History of anthropogenic nitrogen input to the German Bight/SE North Sea as reflected by nitrogen isotopes in surface sediments, sediment cores and hindcast models

摘要

The German Bight/SE North Sea is considered a hot-spot of river-induced eutrophication, but the scarce observational data of river nitrate loads prior to the 1970s complicate the assessment of target conditions for environmental management and legislation. Stable nitrogen isotope ratios (δ~(15)N) in sediment records can be used to decipher historical river nitrate contributions. To better constrain pre-1970s conditions, we determined δ~(15)N in archive sediment samples (1950-1969) and dated cores from the Helgoland depositional area. We also modeled the δ~(15)N in past situations (1960 and 1860) using an N-isotope-tracking ecosystem model. The modeled spatial distribution of δ~(15)N in sediments for 1960 conditions and the observed spatial pattern of δ~(15)N in archive sediment samples (1950-1969) represent a period of moderate eutrophication. The modeled spatial distribution of δ~(15)N in sediments for 1860 conditions (pre-industrial) showed a moderate δ~(15)N gradient from the Elbe river mouth (δ~(15)N<4<) to the open sea (δ~(15)N~;5<). This pattern contrasts with the δ~(15)N pattern in modern surface sediments, which exhibits a steep and inverted δ~(15)N gradient from the Elbe river mouth (δ~(15)N>9<) to the open sea (δ~(15)N<7<). Modeled δ~(15)N for 1860 conditions are consistent with δ~(15)N values observed in dated sediment cores that span the last 900 years. Value of δ~(15)N in sediment cores increased from approximately 1860 to 2000 by 2.5