Carbon budget and pelagic community compositions at two coastal areas that differ in their degree of eutrophication, in the Southern Baltic Sea

The coastal areas of the Southern Baltic Sea are highly productive ecosystems due to a high nutrient loading via rivers, non-point sources and frequent upwelling. The organic carbon production of these areas contributes to final carbon deposition in deeper basins of the Southern Baltic Sea by horizontal near-bottom transport processes, followed by littoral secondary production and biogeochemical cycles in the deeper areas. Seasonal investigations concerning the major driving forces of primary and bacterial production at two Southern Baltic Sea coastal areas with different degrees of eutrophication have been performed. The hypertrophic study site, Koserow, influenced by high nutrient loading from the River Oder, was characterised by high primary production rates and high phytoplankton biomass, compared with the meso-/eutrophic site, Tromper Wiek. At both sites a typical seasonal phytoplankton succession was found. Numerical integration of phytoplankton photosynthesis showed that the plankton community at Koserow acted as a potential net source of organic matter whereas Tromper Wiek seemed to be a potential sink of organic carbon. At both study sites, bacterial secondary production was limited by temperature only, which could be explained by a high proportion of detritus associated bacteria, not directly dependent on DOM exudation of the phytoplankton community. This was supported by underwater camera observations and investigations of the benthic fauna, both indicating a high potential for aggregation and sedimentation.