Seasonal and spatial variation of suspended and sedimented nutrients (C, N, P) in the pelagic system of the Gulf of Riga

The temporal and spatial variation of suspended and sedimented C, N and P in the pelagic system in the central and southern part of the eutrophicated Gulf of Riga were investigated during late spring, summer and early autumn from 1993–1995. The temporal variability at a central station was as high or greater than the spatial variability. Thus no discernible influence of the highly eutrophicated river Daugava on the plankton dynamics and the pelagic–benthic coupling of the Gulf of Riga was observed. High sedimentation rates of carbon-rich organic material were recorded at the end of the spring bloom, while P and N were retained on average 5 and 1.2 times more effectively, respectively, than C in the pelagic system. Low sedimentation rates of P, indicating effective recycling in the mixed surface layer, were typical for most of the productive season. The low C/N ratios of the settled material during summer were probably caused by accumulation of heterotrophic dinoflagellates into the sediment traps rather than due to increased vertical export of N. It is suggested that the major supply of organic matter to the benthos is determined by the time window characterised by low heterotrophic diversity and biomass in early spring. The trophic complexity of the planktonic community in the southern and central Gulf of Riga with its continuous discharge of nutrients by large rivers such as Daugava, results in a high buffering capacity of the pelagic system towards external perturbations such as increased nutrient supply.