Nutrient pools, transformations, ratios, and limitation in the Gulf of Riga, the Baltic Sea, during four successional stages

Basin-wide spatial distribution of different nutrient (N, P) fractions, chlorophyll a and basic hydrography in the Gulf of Riga was studied on five cruises during four stages of the annual succession, in 1993–1995. On the basis of the spatial distributions of the nutrient fractions, a general hypothesis was depicted of the annual transformations in nutrient pools, and their consequences for the nutrient limitation of planktonic production. Total nutrient pools indicate a degree of eutrophication comparable to the middle and eastern Gulf of Finland. In contrast to previous evidence, the Gulf of Riga seems not to be P-limited during the productive season. Our results suggest that the basin is basically N-limited, with hydrographically mediated P-limited phases potentially emerging during the annual succession. In early stages of the productive season, N limitation seems to predominate, with possible P limitation only in the SE parts of the Gulf, affected by the River Daugava spring runoff. During the late summer/early autumn periods when mineralization processes in the water column cumulatively liberate especially ammonium, the depth of the mixed surface layers seems to affect the limitation pattern. Thin or moderate mixed layer favors N limitation and deep mixing favors P limitation, the conditions thus resembling those previously observed in the estuarine eastern Gulf of Finland. With deep mixing, it seems also likely that phytoplankton is not able to fully utilize available nutrients due to the shallow euphotic zone. Therefore, strongly heterotrophic late summer/early autumn stages are likely to emerge. As the simple and exposed topography of the basin makes it exceptionally prone to physical forcing, it is obvious that episodic mixing events can cause considerable interannual and within-season variability in the production preconditions and nutrient limitation of the planktonic community, especially towards late summer and early autumn.