To study the effects of nutrient enrichment (N and P) and zooplankton grazing over the phytoplankton community structure in El Andino Reservoir (Venezuela), in situ mesocosms were isolated for 6-7 days. Mesocosms consisted in polyethylene bags (42 centimeters diameter and 71 cm length) filledwith 10 liters of filtered epilimnetic reservoir water. Experiments were carried with a frequency of one experiment per month (January to December, 1993). The experimental design was as follows: (1) no nutrients, no zooplankton; (2) with nutrients (150 NH4Cl micromol/ml and 10 KH2PO4 micromol/ml, 1ml per liter of sample), no zooplankton; (3) no nutrients, with zooplankton present in the water column (hauled from 6 to 0 meters with a zooplankton net, mesh size 80 microm); (4) with nutrients as in (2), with zooplankton as in (3).

The nutrient enrichment caused an increase in phytoplankton biomass (measured as chlorophyll-A) in the mesocosms, also the increase of all algal groups abundance, except Cryptophyta and Pyrrophyta. In spite of this, relative proportions of Cyanobacteria decreased in most cases, while Chlorophyta and Bacillariophyta increased, probably due to their greater competitive ability for phosphorus. After enrichment, SCENEDESMUS was the dominant species from January to June, while from July to December DACTYLOCOCCOPSIS and LYNGBYA dominated in the enriched mesocosms.

Herbivorous zooplankton mainly grazed on diatoms, although particulate matter was present in almost all the gut contents analysed (cleared with Hoyerıs medium). Particulate matter probably consisted of micro-algae, detritus, triturated algae, and inorganic material. Zooplankton excretion caused a slight increase in phytoplankton biomass and P concentrations.

The results suggest that, at the initial stages of a eutrophication process, phytoplankton would increase its abundance and biomass, but would not change its community structure. Since there was a strong correlation between total phosphorus and chlorophyll-A (bottom-up control), it is suggested that eutrophication could be avoided by controlling P input to the reservoir.