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

Abstract

Venezuela has more than 110 operating reservoirs. However, limnological information is only available for about 20%, despite the fact that several of them are subject to negative impacts (eutrophication) caused by anthropogenic activities in their drainage basins. We analyzed the relationships between nutrients 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, with the most enriched located in unprotected drainage basins. The systems could be separated according to low (<20μg/l) and high (>20μg/l) total phosphorus concentrations. Furthermore, in reservoirs with low NO3:NH4 ratios, Cyanobacteria were dominant, whereas other phytoplankton groups were dominant in high NO3:NH4 ratios. Our results showed a significant linear relationship between chlorophyll a concentrations and nutrients, phosphorus and nitrogen. This is because both nutrients can be limiting for phytoplankton growth, at least in some systems. Following these results, we suggest 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.