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 i n 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 o r greater than the spatial variability. Thus no discernible influence of th e highly eutrophicated river Daugava on the plankton dynamics and the pelag ic-benthic coupling of the Gulf of Riga was observed. High sedimentation ra tes 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 effectiv ely, 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 mater ial during summer were probably caused by accumulation of heterotrophic din oflagellates 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 heterotrophi c diversity and biomass in early spring. The trophic complexity of the plan ktonic community in the southern and central Gulf of Riga with its continuo us discharge of nutrients by large rivers such as Daugava, results in a hig h buffering capacity of the pelagic system towards external perturbations s uch as increased nutrient supply. (C) 1999 Elsevier Science B.V. All rights reserved.