Modeling approach of nitrogen and phosphorus exchanges at the sediment–water interface of an intensive fishpond system

Abstract

The sediment–water interface in aquaculture ponds is both a sink and a source of various substances that are potentially toxic for cultured species. The sediment to water nitrogen and phosphorus exchanges were studied at the sediment–water interface in an intensive earthpond fish-farm on the French Atlantic coast. This was to define the contribution of diffusive fluxes to the total dissolved nitrogen and phosphorus produced during the 1997–1998 rearing period. Fluxes of particulate organic nitrogen (PON) and particulate phosphorus (PP) were modeled and validated using a set of observations. Diffusive fluxes were modeled using an empirical function of temperature based on in situ sediment porewater concentration profiles. An average of 15% of PON and 10% of PP, produced by fish food waste and fish faeces were sedimented in fishponds. Ammonia and phosphate diffusive fluxes (μmol m −2 h −1) were expressed as a function of temperature ( T). J mod NH 4 + =(0.144T+3.49)×10 −6 exp(0.11T+16.81) J mod PO 4 3− =(0.086T+1.8)×10 −6 exp(0.09T+15.76) PON and PP stocks in the sediment decreased during the summer and increased during the winter. However, sedimentation and mineralization–diffusion processes were approximately balanced over the 2-year period. Ammonia and phosphate diffusive fluxes accounted for only 1–4% and 4–15%, respectively, of the total dissolved nitrogen and phosphorus components produced during the rearing period.