Short term variations in phytoplankton in Turpialito Bay were studied. Wind velocity, salinity, temperature,
nutrient concentration, and phytoplankton biomass were determined at 0 and 6 m every six hours from the 07/24th to the 07/
31st/2010. Wind velocity (0-13 m s-1), salinity (30-35.6) and temperature (22.8-29ºC) only showed significant differences
with respect to sampling time. Nutrient concentrations varied daily (averages of 0.89, 0.03, 0.84, 0.06, and 3.66 ìmol L-1 for
ammonia, nitrite, nitrate, phosphate and silicate, respectively), which indicated that their source is not constant. Concentration
of chlorophyll a (chlor a; 0.11 – 0.96 mg m-3), and microalgal abundance (0,26 – 3,28 X 103 cel. ml-1) varied significantly with
respect sampling day. This result indicated that the daily fluctuations in environmental variables did affect the phytoplankton.
A total of 80 species were identified. Dinoflagellates (46 taxa) were the most diverse group. The community was dominated
by: Ceratium kofoidii (Jorgesen), Prorocentrum micans (Ehrenberg), Scripsiella sp. (dinoflagellates), Navicula sp.1 (diatom),
Oscillatoria sp. (cyanobacteria). On the other hand, the PCA showed a positive correlation among the concentrations of
ammonia, silicate and chlor a. These variables exhibited a negative correlation with phosphate concentration. This fact
suggests a constant supply of ammonia and silicate and a puntual or irregular supply of phosphate. In the other component,
wind velocity, temperature, nitrite concentration and diversity correlated positively among themselves. An increase of wind
velocity in shallow waters would result in a thermally uniform water column and a higher diversity. The results suggest that
in Turpialito Bay during the rainy season, the phytoplankton community changes in periods shorter than one week.