Dissimilatory nitrate reduction in anaerobic sediments leading to river nitrite accumulation

Recent studies on Northern Ireland rivers have shown that summer nitrite (NO2-) concentrations greatly exceed the European Union guideline of 3 mu g of N liter(-1) for rivers supporting salmonid fisheries. In fast-bowing aerobic small streams, NO2- is thought to originate from nitrification, due to the retardation of Nitrobacter strains by the presence of free ammonia. Multiple regression analyses of NO2- concentrations against water quality variables of the six major rivers of the Lough Neagh catchment in Northern Ireland, however, suggested that the high NO2- concentrations found in the summer under warm, slow-how conditions may result from the reduction of NO3-. This hypothesis was supported by field observations of weekly changes in N species. Here, reduction of NO3- was observed to occur simultaneously with elevation of NO2- levels and subsequently NH4+ levels, indicating that dissimilatory NO3- reduction to NH4+ (DNRA) performed by fermentative bacteria (e.g., Aeromonas and Vibrio spp.) is responsible for NO2- accumulation in these large rivers. Mechanistic studies in which N-15-labelled NO3- in sediment extracts was used provided further support for this hypothesis. Maximal concentrations of NO2- accumulation (up to 1.4 mg of N liter(-1)) were found in sediments deeper than 6 cm associated with a high concentration of metabolizable carbon and anaerobic conditions. The N-15 enrichment of the NO2- was comparable to that of the NO3- pool, indicating that the NO2- was predominantly NO3- derived. There is evidence which suggests that the high NO2- concentrations observed arose from the inhibition of the DNRA NO2- reductase system by NO3-.