Nitrification in a highly polluted european estuary (The scheldt estuary) and consequences on changing the natural isotopic ratio of 15N in particulate organic matter

The Scheldt estuary is a highly polluted macrotidal estuary draining one of the most densily populated areas in the world (425 inhab/km²). The present nitrogen load to the estuary is approximately of 50 kT N / year from which about 20 % is under the form of ammonium. This ammonium is almost completely nitrified in the estuary, even in winter conditions, when low temperatures are known to limit the activity of nitrifying bacteria. This is probably linked to the fact that water residence times are very long (75 days on the average) sothat even with reduced nitrification rates, depletion of ammonium is still possible. The oxidation of ammonium to nitrate by nitrifying bacteria results in an enrichment of N-NH4 in its heavy isotope 15N. Indeed, the heavy isotope 15N is discriminated against the light 14N isotope during the oxidation process. Measurements of the delta 15N in particulate organic matter and copepods shows that the heavy isotopic 15N signature of ammonium is most probably incorporated in the entire microbial food web. This indicates that, at least for some periods of the year, the microbial food web of the estuary is based on microorganisms (phyto- and bacterio-plankton) finding their N requirements by assimilating NH4+ rather than any other inorganic or organic N source.