A diagenetic model discriminating denitrification and dissimilatory nitrate reduction to ammonium in a temperate estuarine sediment

Abstract

A diagenetic model is presented which considers nitrate reduction by both denitrification and Dissimilatory Nitrate Reduction to Ammonium (DNRA). This work builds on an existing model (Kelly-Gerreyn et al. 1999; Mar Ecol Prog Ser 177:37-50). Previous models have assumed nitrate reduction to be solely due to denitrification. This paper questions the reliability of this assumption in coastal areas and suggests that DNRA can account for a high proportion of nitrate reduction. Data from a North Sea estuary (the lower Gt. Ouse, Norfolk, UK) containing high nutrient concentrations (mean 406 μM NO3-) are used to derive a relationship between temperature and the proportioning of nitrate reduction driven by nitrate from the overlying water into denitrification and DNRA. The relationship is assumed to apply to total nitrate reduction. The result is a function which shows that DNRA and denitrification occur at all temperatures but that DNRA is the favoured pathway at the extremes of the observed temperatures (<14 and >17°C) while denitrification is favoured only in a narrow range of temperatures (14 to 17°C). The mechanism is probably an adaptive response of different nitrate-reducing bacteria to temperature. This temperature relationship is implemented in the model and used to successfully simulate both observed rates of uncoupled denitrification (4 to 228 μmolN m-2 h-1), denitrification fuelled by nitrate in the overlying water (Dw), and calculated rates of DNRA fuelled by nitrate in the overlying water (DNRAw) (measured nitrate flux - measured Dw rate). In contrast, standard diagenetic formulae for nitrate reduction (i.e. by denitrification only) cannot satisfactorily reproduce the Dw rates observed in these sediments. It is concluded that temperature is an important controlling factor for partitioning nitrate reduction into DNRA and denitrification in the lower Gt. Ouse sediments. This temperature effect implies that during an extended warm summer in temperate estuaries receiving high nitrate inputs, nitrate reduction may contribute to, rather than counteract, a eutrophication event. Diagenetic models of the nitrogen cycle in coastal areas should include DNRA.