The pathways influence of agricultural expansion on water quality of fish farming in Ilha Solteira reservoir, São Paulo, Brazil

Abstract

It is widely accepted that fish farms (FF) have important contribution to input of nutrients into water, however fundamental knowledge regarding the influence of land use and land cover (LULC) on the water quality of fish farms is still lacking. The aim of this study was to assess the linkages between the LULC types and the water quality parameters in FFs. Three specific objectives were set: (1) to characterize the current water quality standards in four FFs (FF1, FF2, FF3, FF4) installed in the Ilha Solteira reservoir; (2) to delineate and analyze the LULC changes in sub-basins of tributaries where FFs are installed; (3) to elucidate the interactions between LULC and water quality in FFs. Results indicated signs of eutrophication in FF2 in both the dry and wet seasons. The mainly LULC was decrease of pasture and increase of sugarcane crops were observed in the four FFs' sub-basins between 2005 and 2018. Sugarcane land uses showed a significant positive correlation with SO4, Mg and K (r > 0.95, p < 0.005). The most likely source of these highlighted parameters may come from mineral fertilization, decomposition of the plinth in sugarcane and the application of vinasse to irrigate and fertilize crops. Concentrations of SO4, Mg and K observed in this present study do not cause direct toxic effects to Nile tilapia in fish farms. However, indirect impacts to water quality should be expected due to enhance primary production and promotes water eutrophication, as observed in FF2. Based on our results, we suggest that the criteria “basin characteristics”, including future sugarcane expansions, should be adopted to gather information in identifying areas more suitable for aquaculture parks that the Brazilian government intends to install in the large reservoirs.