Trophic planktonic dynamics in a tropical estuary, Gulf of Nicoya, Pacific coast of Costa Rica during El Ni？o 1997 event

the trophic dynamics of the plankton from the gulf of nicoya, a tropical estuary in the pacific coast of costa rica, was conducted during dry and rainy seasons of 1997. water temperature, oxygen and salinity were measured at two different light penetration depths. inorganic nutrients, total and nannophytoplanktonic chlorophyll a were determined at each depth and additional samples were taken for phytoplankton taxonomic work. oblique zooplankton hauls were done and abundance, composition and dry weight of zooplankton were determined. indirect grazing rate estimate was calculated using the pigment budget. the study period coincided with an el ni？o event detected in the punta morales zone and reflected by a change in the physico-chemical water characteristics. some nutrient concentrations were correlated to the salinity, and significant differences were observed between the transition and rainy seasons in the phytoplankton biomass of the two fractions. a total of 43 phytoplankton taxa were identified and flagellates, centric and pennate diatoms represented 90% of the total phytoplankton abundance. in the zooplanktonic samples, copepods were the most abundant group and acartia lilljeborgii, pseudodiaptomus sp. and paracalanus parvus were the dominant herbivores; dry weight mean values of herbivorous copepods were between 0.6 and 50 mg m-3. grazing rate showed a mean of 0.67 ± 0.83 d-1. this study showed a seasonal behavior with changes in nutrient concentrations, availability of phytoplankton and rising temperatures through el ni？o. predation and grazing pressure produced changes in the structure of plankton (phyto and zooplankton) populations in the internal zone of gulf of nicoya