Nutrient behavior and productivity in a river system affected by fishpond effluents

The expanding culture of fish in coastal areas has raised issues concerning resource use conflicts and water quality problems. A major concern is poll ution resulting from nitrogen and phosphorus lost to the environment from f eed wastage and fish excretion. The purpose of the study was to assess the environmental conditions in a river system affected by fishponds, Nutrient fluxes and productivity measurements were obtained from Sta, Rita River, a river estuary used as a discharge and an irrigation area of approximately 3 16 ha of fishponds, During the dry season, when freshwater influence is abs ent, salinity values inside the river were higher than outside (in the bay) . Nutrient trends during ebb and flood periods were similar except for some lag in the profiles that indicate tidal oscillation, The absence of river flow during the dry period resulted in a negligible flux of nutrients along the river. During the met season, the increase in nitrate concentrations i ndicates river inputs while the decrease in phosphate values imply less of freshwater influence but more from ponds and domestic wastes. Similarity in productivity estimates signifies good agreement between direct and indirec t methods. The absence of flushing during the dry season translates to trap ping of nutrients discharged from the ponds in the drainage canals. (C) 199 9 Elsevier Science Ltd. All rights reserved.