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Affluents of Orinoco River flowing from the precambrian Guayana shield (southern basin) are among the most important Venezuelan rivers, considering their length and water volume. Two of them, Caroní and Paragua, were dammed in 1968 to form the largest freshwater lake in the country (Guri Reservoir)(Fig. 1). lt provides a substantial part of the Venezuelan requirement for electrical power (10,000 MW). The filling ofthe reservoir to the 270 masl was completed in 1986. The high content in humic substances and the low electrolyte concentration of inflowing water confer unique properties to Guri Reservoir. The effect of impoundment may influence the chemistry of the water and favours the development of a truly planktonic community. Productivity of the system is in consequence af-fected. There are few studies on primary production of southamerican blackwaters (HAMMER 1965, ScHMIDT 1973, LEWIS & …