Nutrients vs toxicity in sediments : A case study of two semi-closed basins in Rijeka Bay, Croatia

River estuaries and semi-enclosed bays with poor water exchange are considered as sensitive coastal areas. The determination of nutrients and potential toxicity in sediments represent important elements in their environmental quality assessment process. Sediments act both as sink and as a source of toxic environmental compounds. In this study, spatial variations of sediment nutrients and toxicity in coastal areas with limited water exchange (Rjecina River basin, Bakar Bay) in vicinity of the industrial and urban centre of Rijeka was examined. In sediment of Rjecina basin, both total nitrogen and total phosphorus exhibited a clear spatial pattern characterized by the natural and anthropogenic enrichment of the sediment along the river flow. The N:P ratio ranged from high upstream to low in marine sediments. Potential sediment toxicity varied from highly toxic to not toxic, in accordance to the river flow and the anthropogenic input. In Bakar Bay, nitrogen and phosphorus originated from different sources. The main sources of nutrients were situated in the northern part of the bay while a higher homogeneity of environmental conditions was present in its southern part. No correlation between the concentration of nutrients and sediment toxicity was found in both investigated areas. Therefore, the local impact of different contamination sources created a heterogeneous scenario from both the geochemical and ecotoxicological point of view. Moreover, this study confirmed the Microtox (R) assay as a sensitive sediment toxicity screening tool and supported the idea of sediment toxicity as an independent parameter considered as vital in assessing ecological risks for biota and describing the environmental quality of sites with multiple sources of contamination.