Impact evaluation of a pisciculture in the Tucuruí reservoir (Pará, Brazil) using a two-dimensional water quality model

The Tucuruí reservoir, Pará State, Brazil, has one of the largest flooded areas in Brazil and its viability for aquaculture is being evaluated by local government in pilot projects. In this context, the purpose of this study is to assess the impact of a pisciculture in the water quality of the reservoir, by combining field data with model simulations. Hydrodynamics and water quality was modeled using CE-QUAL-W2. The model was validated using about 5 years of field data of temperature, nitrate, ammonia, phosphorus, total suspended solids (TSSs), dissolved oxygen and chlorophyll a. The model was able to reproduce horizontal and vertical gradients and their temporal variability. After validation, the model was used to simulate the response of water quality to various management scenarios of the pisciculture. Results show that solely based on phosphorus index, an increase of 30% occurs from 2007 to 2011, and at least 10% from 2008 to 2011. Based on this, the system tends to change its trophic state from oligotrophic to mesotrophic. However, if we consider chlorophyll a (Chla) as an indicator, no change in the trophic state is observed, since even with a 100% increase of the phosphorus load, the concentration of Chla remains oligotrophic (<4 μg/L) during the simulated period. Thus, the model results suggest that Chla should be used as the key indicator to assess the trophic state of the system, since the nutrients may be in excess in the environment without promoting primary production.