Riverine input of nutrients to the Gulf of Riga — temporal and spatial variation

Riverine transport is the, most important pathway for input of nutrients to the Gulf of Riga. The present study focused on updating existing estimates of the riverine nutrient contributions and on improving the favailable information on temporal and spatial variation in such input. The results show that the gulf received an average of 113,300 tons of nitrogen, 2050 tons of phosphorus and 64,900 tons of dissolved silica (DSi) annually during the time period 1977–1995. There was large interannual variation in loads, e.g., a factor two difference was found between the two most extreme years (1984 and 1990); this was attributed mainly to natural variation in water discharge. The seasonal distribution of nutrient loads exhibited a distinct pattern for practically all studied constituents, especially nitrate. Loads were high during the spring-flow and relatively low during the low-flow summer period. Examination of the spatial variation of nutrient loads showed that the Daugava River alone accounted for approximately 60% of the total riverine load. The highest area-specific loads of nitrate and phosphate were observed in the agriculturally dominated Lielupe River, and the highest loads of organic-nitrogen (org-N) and total phophorus (tot-P) were found in the Pärnu River. However, the values for all the studied rivers and constituents were rather low (phosphorus) or moderate (nitrogen and silica) compared to those reported for many other drainage areas of the Baltic Sea. This was true despite the inefficient sewage treatment and intensive agriculture in the studied basins in the 1970s and 1980s.