Species-specific phytoplankton sedimentation in relation to primary production along an inshore-offshore gradient in the Baltic sea

Abstract

The temporal and spatial variability in the quality and quantity of settling phytoplankton material in relation to concurrent primary production was studied using sediment traps at three coastal stations from a semi-enclosed bay (Pojo Bay) through the outer archipelago to the open Gulf of Finland. The flux of settling phytoplankton was high (9.3 g C m-2 period-1) in Pojo Bay, especially in spring, and lower in the archipelago (8.1 g C m-2 period-1) and open-sea area (5.2 g C m-2 period-1), although the primary production followed the opposite pattern. A large influx of allochthonous material into Pojo Bay in spring brought allochthonous phytoplankton cells into the traps, but limited primary production. Diatoms were the most abundant settled phytoplankton at all stations, but the species composition varied between Pojo Bay (Aulacoseira spp., Rhizosolenia minima) and the outer stations (Skeletonema costatum, Chaetoceros spp.). At the outer stations, migrating dinoflagellates (Peridiniella catenata) comprised part of the settling material in spring. The high settling flux of the cyanophyte Aphanizomenon flos-aquae is discussed. The species composition of the phytoplankton assemblage influenced the proportion of the total organic carbon sedimentation that consisted of phytoplankton carbon.