Sedimentation and particulate nutrient dynamics along a coastal gradient from a fjord-like bay to the open sea

Seasonal changes of total particulate material (TPM), particulate organic c arbon (POC), nitrogen (PON) and phosphorus (PTP) concentrations in the wate r column, in sediment traps and on the sediment surface were studied in the SW coast of Finland, Baltic Sea, from March to November 1992. Sampling was carried out along a coastal gradient from the fjord-like, semi-enclosed Po jo Bay to the outer archipelago and open sea area. In Pojo Bay, TPM sedimen tation rates were high and relatively constant, and had low organic carbon contents throughout the seasonal cycle. Resuspension was estimated to contr ibute > 90% of total sedimentation of POC and PON. Clear seasonality in sed imentation with high settling rates of primary organic material in spring, low sedimentation rates during summer and a considerable increase of resusp ension during autumn was found in the outer archipelago and open sea. The C :N:P ratios of suspended, settled and sediment surface material indicated g reater sedimentary loss of N (as compared to P and C) and closer coupling b etween pelagial and benthos in the archipelago and open sea area than in Po jo Bay. The sedimentation of P was 20-50% more effective (as compared to N and C) in Pojo Bay than elsewhere. These results indicate that the shift of planktonic nutrient limitation (from P to N limitation) is enhanced due to the more efficient sedimentation of the main limiting element along the es tuarine gradient. The primary sedimentation of organic carbon (approximatin g export flux from the pelagic system) during the whole study period was es timated to be 30-48% of the total net primary production. This indicates th at despite the differences in the salinity, nutrient dynamics and planktoni c community structure along the coastal gradient, a relatively constant fra ction of the annual primary production is exported from the pelagic system by sedimentation.