MEPS 408:7-18 (2010)  -  DOI: https://doi.org/10.3354/meps08558

ABSTRACT: The German Bight is a hot-spot of eutrophication in the North Sea due to nitrate loads discharged by several large rivers into this semi-isolated embayment. River nitrate loads have a distinctly higher ¹⁵N/¹⁴N ratio than nitrate in open North Sea waters, and to trace the sphere of river influence we analysed stable isotope signatures of water column nitrate in the area on a grid of stations in winter and early spring 2007. Overall spatial patterns of ¹⁵N/¹⁴N and ¹⁸O/¹⁶O in nitrate reflect the predominant influence of nitrate discharged by the Rhine and Elbe rivers on the German Bight nitrate pool. On a smaller scale, however, and in offshore stations, nitrate assimilation of an incipient phytoplankton bloom is indicated by parallel enrichment of ¹⁵N and ¹⁸O in nitrate. Intriguingly, the enrichment ratio of ¹⁸O_NO3 to ¹⁵N_NO3 is 1.6:1, thus differing from the ratio of 1:1 associated with uptake by marine phytoplankton. This suggests that nitrate isotopic composition is not solely affected by phytoplankton assimilation, but that a substantial portion of nitrate in the outer regions of the German Bight is derived from nitrification, despite low ambient temperatures. Moreover, the data identify remineralisation and nitrification of particulate N in sediments as important sources of dissolved inorganic nitrogen to the German Bight water column, and underscore the role of sediments in recharging water column nutrient inventories.

KEY WORDS: Nitrogen cycle · δ¹⁵N · δ¹⁸O · Stable isotopes · German Bight · Nitrification