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

The sediment-water interface in aquaculture ponds is both a sink and a sour ce of various substances that are potentially toxic for cultured species. T he 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 r earing period. Fluxes of particulate organic nitrogen (PON) and particulate phosphorus (PP) were modeled and validated using a set of observations. Di ffusive fluxes were modeled using an empirical function of temperature base d on in situ sediment porewarer concentration profiles. An average of 15% o f PON and 10% of PP, produced by fish food waste and fish faeces were sedim ented in fishponds. Ammonia and phosphate diffusive fluxes (mu mol m(-2) h( -1)) were expressed as a function of temperature (T). J(modNH4)(-) = (0.144T + 3.49) X 10(-6)exp(0.11T + 16.81) J(modPO4)(3-) = (0.086T + 1.8) X 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 pro cesses were approximately balancedover the 2-year period. Ammonia and phosp hate diffusive fluxes accounted for only 1-4% and 4-15%, respectively, of t he total dissolved nitrogen and phosphorus components produced during the r earing period. (C) 2001 Elsevier Science B.V. All rights reserved.