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

Abstract. The western Black Sea shelf is particularly sensitive to river-induced eutrophication due to river discharge from the Danube River, and accordingly, eutrophication peaked in the 1980s and 1990s due to human-induced nutrient input. Nutrient input to the western Black Sea shelf and eutrophication decreases 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 hindered by the scarcity of information on earlier Danube nutrient loads. Thus, to define pristine conditions 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 ∼ 7000 years with special focus on the past 200 years, using sedimentary records of TOC, TIC, nitrogen, and δ15N to quantify the share of anthropogenic nitrogen. Our results demonstrate that climate effects determine the relative contribution of riverine nitrogen and pelagic nitrogen fixation to fuel marine primary production on the NW shelf. This balance is not only controlled by the riverine nutrient load, but also by the freshwater volume itself, which controls the intensity of thermohaline stratification and thereby the timing and intensity of nutrient recycling from the deep basin back into the euphotic surface water. In the cold and dry Sub-Boreal climate pelagic N-fixation dominates over riverine N discharge, while in the warm and wet Atlantic climate riverine N discharge appears as dominant N source to sustain primary production on the NW shelf. Stable nitrogen isotopes further demonstrate the increased deposition of nitrogen from human activities across the shelf and the concomitant changes in deposition rates of organic matter, which can be tracked back to perturbations in the plankton due to the human-induced eutrophication. Finally, our stable isotope data indicate that human-induced eutrophication can be traced back to the 11th century CE and highlight that the Danube nutrient load was not pristine for at least the past 900 years.