The role of inorganic and organic nutrients on the development of phytoplankton along a transect from the Daugava River mouth to the open Baltic, in spring and summer 1999

The importance of dissolved silicate (DSi), dissolved inorganic nitrogen (DIN), dissolved organic nitrogen (DON), phosphate and dissolved organic phosphorus (DOP) on algal growth is analysed for the Gulf of Riga and the adjacent open Baltic Sea. The results of three cruises (May, June, and July, 1999) along a transect across the Gulf of Riga from the entrance to the Daugava River to the open Baltic are presented. Nutrient-limitation was identified on the basis of available nutrient concentrations and stoichiometric analysis. In spring, phosphate appeared to be the algal-growth-potential-limiting nutrient at the entrance of the Daugava River, DSi in the central Gulf, and DIN at the open Baltic station. There was no correlation between limiting nutrient and spring phytoplankton community structure. Both the DIN and phosphate pools of the upper mixed layer were exhausted by mid-May, except at the river mouth. In summer there was a good correlation between phytoplankton biomass and DOP along the transect. Contrary to the situation in the open Baltic, the lower layer DIN/phosphate ratio in the Gulf of Riga significantly exceeds the Redfield ratio, and upwelling likely does not favour nitrogen-fixing species. Therefore, the upper layer DOP pool should be regarded as potentially the main source of phosphorus for nitrogen-fixing cyanobacteria in the Gulf of Riga.