Nutrient runoff and transfer from land and rivers to the Gulf of Riga

In this paper, we have synthesized and integrated results regarding nutrien t loads and eutrophication of the Gulf of Riga (GoR) that were obtained in three projects that were part of a six-year research programme (1993-1998). In particular, we focused our attention on the factors that control the te mporal variability in the load of nutrients in the drainage basin and river s, as well as the effects of such nutrients on the environment of the recip ient, the Gulf of Riga. The results indicate that the rivers play a crucial role in the total input of nutrients to the GoR, and exceed the combined contribution from atmosph eric deposition, point emissions from cities and industries along the coast , and nitrogen fixation by marine organisms. It was found that natural vari ability in water discharge was the main factor controlling the temporal var iability in the riverine load of nutrients (i.e. differences in load betwee n seasons and years). Moderate nutrient losses to surface waters, especiall y from agriculture, and weak riverine response to the sudden decrease in ag ricultural production after the disintegration of the Soviet Union in the e arly 1990s were also found. It is suggested that this is most likely relate d to the inertia in, and buffering capacity of, agricultural soils, i.e. de pending of factors such as hydrological conditions, the size of ground wate r aquifers and water-saturated soils, creating favourable conditions for nu trient retention processes in the agricultural landscape. Regardless of the relatively low area-specific riverine inputs, the pollution loads in the r ivers have contributed significantly to eutrophication of the Gulf of Riga. This seems to be true despite the relatively rapid distribution of the loa ds beyond the littoral zone of the gulf, resulting in a moderate all-over e utrophication of the entire recipient, characteried by a continuously shift ing gradient between the southern and northern parts of the gulf. Furthermo re, the marine eutrophication seems to be less prominent and less confined to the southernmost basin than previously assumed.