Reconstructing changes in nitrogen input to the Danube-influenced Black Sea Shelf during the Holocene

Abstract. The western Black Sea shelf, where Danube River contributes the largest river discharge into the Black Sea, is particularly sensitive to river-induced eutrophication, which peaked in the 1980s and 1990s due to human-induced nutrient input. Nutrient input to the western Black Sea shelf and eutrophication is decreasing since the mid-1990s due to the collapse of eastern European economies after 1989 and ongoing mitigation measures to reduce nutrient emissions. The assessment of nutrient inputs to the Black Sea prior to the 1960s however is complicated by the scarcity of information on earlier Danube nutrient loads. Thus, to define what pristine conditions have looked like to provide a reference for nutrient reduction targets remains challenging. In this study, we aim to trace modern and historical nitrogen sources to the western Black Sea Shelf during the last ~5,000 years with special focus on the past 100 years, using sedimentary records of TOC, TIC, nitrogen, and δ¹⁵N.

Our results demonstrate that the balance of riverine nitrogen discharge into the Black Sea on the one hand, and nitrogen fixation in the pelagic system on the other, seem largely determined by climate effects. Specifically, this balance of riverine N input and N fixation is not only controlled by the concentration of nutrients discharged by rivers, but also by the freshwater volume, which controls the intensity of thermohaline stratification and thereby the timing and intensity of nutrient recycling from the deep basin back into the euphotic epipelagic. Based on analytical data of geochemical and isotopic properties of dated sediment cores, we identified a gradient from the nearshore sediment directly at the Danube Delta, where riverine N is dominant to offshore sediment in 80 m water depth, with pelagic N fixation being dominant in the past. Our results based on stable isotopes also demonstrate the increased deposition of nitrogen from human activities in all stations across the shelf and the concomitant changes in deposition rates of organic matter as indication for perturbations in the epipelagic community due to the human-induced eutrophication. Finally, our stable isotope data indicate that human-induced eutrophication can be traced back to the 12^th century CE, which raises the question which point in time is a feasible reference for nutrient reduction goals as the Danube nutrient loads was not pristine since at least in the 800 years.