Three daily samplings of the phytoplankton community were made at two consecutive days in March, April, May, September, October, November and December 1997, at Punta Morales, Golfo de Nicoya, Costa Rica. Samples were collected during each tide at depths of 50 % and 10 % of light penetration using a Niskin bottle. A total of 43 taxa were identified. Centric diatoms, pennates and flagellates represented 90% of total phytoplankton abundance. In the phytoplankton fraction (cells > 30 μm), diatoms were the most abundant group, and Skeletonema costatum (32%) dominated. In nannophytoplankton (cells < 30 μm), Chaetoceros (23.7%) was the most abundant taxon, followed by flagellates (23%) and Cylindrotheca closterium (13.1%). These results agree with previous surveys and suggest that a typical net phytoplankton community persist through time in the Punta Morales zone. The number of nannophytoplankton fraction cells varied seasonally and suggests quantitative changes in species abundance, with possible modifications of cellular size or chain length in filamentous species. The codominance between S. costatum and Chaetoceros spp. during the rainy season suggested the ocurrence of an early ecological sucession, and nutrients could be the factor generating such population changes.
Keywords: Tropical estuaries, Costa Rica, net phytoplankton, nannophytoplankton, sucession