Development of biomanipulation strategies for the remediation of eutrophication problems in an urban reservoir, Lago Paranoa, Brazil
The feasibility of improving water quality by food web manipulation in an urban eutrophic tropical reservoir, Lago Paranoa (Brasilia, Brazil) was investigated. The potential of promising biomanipulation strategies was experimentally tested in limnocorrals: reduction of internal nutrient loading by controlling tilapia overpopulation, and suppression of nuisance cyanobacteria by stocking non reproducing filter-feeding silver carp. A four-month test in two large littoral isolated areas of the reservoir (1,000 m2 each) stocked with high vs. low tilapia biomass revealed that fish over-population promoted blooms of Microcystis and decreased water clarity by supplying phytoplankton with additional nutrients (mainly phosphorus, "P"). Since reduction of tilapia biomass from 150 to 40 g/m3 resulted in significant water quality improvements, the release of commercial fisheries using cast-nets was recommended. Field experiments III floating net-cages (10 m3) were conducted to evaluate silver carp adaptation to Lago Paranoa and to determine fish growth rates when maintained feeding exclusively on the abundant plankton in the reservoir. High growth rates (up to 6 g/day) and survivorship (> 90%) of young-of-the-year (72 g), juveniles (300 g) and adult silver carp (1100 g) during both dry and rainy season were observed. Efficient ingestion of nuisance algae was evidenced by the dominant presence of Microcystis aeruginosa and Botryococcus braunii colonies in the fore-intestine of experimental fishes. Results indicate the great potential for silver carp cage culture as a low-cost and environmentally beneficial economic activity. Following indications from literature and previous studies in Lago Paranoa, the optimum range of silver carp biomass which maximize phytoplankton control was determined in ten large replicated limnocorrals (50 m3 ). Although a significant decrease in Microcystis abundance was achieved by stocking silver carp at all biomass levels, net-phytoplankton biomass was only significantly suppressed at moderate fish stocking densities of 40 and 60 g/m3 . Two additional limnocorral experiments during dry and rainy seasons were performed to test the effectiveness of both biomanipulation strategies when adopted separately or simultaneously. Water quality improvements through control of tilapia abundance (from 100 to 40 g/m3) and stocking with silver carp (at moderate stocking rates of 40-50 g/m3) induced significant decreases in total phosphorus (21- 31 %), cyanobacteria density (40-44%) and phytoplankton biomass (22-38%). As those strategies were found to act independently, the combination of both enhanced water quality benefits by reducing total phosphorus by 38%, cyanobacteria density by 75% and phytoplankton biomass by 60%. To predict the internal P-loading reduction that could be achieved by implementing both biomanipulation strategies on a whole-reservoir scale, P excretion rates were quantified in indoor tanks for 16 and 40 g tilapia (0.527-1.576 ug SRP/g ww/h) and silver carp (0.391-0.737 ug SRP/g ww/h). It was estimated that the tilapia contribution to internal P-Ioading (5.4 mg TP/m2/day for 1,300 kg/ha) is equivalent to external P input to the Bananal Branch (6.0 mg TP/m2/day). A 60% reduction in tilapia biomass plus stocking of silver carp at densities of up to 60 g/m3 after three years would not change internal P-Ioading (5.6 mg TP/m2/day) but would (1) reduce by 60% the input of "new" phosphorus to the water column from tilapia bottom feeding, and (2) increase grazing on phytoplankton following silver carp introduction.