Nitrogen is a key element in aquatic environments and an important pond management variable. In aquaculture systems, nitrogen accumulation eventually leads to a deterioration of the system. The interactions between various N-species are complex and difficult to integrate. Modelling can improve our ability to evaluate this complex system. This paper integrates existing knowledge about nitrogen transformations in fish ponds into a model that calculates the amount of various N-compounds in the water column and in the sediment. The model is also used to gain insight into the relative importance of transformation processes between the various N-compounds. The model was divided into three modules: fish, phytoplankton and sediment-water. The fish module is based on physiological and bio-energetic principles. The phytoplankton dynamics module is based on physico-chemical principles of alga growth. The water-sediment module is based on the bacterial transformations and chemical fluxes of N-species across the water-sediment interface. Relationships and parameters were taken from the literature, except for a few parameters that were estimated by fitting model predictions to observed data. The model was implemented in Turbo Pascal (7.0) using a fixed time step of 1 h and it was calibrated using a set of data from an earthen fish pond stocked with Colossoma macropomum. The validation was performed using data from earthen ponds stocked with Oreochromis niloticus. The difference between the calibrated and validated model was related to the fish species. All concentrations of the various N-species present were simulated well, except the N retained in organic matter in the sediment (average relative error −0.34). Sensitivity analysis revealed that the concentrations of inorganic-N compounds, both in the water column and in the sediment, are more affected by changes in specific parameters included in the fish and phytoplankton modules than other forms of nitrogen in the pond. The model works well, except for organic matter accumulation in the sediment. Further research should concentrate on a better understanding of the bottom organic matter dynamics, to make the model a more comprehensive predictive tool.

This study quanti¢ed the accumulation of nitrogen (N) in the water column, sediments, ¢sh and seepage water during a production cycle of Colossoma macropomum. By combining estimates of the deposition rates of uneaten feed, faeces and dead phytoplankton with measurements of N accumulation in the sediment, the rate of decomposition of organic matter in the sediment was estimated. The ¢rst-order rate constant for organic matter decomposition was 0.237 AE 0.019 day À1 . Total N recovery during the ¢rst weeks of the experiment was about 65%. Later, the N recovery was close to 100%. The cumulative recovery at the end of the experiment was almost 100%, meaning that the N budget in the system studied can be fully explained without consideration of N volatilization, due to either denitri¢cation or ammonia volatilization. In the beginning of the growth cycle, the major £ux of N was sedimentation. Intensive microbial degradation process occurred about 3^4 weeks later, leading to a release of inorganic N and an approach towards a steady state as to the accumulation of organic N. Feed was irregularly applied during the experiment but ¢sh growth was constant, showing that the ¢sh utilized detrital or planktonic feed during periods of low feeding. Nitrogen accumulated in the pond during periods of excessive feeding and was utilized by the ¢sh during periods of low feeding. This cycling should be further studied and may be an important pond management technique.

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In aquaculture, ponds with high loads of organic inputs, organic matter accumulates at the bottom over time. Uneaten feed, senescent phytoplankton and faeces are the principal sources of accumulated material, but quanti¢cations are scarce. The sedimented organic matter develops into a £occulent layer in which di¡erent processes transform the material into inorganic forms. A better understanding of factors in£uencing organic matter accumulation/decomposition in the sediment is needed to better understand and manage the dynamics of nitrogen in ¢sh ponds. In this study, the rate of mineralization of organic nitrogen and the nitrogen £ux between the sediment and the water column were measured. Organic matter accumulation in ¢sh ponds was quanti¢ed, and the data were used to construct, calibrate and validate a dynamic simulation model of organic matter deposition/decomposition in ¢sh ponds. The accumulating material consisted of dead phytoplankton, ¢sh faeces and uneaten feed. Through model calibration, the proportion of these materials in the total accumulated organic matter was determined. In the model, gross photosynthetic rate was estimated from an empirical relationship with feed input. After calibration, the model was validated using independent data. The model simulated well the concentrations of organic carbon and nitrogen in the sediments but it may be developed further, especially by considering the e¡ects of resuspension.

La sobreexplotación pesquera y la búsqueda de alternativas ante la pobreza crónica han llevado a explorar el campo de la maricultura por medio de un enfoque ecosistémico se abrieron dos procesos paralelos: una estrategia e; investigación en laboratorio para el levantamiento larval de peces marinos tropicales y una estrategia de comanejo para desarrollar el cultivo de peces marinos tropicales. El pargo la mancha muestra gran potencial de producción de larvas y posterior cultivo en el mar. No mostró respuestas positivas a la inducción por medio de hormonas, el desarrollo de un ambiente adecuado para su reproducción espontánea, permitió tener desoves frecuentes y de calidad. La fase de levantamiento larval mostró contratiempos por el tamaño reducido de las larvas de pargo la mancha el manejo en tanques apropiados y el uso de enriquecedores permitieran mejorar los procesos y llegar a incrementar la producción de alevines. El enfoque ecosistémico permitió abrir los espacios necesarios que permitieron la operación de un proyecto piloto. La granja se creó con un enfoque productivo, pero también para turismo rural.Overfishing and the search for alternatives to it, led us to the explore mariculture as an option using an ecosystem-based approach through two parallel processes: an in-laboratory research strategy for larval rearing of tropical marine fish, and a strategy of co-management to develop tropical marine fish aquaculture. The spotted rose snapper shows great potential for larvae production and cage culture. This species showed no responses to hormone induction; however, the set up of a suitable environment allowed frequent, high-quality spontaneous spawnings. The larval rearing phase presented problems due to the small size of the larvae; although, management in appropriate tanks and the use of enrichments improved the processes, increasing the production of juveniles. This ecosystem approach allowed the operation of a profitable farm pilot project with a productive approach, but with rural tourism in mind as well.

RESUMEN.Se evaluó el efecto del flujo de agua sobre la eficiencia de tapetes microbianos para remover nitrógeno amoniacal total (NAT, N-NH4 + + N-NH3), nitritos (N-NO2 . En este trabajo se demostró que el flujo de agua posee un efecto importante en la capacidad de los tapetes microbianos para la biorremediación de amonio y nitritos en sistemas recirculados. Se recomienda continuar con la identificación de otras variables que podrían afectar el óptimo funcionamiento de los tapetes microbianos. Palabras clave: Litopenaeus vannamei, tapetes microbianos, biorremediación, Sistemas de Recirculación en Acuicultura (RAS). . This study showed that the water flow rate has a significant effect on the microbial mats capacity for the bioremediation of ammonium and nitrite in recirculating systems. It is recommended to continue with the identification of other variables that could affect the optimal functioning of microbial mats in RAS.