Sources, turnover and bioavailability of dissolved organic nitrogen (DON) in the Colne estuary, UK

The significance of dissolved organic nitrogen (DON) as a eutrophication pressure in the Colne estuary, eastern England, was studied. DON comprised a significant proportion (19%) of the annual total dissolved N load to the Colne estuary. DON concentrations in the upper Colne estuary correlated seasonally with river flow; in contrast, DON inputs from the Colchester sewage treatment works (STW) were constant seasonally, and during summer exceeded the input of DON from the river. Bioassays showed that only 2.2 to 5.2% of DON were bioavailable at any given time, and bioavailability was significantly lower in STW DON than in fluvial DON. Degradation of DON within the estuarine water column followed first-order kinetics and was very slow (mean 0.4% of DON removed d–1) although removal by the bottom sediments was faster (mean 3% DON d–1). The low bioavailability and slow breakdown rate of DON, compared to the typical freshwater flushing time of water in the estuary, meant that the DON load, though considerable, did not represent a significant eutrophication pressure within the estuary. Most DON was degraded outside the estuary in the coastal zone.