonema costatum) than any other meiobenthic species, and 10 and 100 times more than the other 2 ostracod species, respectively. The uptake of phytodetritus by the 3 ostracod species was investigated in 2 separate experiments with special reference to (1) effects of the presence of amphipods and (2) variable density of ostracods. The amphipods had negative effects on the uptake rate of phytodetritus by ostracods. Mechanical disturbance, which may have caused burial of the phytodetritus, is one plausible explanation. There was no evidence of competition for phytodetritus among the ostracod species, i.e. the absence/presence of C. neglecta did not affect the uptake of phytodetritus by P. fennica and H. sorbyana. A density-dependent uptake of phytodetritus was, however, observed in C. neglecta, where the uptake of phytodetritus was stimulated at an intermediate density. KEY WORDS:14 C radio-labelled phytodetritus · Ostracoda · Amphipoda · Meiofauna · Disturbance · CompetitionResale or republication not permitted without written consent of the publisher

No abstract