Abstract
The northwestern Black Sea Shelf region suffered from severe eutrophication in recent decades, mainly due to the Danube River's increased nutrient loads, as well as nutrient remineralization in sediments. During two cruises (spring 2012 and summer 2013) in the Danube River Delta–Black Sea transition zone, we analyzed dissolved nutrients, dual isotope signatures of nitrate, and δ15N of suspended particulate matter across the salinity gradient, and two pore water nutrient profiles (only 2013) in the outer river plume. We aimed to investigate nitrogen turnover and nutrient dynamics in the present Black Sea Shelf region in order to elucidate the contemporary role of nitrogen inputs of the River Danube in the light of the past, severely eutrophic, situation in the Delta region. Data indicate intense drawdown of river-borne nitrate, phosphate, and dissolved silica in the river plume. Isotopic patterns of nitrate and suspended N illustrate exceptionally clearly the dominance of nitrate assimilation over the entire salinity gradient. An unusual 1.9 : 1 enrichment of δ18ONO3 to δ15NNO3 questions the usually uniform enrichment that is attributed to phytoplankton. Accordingly, we investigated the potential effects of nitrate regeneration and uptake by heterotrophic plankton on the isotopic composition of nitrate in the Delta. Overall, our data suggest that today, the nitrogen input from the Danube River is taken up within the salinity gradient in large quantities, and its immediate effects on the eutrophication status of the Black Sea shelf are relatively limited.
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