Hydrobiologia 434: 81–96, 2000. © 2000 Kluwer Academic Publishers. Printed in the Netherlands. 81 Nutrient enrichment and zooplankton effects on the phytoplankton community in microcosms from El Andino reservoir (Venezuela) Ernesto J. Gonz´ alez Universidad Central de Venezuela, Instituto de Biolog ´ ia Experimental, Apartado 471069, Los Chaguaramos, Caracas 1041 - Venezuela. Fax: 58-2-7535897. E-mail: ergonza@reacciun.ve Received 3 September 1999; in revised form 25 April 2000; accepted 25 May 2000 Key words: microcosm, nutrient enrichment, zooplankton grazing, biomanipulation, El Andino reservoir, Venezuela Abstract To quantify the effects of nutrient enrichment (N and P) and zooplankton grazing on the phytoplankton community structure of El Andino reservoir (Venezuela), in situ microcosms were installed for 6–7 days. Microcosms consisted of polyethylene bags (42 cm × 71 cm, non-cylindrical shaped) filled with 10 l of filtered epilimnetic water. Ex- periments were carried out on a monthly basis from January to December 1993. The lack/addition of nutrients was cross-classified with the absence/presence of zooplankton, resulting in an experimental design of four treatment levels: (1) no nutrient addition, zooplankton absent (C); (2) nutrient addition (150 NH 4 Cl μmol ml −1 and 10 KH 2 PO 4 μmol ml −1 ; 1 ml per l of sample), zooplankton absent (N); (3) no nutrient addition, zooplankton present (collected from the reservoir water column using a 6-m vertical tow with a 80-μm plankton net) (Z); and (4) nutrient addition (as in [2]), zooplankton present (as in [3]) (NZ). Treatments were triplicated, and samples were collected at the start and end of each experiment. Significant differences between treatments were determined using a two-way ANOVA at p<0.05. Nutrient enrichment caused an increase in phytoplankton biomass, with the increase of all algal groups, except Pyrrhophyta. In spite of this, relative proportions of Cyanobacteria decreased in most cases. Chlorophyta and Bacillariophyta increased, probably due to their greater competitive abilities 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 microcosms. Zooplankton affected the phytoplankton community in microcoms through grazing and nutrient (mainly P) regeneration. Cladocerans (Ceri- odaphnia cornuta, Moina micrura and Diaphanosoma sp.) mainly grazed on diatoms, although particulate material was present in almost all the gut contents analyzed. Particulate material probably consisted of micro-algae, detritus, bacteria, triturated algae and mineral particles. Ostracoda mainly fed on Peridinium and particulate material, whereas Thermocyclops sp. and rotifers (Brachionus spp. and Keratella spp.) mainly ingested particulate material. On the other hand, zooplankton excretion caused a slight increase in phytoplankton biomass and P concentrations in microcosms with the animals present. The effects of nutrient and zooplankton did not interact in most cases. Experimental results suggest that, at the initial stages of a eutrophication process, phytoplankton could increase their abundance and biomass, but might not change its community structure. Since there was a strong correlation between phosphorus and chlorophyll-a (bottom-up control), it is suggested that eutrophication could be avoided by controlling P input to the reservoir. Introduction Many experiments have been carried out to assess ef- fects of nutrient enrichment and zooplankton grazing on phytoplankton communities (González & Ortaz, 1998). Many of them use phytoplankton populations isolated in microcosms (e.g. Henry & Tundisi, 1982; Henry et al., 1985; Bergquist & Carpenter, 1986; Tun- disi & Henry, 1986; Elser & Goldman, 1991; Elser, 1992; Oliveira, 1992; Queimaliños & Mondenutti,