Numerical modeling of estuarine transport, water quality, and the impact of shrimp mariculture in the Rio Chone esturary, Ecuador

The Rio Chone estuary in Ecuador has been heavily impacted by a range of anthropogenic impacts, particularly the conversion of mangrove forests to shrimp mariculture ponds. The combination of seasonal river flow, climate forcing and the influx of highly saline pond effluents have created a slow circulation regime in the estuary, especially during the dry season when the estuary can undergo a salinity inversion from mouth to head. Both shrimp production and water quality in the estuary have declined, specifically in the upper regions of the estuary where low dissolved oxygen is often a problem.; A combined physical and biological modeling study was undertaken to characterize circulation and water quality in the estuary, as well as the production of shrimp in the ponds. A two-dimensional hydrodynamic model, modified to include dissolved inorganic nitrogen (DIN) and biochemical oxygen demand (BOD), was applied to the Rio Chone estuary and simulated under a range of seasonal forcing conditions. A one-dimensional model of biological and chemical interactions within a shrimp pond was developed to estimate the sensitivity of shrimp growth to estuarine water quality, and the sensitivity of the pond effluent water quality to a range in pond management decisions. Inputs from the shrimp ponds for DIN, BOD and salt were included as sources in the hydrodynamic/water quality model to evaluate the seasonal impact of a range in pond effluent concentrations on estuarine water quality.; Results indicate that pond processes are sensitive to estuarine DIN and BOD concentrations, and that effluents from these ponds are impacting both seasonal circulation and water quality in the estuary. Simulations of the conversion of mangroves to ponds suggest that the conversion of more then 40% of the upper estuary mangroves to shrimp ponds leads to excessive DIN concentrations in this area. Overall modeling results suggest that consideration should be given to the hydrodynamic regime of the estuary when selecting locations for shrimp pond siting.