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The roles of anaerobic and aerobic microbial processes on nitrogen loss from intertidal and estuarine sediments

Final Activity Report Summary - NITROQUEST (The roles of anaerobic and aerobic microbial processes on nitrogen loss from intertidal and estuarine sediments)

A new, naturally occurring process, carried out by bacteria, has been discovered in the Ocean. The bacteria convert ammonium (an inorganic form of nitrogen (N)) to nitrogen gas (N2), which is emitted to the atmosphere. This means that ammonium (NH4), which is used for living organisms in the water, is not available. Although it is likely, we do not know if this process is dominant in continental shelf sediments at intertidal areas. These areas are increasingly subject to high nutrient (nitrogen, phosphorus, etc) loads from anthropogenic (domestic, industrial and agricultural) activity, which can result in enrichment (eutrophication) of the waterbody and, ultimately, ecosystem destruction, including fishkills. Knowledge of the rates of N-cycling in sediments from areas around the points of discharge of anthropogenically-generated N is of immediate value to basic science and to, for example, policy makers.

The primary objective of the research for this Intra-European Fellowship - NitroQuest - was to investigate the role, and rates, of specific bacterial processes in intertidal sediments. Three sites along the Weser river in Northern Germany, as well as a tidal sand flat at Janssand in the German Wadden Sea, and four sites around the island of Helgoland in the North Sea were chosen for investigation. Experiments using sediment samples incubated with various chemical states of the rare '15N' isotope and the 'unlabeled' '14N' isotope of nitrogen were carried out and the amount of N2 gas produced was measured. The results indicate that the sediments - especially sandy sediments, which comprise a large majority of the sediments on the continental slope of northern Europe - are a large sink for land-derived nitrogen along the coastal zone. Denitrification (the transformation of nitrate to N2 gas) and anammox were detected in riverine, intertidal and estuary sediments and our measurements indicated that those processes were confined to different depths in the sediment. The data also suggest that sediment mixing, due to the tidal influence, was strong, and resulted in increased nitrogen loss (as N2 production). In fact, the rates of nitrogen loss in mixed sediments were up to 30 times higher than in unixed samples.

In summary, and in contrast to conventional views, our results indicate that the sediments sampled are the site of some of the highest N-loss rates in the coastal zone of Wadden Sea and North Sea. Thus, such sediments could play a significant role in N-removal from shelf environments, which account for 50-70% of oceanic N-loss.