Morphometry and sedimentation as regulating factors for nutrient recycling and trophic state in coastal waters

The aim of this work is to quantify the importance of morphometry and sedimentation/resuspension on nutrient recycling and trophic characteristics in coastal waters. Extensive field work has been carried out in 23 coastal areas in the Swedish and Finnish part of the Baltic Proper. Sediment traps were deployed for two one-week periods in all areas. On average, 56% of the total sedimentation in sediment traps 3 m below the water surface (SedS) and 62% of the total sedimentation on sediment traps 1 m above the bottom (SedB) was resuspended material. Coastal morphometric parameters, surface water retention time and bottom dynamic conditions were determined for all areas. There is a marked relationship between SedS and inorganic-N concentration in the surface water. The relationship was improved significantly by using sedimentation of the resuspended fraction at 3 m water depth (SedR) instead of SedS.This led to the hypothesis that increased concentration of inorganic nitrogen in the surface water results from increased mineralisation of resuspended organic particles. A model describing SedS is presented where inorganic nitrogen concentration, the water surface area and the surface water retention time can explain 82% of the variation in SedS. In another model inorganic nitrogen and water surface area can explain as much as 93% of the variation in SedR.These results emphasise the importance of resuspension for nutrient recycling and trophic state in coastal waters. The importance of coastal morphometry and surface water retention time on total sedimentation and nutrient recycling makes it possible to classify coastal areas in terms of potential nutrient recycling capacity/trophic state from these simple sensitivity parameters.