Stable isotope composition and turnover of nitrate in the German Bight

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 higher15N/14N ratio than nitrate in open North Sea waters, and to trace the sphere of river influence we...

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Bibliographic Details
Published in:Marine ecology. Progress series (Halstenbek) 2010-06, Vol.408, p.7-18
Main Authors: Dähnke, Kirstin, Emeis, Kay, Johannsen, Astrid, Nagel, Birgit
Format: Article
Language:English
Subjects:
Bays
Freshwater
Groundwater
Isotopes
Marine
Nitrates
Nitrification
Nitrogen
Salinity
Sea water
Seas
Sediments
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Summary: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 higher15N/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 of15N/14N and18O/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 of15N and18O in nitrate. Intriguingly, the enrichment ratio of18ONO3to15NNO3is 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.
ISSN:0171-8630
1616-1599
DOI:10.3354/meps08558