AbstractThe Caroni River, a tributary of the Orinoco River, in south‐eastern Venezuela, and originating in the Guayana Shield, has been impounded by the construction of two dams. The largest, Guri, is 90km upriver from the confluence with the Orinoco. The final stage of the dam and hydroelectric power plant of the Guri complex, was completed in 1986. The Guri Reservoir (62° 32′–63° 18′ long. W and 6° 55′–7° 52′ lat. N) is a blackwater lake, due to the high concentration of humic substances in the water. The surface of the reservoir is 4250km2at normal maximum operating water level of 270 m a.s.l., has a maximum depth of 150 m and a storage capacity of 135.7 km3. The waters of the Caroni, flowing through an old geological basement (Pre‐Cambrian), highly weathered and eroded, are poor in mineral nutrients, making Guri's waters oligotrophic, that is biologically manifested in low organic production (Lewis and Weibezahn, 1976; Gonzalezet al., 1991). Its hydrological regime is closely related to the regional dry‐wet climatic pattern. Chemically, the waters are characterized by low pH and alkalinity, relatively high content of Si, Al und Fe, low anion concentrations with sodium dominating the series, low content of nitrogen, and very low concentrations of phosphorus. Phytoplankton is represented predominantly by Chlorophyta and Chrysophyta, with low biomass and depressed production (Gonzalezet al., 1991). Zooplankton (only Copepoda were studied) is represented by 8 spp. and densities are low (Gonzalez, 1992). Suspended particulates concentrations present descending gradient from the tail of the reservoir towards the dam, but in absolute ter