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

The western Black Sea Shelf where the Danube is discharging into the Black Sea is especially sensitive to river-induced eutrophication, which peaked in the 1980s and 1990s due to human-induced nutrient input and is decreasing since the mid-1990s due to ongoing mitigation measures. The scarcity of information on Danube nutrient loads prior to the 1960s complicates the assessment of earlier nutrient input and thus to define pristine conditions to provide a reference for nutrient reduction goals. In this study, we aimed 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 on the other hand is generally determined by climate effects. Specifically, this balance of riverine N input and N fixation is not only controlled by the amount of nutrients discharged by rivers, but also by the freshwater itself, which controls the intensity of thermohaline stratification and thereby the intensity of nutrient recycling from the deep basin back into the euphotic epipelagic. Our results show a gradient from the nearshore sediment directly at the Danube Delta, where riverine N is dominant to offshore sediment in 80 m water depth, where pelagic N fixation was 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 AD, which raises the question which point in time is a feasible reference for nutrient reduction goals as the Danube was not pristine since at least 800 years.