Abstract
In this paper, we have synthesized and integrated results regarding nutrient 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 temporal variability in the load of nutrients in the drainage basin and rivers, as well as the effects of such nutrients on the environment of the recipient, 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 atmospheric deposition, point emissions from cities and industries along the coast, and nitrogen fixation by marine organisms. It was found that natural variability in water discharge was the main factor controlling the temporal variability in the riverine load of nutrients (i.e. differences in load between seasons and years). Moderate nutrient losses to surface waters, especially from agriculture, and weak riverine response to the sudden decrease in agricultural production after the disintegration of the Soviet Union in the early 1990s were also found. It is suggested that this is most likely related to the inertia in, and buffering capacity of, agricultural soils, i.e. depending of factors such as hydrological conditions, the size of ground water aquifers and water-saturated soils, creating favourable conditions for nutrient retention processes in the agricultural landscape. Regardless of the relatively low area-specific riverine inputs, the pollution loads in the rivers have contributed significantly to eutrophication of the Gulf of Riga. This seems to be true despite the relatively rapid distribution of the loads beyond the littoral zone of the gulf, resulting in a moderate all-over eutrophication of the entire recipient, characteried by a continuously shifting gradient between the southern and northern parts of the gulf. Furthermore, the marine eutrophication seems to be less prominent and less confined to the southernmost basin than previously assumed.
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Stålnacke, P., Vagstad, N., Tamminen, T. et al. Nutrient runoff and transfer from land and rivers to the Gulf of Riga. Hydrobiologia 410, 103–110 (1999). https://doi.org/10.1023/A:1003736620843
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DOI: https://doi.org/10.1023/A:1003736620843