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, chl orophyll a and basic hydrography in the Gulf of Riga was studied on five cr uises during four stages of the annual succession, in 1993-1995. On the bas is of the spatial distributions of the nutrient fractions, a general hypoth esis was depicted of the annual transformations in nutrient pools, and thei r 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-Iimited during the productive season. Our results s uggest that the basin is basically N-limited, with hydrographically mediate d P-limited phases potentially emerging during the annual succession. In ea rly stages of the productive season, N limitation seems to predominate, wit h possible P limitation only in the SE parts of the Gulf, affected by the R iver Daugava spring runoff. During the late summer/early autumn periods whe n mineralization processes in the water column cumulatively liberate especi ally ammonium, the depth of the mixed surface layers seems to affect the li mitation pattern. Thin or moderate mixed layer favors N limitation and deep mixing favors P limitation, the conditions thus resembling those previousl y observed in the estuarine eastern Gulf of Finland. With deep mixing, it s eems also likely that phytoplankton is not able to fully utilize available nutrients due to the shallow euphotic zone. Therefore, strongly heterotroph ic late summer/early autumn stages are likely to emerge. As the simple and exposed topography of the basin makes it exceptionally prone to physical fo rcing, it is obvious that episodic mixing events can cause considerable int erannual and within-season variability in the production preconditions and nutrient limitation of the planktonic community, especially towards late su mmer and early autumn. (C) 1999 Elsevier Science B.V. All rights reserved.