Sedimentation and Pelagic Retention of Particulate C, N and P in the Coastal Northern Baltic Sea

Abstract

Sedimentation and pelagic retention capacity of particulate carbon, nitrogen and phosphorus as well as the impact of resuspension and preservation on sedimentation rates were studied during spring and summer in the SW coast of Finland, Baltic Sea. Formaldehyde preservation resulted in significantly higher carbon, nitrogen and chlorophyllasedimentation rates when compared to traps without preservative. This was mainly attributable to contamination by vertically migrating micro-organisms in traps with formaldehyde and mineralization inside the non-preserved trap cylinders. During the spring bloom, stoichiometric ratios of settled and suspended particulate material indicated that phosphorus was retained in the pelagic system almost twice as effectively as carbon and nitrogen, while carbon and nitrogen were settling at equal rates. During summer, when sedimentation rates were low, nutrient ratios of settled and suspended matter indicated that both N and P were recirculated in the pelagic system more effectively than carbon while P was retained 1·4 times more effectively than nitrogen. High current velocities and increase of deep-water salinity correlated with increased sedimentation of particulate material, with low N:P, C:P and high C:N ratios indicating input of resuspended phosphorus during upwelling events. Wind velocity and direction did not have any clear relationship to the quality of settled material, denoting that wind-induced wave action and transport of littoral material had minor impact on sedimentation rates. Primary sedimentation of carbon corresponded to 57% of the total primary production during the study period from April to August, while that of nitrogen was 42% of estimated nitrogen assimilation, indicating that the overall export of carbon was more effective than that of nitrogen from the pelagic system.