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 Δ 15 N and Δ 18 O 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 µmol L ‐1 to 232 µmol L ‐1 ; Δ 15 N varied from 8.2‰ to 16.2‰, and the Δ 18 O values ranged from 20.1‰ to 3.2‰. The nitrate concentrations in the German Bight were between 2 µmol L ‐1 and 34 µmol L ‐1 , with Δ 15 N between 8.0‰ and 12.2‰ and Δ 18 O 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.