EUTROPHICATION OF RESERVOIRS IN VENEZUELA: RELATIONSHIPS BETWEEN NITROGEN, PHOSPHORUS AND PHYTOPLANKTON BIOMASS

Resumo

Venezuela has more than 110 operating reservoirs, however limnological information is only available for about 20%, in spite of the fact that several of them suffer from negative impacts (eutrophication) caused by anthropogenic activities that occur within their drainage basins. We analyzed the relationships between the principal physicochemical variables and phytoplankton biomass (as chlorophyll a). A total of 14 reservoirs from the north central and northeastern regions of Venezuela were assessed. The reservoirs showed different degrees of eutrophication, as determined by their nutrient concentrations, with the most eutrophicated located in areas with unprotected drainage basins. The results of the analyses showed a good linear association between the concentrations of chlorophyll a and nutrients, especially phosphorus. The reservoirs could be separated into two groups with respect to their concentrations of total phosphorus: those with low (<20µg/l) and high (>20µg/l) concentrations. Similarly, the NO₃:NH₄ ratio permitted us to differentiate between reservoirs dominated by cyanobacteria and those dominated by other phytoplankton groups. We conclude that the control or mitigation of eutrophication in Venezuelan reservoirs should be based on an improved management of the drainage basins rather than simply that of the reservoirs themselves.