EFFECTS OF N AND P ENRICHMENT ON THE PHYTOPLANKTON COMMUNITY UNDER MICROCOSMS CONDITIONS IN A TROPICAL EMBAYMENT (LA MARIPOSA, VENEZUELA)

The effect of nutrient enrichment (N and P) on the phytoplankton commu nity of La Mariposa reservoir (Venezuela) was studied. Polyethylene ba gs (microcosms) were filled with 101 of filtered epilimnetic water of the reservoir, and enriched with different nutrient proportions, simul ating an eutrophication process. Chlorophyll 'a', total nitrogen, tota l phosphorus, and phytoplankton abundance were determined. There was a n increase in phytoplankton abundance because of nutrient enrichment, specially when both nutrients were present. After the experiments, Cry ptophyta, Euglenophyta, and Pyrrophyta dissappeared from the microcosm s, whereas Chlorophyta and Cyanobacteria increased. Numeric proportion s of diatoms decreased at higher nitrogen concentration. Although the results obtained with microcosm experiments could be limited in ecolog ical significance, they permit an useful approach to the natural situa tion.