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 inf
ormation 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 t
ime 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 discha
rge. The seasonal distribution of nutrient loads exhibited a distinct patte
rn 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 riveri
ne load. The highest area-specific loads of nitrate and phosphate were obse
rved in the agriculturally dominated Lielupe River, and the highest loads o
f organic-nitrogen (org-N) and total phophorus (tot-P) were found in the Pa
rnu 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 tr
ue despite the inefficient sewage treatment and intensive agriculture in th
e studied basins in the 1970s and 1980s. (C) 1999 Elsevier Science B.V. All
rights reserved.