DISSIMILATORY NITRATE REDUCTION IN ANAEROBIC SEDIMENTS LEADING TO RIVER NITRITE ACCUMULATION

Recent studies on Northern Ireland rivers have shown that summer nitri te (NO2-) concentrations greatly exceed the European Union guideline o f 3 mu g of N liter(-1) for rivers supporting salmonid fisheries. In f ast-bowing aerobic small streams, NO2- is thought to originate from ni trification, due to the retardation of Nitrobacter strains by the pres ence of free ammonia. Multiple regression analyses of NO2- concentrati ons against water quality variables of the six major rivers of the Lou gh Neagh catchment in Northern Ireland, however, suggested that the hi gh NO2- concentrations found in the summer under warm, slow-how condit ions may result from the reduction of NO3-. This hypothesis was suppor ted by field observations of weekly changes in N species. Here, reduct ion of NO3- was observed to occur simultaneously with elevation of NO2 - levels and subsequently NH4+ levels, indicating that dissimilatory N O3- reduction to NH4+ (DNRA) performed by fermentative bacteria (e.g., Aeromonas and Vibrio spp.) is responsible for NO2- accumulation in th ese large rivers. Mechanistic studies in which N-15-labelled NO3- in s ediment 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, ind icating that the NO2- was predominantly NO3- derived. There is evidenc e which suggests that the high NO2- concentrations observed arose from the inhibition of the DNRA NO2- reductase system by NO3-.