A nitrate sink in estuaries? An assessment by means of stable nitrate isotopes in the Elbe estuary

To elucidate the fate of river-borne nitrate in the estuarine environment, we measured nitrate concentrations and d15N and d18O of nitrate along the salinity gradient in the estuary of the river Elbe, one of the largest German rivers discharging into the North Sea. Nitrate concentrations in river waters ranged from 78 mmol L21 to 232 mmol L21; d15N varied from 8.2% to 16.2%, and the d18O values ranged from 20.1% to 3.2%. The nitrate concentrations in the German Bight were between 2 mmol L21 and 34 mmol L21, with d15N between 8.0% and 12.2% and d18O between 0.3% and 9.5%. Both riverine and marine end-member concentrations showed seasonal variations, with lower nitrate concentrations and more enriched isotope values during spring and summer compared to winter months. We found no indication in either concentrations or isotopic composition for a significant loss of nitrate within the estuary, but we found a significant increase of nitrate in the maximum turbidity zone in summer. We attribute this to nitrification reflected in a change in the oxygen isotopic composition. The entire riverine nitrate load is entrained into the North Sea by conservative mixing; this conflicts with both the presumed role of estuaries as effective N-sinks and with historical data from the Elbe estuary. Fundamental changes in the biogeochemical processes of the estuary have occurred over the past several decades due to extensive dredging and removal of sediment favorable for denitrification in the Elbe estuary that connects the port of Hamburg with the North Sea. Estuaries have a prominent role in regulating material fluxes from land to sea (Crossland et al. 2005), and the capacity of estuaries for reducing riverine nutrient loads to continental shelf seas has been appreciated as one of the most valuable functions of all global ecosystems (Costanza et al. 1997). According to current understanding of reactive nitrogen transport from land to sea, the estuaries of major rivers are thought to be sites of massive nitrate losses (Brion et al. 2004; Seitzinger et al. 2006), removing up to 50% of reactive nitrogen (OsparCom 2000). In spite of its salient relevance as a natural attenuation mechanism combating eutrophication of coastal seas and the intrinsic economic relevance of this specific ecosystem service, the cycling of nitrogen in contemporary estuaries is still subject to open questions. Most older studies are based on tidal input and output, which are prone to a large degree of uncertainty or are based on mass fluxes alone, which is problematic when sources (e.g., nitrification) and sinks (assimilation and burial, denitrification) may be balanced. A few newer studies suggest that estuarine removal of reactive nitrogen may be significantly overrated, with estimates of removal efficiency ranging from ,5% in the Humber estuary (Jickells et al. 2000) to ,20% in the Rowley estuary (Tobias et al. 2003). More than concentration data alone, measurements of stable isotopes in reactive nitrogen species provide a powerful tool to assess internal turnover and sources in estuaries (Middelburg and Nieuwenhuize 2001; Sebilo et al. 2006). The combined use of d15N-NO 3 and d 18O-NO 3 has recently been acknowledged as a powerful tool for valuation of biological turnover (Wankel et al. 2006). The goal of this study was to account for nitrate loads from the Elbe River and to assess nitrate losses in the estuary of this river to better constrain the effect of riverborne nutrients on eutrophication in the German Bight and the southeastern North Sea. Differing from previous studies in the Elbe estuary, we used a combined approach based on the mixing curves of nitrate concentration versus salinity and determined d15N and d18O of nitrate to shed light on possible transformation processes in the estuarine cycling of reactive nitrogen. Faced with unexpected data, we compared the present situation with archival data sets of nitrate concentrations in the Elbe estuary and found that substantial changes must have occurred in nutrient cycles of the Elbe estuary over the past decades. Materials and Methods Study site—The Elbe estuary (Fig. 1) is a turbid estuary with suspended matter concentration ranging from 1 Corresponding author ([email protected]). Acknowledgments We thank the participants of cruises with RV Uthörn for their help with sample collection on the vessel. We also thank M. Bergemann and G. Burghardt from the Abeitsgemeinschaft für die Reinhaltung der Elbe (ARGE) for providing the archive data sets and for many helpful suggestions. We would like to thank Jay Brandes and one anonymous reviewer, whose comments helped to improve the manuscript significantly. Limnol. Oceanogr., 53(4), 2008, 1504–1511 E 2008, by the American Society of Limnology and Oceanography, Inc.