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
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 (delta N-15) in sediment records can be used to decipher historical river nitrate contributions. To better constrain pre-1970s conditions, we determined delta N-15 in archive sediment samples (1950-1969) and dated cores from the Helgoland depositional area. We also modeled the delta N-15 in past situations (1960 and 1860) using an N-isotope-tracking ecosystem model. The modeled spatial distribution of delta N-15 in sediments for 1960 conditions and the observed spatial pattern of delta N-15 in archive sediment samples (1950-1969) represent a period of moderate eutrophication. The modeled spatial distribution of delta N-15 in sediments for 1860 conditions (pre-industrial) showed a moderate delta N-15 gradient from the Elbe river mouth (delta N-15<4 parts per thousand) to the open sea (delta N-15 similar to 5 parts per thousand). This pattern contrasts with the delta N-15 pattern in modern surface sediments, which exhibits a steep and inverted delta N-15 gradient from the Elbe river mouth (delta N-15>9 parts per thousand) to the open sea (delta N-15<7 parts per thousand). Modeled delta N-15 for 1860 conditions are consistent with delta N-15 values observed in dated sediment cores that span the last 900 years. Value of delta N-15 in sediment cores increased from approximately 1860 to 2000 by 2.5 parts per thousand. The increasing trend reflects changes in the abundance and isotopic composition of riverine nitrate loads caused by anthropogenic activities. Sensitivity tests suggest that loads and isotopic ratios of nitrogen forms other than nitrate (ammonium and organic nitrogen) have minor impact on the modeled surface sediments, despite their higher abundance in the riverborne loads in the past. Our results suggest that eutrophication of the German Bight predates the 1960 period of documented rapidly increasing river loads. Pre-industrial levels of delta N-15 modeled with 28\% of the modern annual (1990-1999) atmospheric loads and 10\% of the modern annual river loads agree best with levels of delta N-15 (similar to 6 parts per thousand) observed in sediments of the cores dated to 1860.