Nitrate source identification and quantification in the euphotic zone of the Baltic Proper using nitrogen and oxygen stable isotopes
Zusammenfassung der Projektergebnisse
Within the project different aspects of the nitrate cycle of the Baltic Sea were investigated. At first sea water samples have been collected (February to March) on various cruises to the Baltic Sea for determination of the nitrate isotope signatures, that is δ15N and δ18O in nitrate. They were done to identify the background values of NO3-. For correction of these NO3- measurements oxygen stable isotope ratios of sea water were determined by the University of Stockholm and used to correct the δ18O data. Then, for evaluation of the impact of atmospheric deposition to the nitrogen sources of the central Baltic Sea, precipitation samples from three land-based stations in Germany and Sweden were collected over at least one annual cycle. Spatial (vertical and horizontal distribution) and seasonal variations of the isotope signal were detected. Furthermore, concentrations of dissolved organic nitrogen (DON) in surface waters and their δ15N values were measured from stations along a transect from west to east. Another part of the project was the development and improvement of the "denitrifier method" at the lOW-lab: Numerous tests were done to improve this rather new method, establish it at the lOW lab and share experiences with the scientific community. Our results underline previous findings that nitrate from agricultural impacted land exports isotopically heavy nitrogen to the coast (mean value 8.3 ‰) while the nitrogen from degradation of cyanobacteria or in atmospheric deposition is much lighter. The corresponding δ18O-NO3 are strongly influenced by the share coming from rain or snow. We observed, that with regards to the amount and origin of atmospheric N inputs, seasonal differences between the warm, wet and cold, dry periods were significantly more pronounced than regional variations. While the NO3- load was rather stable throughout the year, atmospheric NH4- loading was enhanced during the warm period and in general exceeded the NO3- load significantly. The DIN isotopic composition of the atmospheric input indicates that during the warm period , atmospheric N represents a mixture of nitrogen from soil emission, biomass burning and fossil fuel combustion, whereas during the cold season N originates mostly from fossil fuels. At the same time, our DIN isotope data imply that isotope-based N budgets may significantly overestimate N2 fixation rates in the Baltic if 15-depleted N (~0 ‰) is assumed to solely originate from N2 fixed by diazotrophic cyanobacteria, without consideration of low-δ15N atmospheric DIN input. Extrapolation of our data to the whole area of the Baltic proper suggests that atmospheric N represents a more important N source than biological N2 fixation in this region.
