A simplified algorithm for calculating benthic nutrient fluxes in river systems

Abstract

Benthic organic matter microbial oxidation is the basic process by which oxidants such as oxygen, nitrate and sulphate are consumed in sediments, while ammonium and phosphate are released. Although these processes play a crucial role in river biogeochemistry, their modelling remains a difficult challenge. Thouvenot et al. [J. Hydrol., 341, 55–78, 2007; 379, 239–250, 2009] have proposed a model of the processes involved in organic matter degradation in a vertical gradient of oxic to anoxic conditions, considering one upper fluid, erodable sediment layer, with transient behaviour, overlaying a compacted sediment layer assumed to be at steady state. In this paper, we present a thorough analysis of the response of Thouvenot's model to the various constraints affecting benthic processes, according to a conceptual interpretative framework relating the nutrient fluxes across the sediment–water interface to the depth-integrated value of certain sediment properties, such as their oxidant demand and their total ammonification rate. Based on this analysis, we propose a new alternative algorithm simplifying the calculation of the benthic fluxes. This algorithm is designed to be suitable for inclusion in any model of in-stream biogeochemical processes.