Effects of spring bloom phytoplankton dynamics and hydrography on the composition of settling material in the coastal northern Baltic Sea

The phytoplankton species succession and sedimentation characteristics were studied on a sheltered and an open coastal station during a spring bloom in the northern Baltic Sea. Biomass (phytoplankton carbon and chlorophyll a), inorganic nutrients and particulate organic carbon and nitrogen (POC and PON) were determined in the suspended material. Sediment traps moored at different depths were used to determine the vertical flux of total particulate material (TPM), POC and PON, chlorophyll a, phaeopigments and phytoplankton carbon. The spring phytoplankton biomass was dominated by dinoflagellates that formed a dense bloom of short duration, whereas diatoms were present in the water column throughout the spring period in more moderate biomasses. The vertical flux of phytoplankton carbon was however dominated by diatoms at all times. The formation of resting stages increased the sedimentation of dinoflagellates, but compared to the suspended biomass of vegetative cells, their sinking rates were low. The ambient silicate concentration did probably not limit diatom growth; these were rather removed from the surface layer through sinking, a situation beneficial for dinoflagellates capable to exploit deep nutrient reserves through vertical migration. Due to rapid sinking soon after bloom formation and high specific loss rates, diatoms can be considered important contributors to the vertical flux of autochtonous material. Dinoflagellates mostly disintegrate in the water column and may settle as phytodetritus, except for the fraction of the populations that form rapidly sinking cysts. In addition to vertical export, advection of water from the stations seems to have been an important loss factor in the phytoplankton community. The two stations differed in that resuspension and input from littoral sources to the vertical flux were more important in the inner and shallower archipelago zone. This was also reflected in the C/N ratio of the settling material and in the bottom surface layer. In our study area, both the hydrographical regime and the species composition of the phytoplankton community were found to affect sedimentation characteristics and the composition of the settling material during the spring period.