Seasonal dynamics of dissolved organic matter on a coastal salinity gradient in the northern Baltic Sea

During the phytoplankton growth season, the vertical distributions of dissolved organic carbon (DOC), nitrogen (DON) and phosphorus (DOP) were followed biweekly along with key physical, chemical and biological variables on a shore-to-open-sea salinity gradient in the Gulf of Finland. Moreover, bioavailable shares of DOC and DON were evaluated with natural bacterial surface and deepwater samples. Dissolved organic matter (DOM) accumulated in the surface layer throughout the productive season, and the accumulated DOM was N-rich (molar C:N ratio of 10) compared to the bulk DOM pool (C:N 20–29). Redundancy analysis showed a negative correlation between phytoplankton and DOM concentrations, suggesting that most DOM release occurred during declining phases of spring and late summer algal blooms. During the spring bloom, bioavailable shares of DOM were small, whereas during and after the late summer bloom of filamentous cyanobacteria, 3–9% and 10–20% of the respective total DOC and DON pools were degraded by bacteria within 2 weeks. Vertical mixing over the thermocline was estimated by constructing a steady state budget for water and salt mass flows between key GoF basin compartments (inflows equal outflows). The flow estimates suggested that the net changes in surface DOC and DON pools underestimated the respective total accumulations by ca 40% and 20%, respectively, over the 4-mo thermal stratification period. Thus, integral export of surface DOC and DON after autumn overturn amounted to ca 710 mmol C m−2 and 40 mmol N m−2, corresponding respectively to ca 12–25% and 11% of reported annual particulate organic carbon and nitrogen sedimentation in our study area.