Modelling of biogeochemical processes in fish earth ponds: Model development and calibration

The biogeochemistry of fish earth ponds is a complex subject due to the interactions between several water column and sediment compounds, particularly nutrient species. Models can improve our ability to understand such complexity. This study combines existing knowledge on biogeochemical processes in earth ponds into a model that calculates the concentration of the compounds that are more likely to negatively affect fish production and cause undesirable environmental impacts, such as nitrogen, phosphorus and oxygen. Aside from inorganic nutrient forms, organic compounds were included in the model due to their relevance for the nutrient cycles in aquatic systems. The model couples the pelagic and benthic compartments, due to the importance of sediment–water interactions in shallow systems. In this first approach in modelling the fishpond environment, the feedbacks between cultivated species and the environment were not accounted for the model, to reduce its complexity and easily identify the interactions between water column and sediment variables and processes. The model was calibrated for an earth pond without fish, using data sets collected during a 2-year trial. The variability of water column compounds was generally well predicted (p < 0.01), however the model could not fully reproduce ammonium and dissolved organic phosphorus concentrations. In sediments, organic phosphorus was accurately simulated (p < 0.05) while nitrogen and carbon pools were occasionally over or underestimated. Model limitations regarding sediment variables are most likely related to the effects of benthic primary producers and macrofauna activity in earth ponds biogeochemistry. Upcoming applications of the model developed herein include its coupling to a fish Dynamic Energy Budget model to be used as a predictive tool for fishpond management. Nevertheless, the model may also be applied to other aquatic systems such as coastal systems or wastewater treatment ponds.