Impact of Nile Tilapia Cage Culture on the phytoplankton community in a Tropical Reservoir

摘要

Tropical reservoir ecosystems are important for multiple purposes, especially for public potable water supply, agriculture and fish culture. This study investigated the physical-chemical characteristics and the spatial and temporal distribution of phytoplankton community to check alterations in water quality due to impacts caused by cage culture of the exotic fish Nile tilapia, Oreochromis niloticus in a reservoir of Northeastern Brazil. The physical-chemical parameters (pH, temperature, electrical conductivity, concentration of dissolved oxygen and light transparency), inorganic nutrients (nitrate, ammonium ions, soluble reactive ortophosphate and total nitrogen and phosphorus) and chlorophyll a biomass were measured in two sites of cage culture (point source and non-point source) during two seasons (dry and rainy). Phytoplankton community showed alternating dominance among the species of Cyanobacteria, Bacillariophyceae and Chlorophyceae between the two sites. There was a considerable variation in the levels of inorganic nutrients, such as, nitrate-nitrogen, ammoniacal nitrogen and orthophosphate during the two seasons. The trophic state index diagnosed mesotrophic category, based on the values of chlorophyll a, total phosphorus and light transparency. The wind driven turbulence of the water column and the inflow of water (flushing and dilution) during the rainy season acted as constraint which did not permit the luxuriant growth of cyanobacterial species. Cage culture of tilapia did not causepollution problems which could interfere with the reservoir water quality and the cyanobacterial bloom formation. The muted impact was due to the small scale sustained aquaculture in the well managed reservoir.