Modelling Nitrogen Cycling in a Mariculture Ecosystem as a Tool to Evaluate its Outflow

A model was constructed to describe an intensive mariculture ecosystem growing sea bass (Dicentrarchus labrax), located in the salt marshes of the Fiers dʹArs Bay on the French Atlantic coast, in order to assess nitrogen cycling within the system and nitrogen outflow from the system. The land-based system was separated into three main compartments: a seawater reservoir, fish ponds and a lagoon (sedimentation pond). Three submodels were built for simulation purposes: (1) a hydrological submodel which simulated water exchange; (2) a fish growth and excretion bioenergetic submodel; and (3) a nitrogen compound transformation and loss submodel (i.e. ammonification, nitrification and assimilation processes). A twoyear sampling period of nitrogen water quality concentrations and fish growth was used to validate the model. The model fitted the observations of dissolved nitrogen components, fish growth and water fluxes on a daily basis in all the compartments. The dissolved inorganic nitrogen ranged widely and over time from 0·5 to 9 g N m-3within the system, depending on seawater supply and water temperature, without affecting fish growth. Fish feed was the most important input of nitrogen into the system. The mean average input of nitrogen in the feed was 205 kg N day-1, of which 19% was retained by fish, 4% accumulated in the sediment and 61% flowed from the system as dissolved components. The farm represented about 25% of the total dissolved nitrogen export from the bay, although the farm surface area was 100 times smaller than that of the bay.