Resumen.-Durante las épocas seca y lluviosa de 1997 se analizó la dinámica trófica del plancton en el Golfo de Nicoya, costa Pacífica de Costa Rica. En dos profundidades de penetración de luz se determinaron la temperatura del agua, salinidad y oxígeno disuelto. En cada profundidad se cuantificaron los nutrientes inorgánicos, biomasa fitoplanctónica total y nanofitoplanctónica, recolectándose muestras para análisis taxonómicos fitoplanctónicos. Se realizaron arrastres oblicuos para estimar abundancias, composición y peso seco del zooplancton. La estimación indirecta de la tasa de herbivoría se realizó utilizando el presupuesto de pigmentos. El estudio coincidió con la ocurrencia de un evento El Niño reflejado en cambios de las características físico-químicas del agua en la zona de Punta Morales. La concentración de algunos nutrientes mostró una correlación con la salinidad y se encontraron diferencias significativas entre las épocas transición y lluviosa en las biomasas fitoplanctónicas. Se identificó un total de 43 taxa fitoplanctónicos, donde flagelados, diatomeas céntricas y pennadas representaron el 90% de la abundancia. Los copépodos fueron el grupo zooplanctónico más abundante y Acartia lilljeborgii, Pseudodiaptomus sp. y Paracalanus parvus, los herbívoros dominantes; los valores del peso seco de herbívoros varió entre 0,6 y 50 mg m -3 . La tasa de herbivoría mostró un promedio de 0,67 ± 0,83 d -1 . Se encontró un comportamiento estacional con cambios en las concentraciones de nutrientes, disponibilidad del fitoplancton e incremento de las temperaturas, donde la predación y presión de herbivoría ocasionaron cambios en la estructura de las poblaciones planctónicas en la zona interna del Golfo de Nicoya.Palabras clave: Plancton, tasa de herbivoría, Océano Pacífico Abstract.-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 Niño 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 ...

Four sampling stations were set 200-500m off Limon Port, Costa Rica, at an average depth of 20 m. From October 1996 to May 1997 horizontal subsuperficial trawls were done for 12 minutes at a speed of about 2KmIh, with a 1 OOO¡.tm plancton neto A total of 28 samples were processed and 104 physieal-chemistry parameter measurements were taken. Average salinity was 30.1 ± 3.7 and dissolved oxygen 6.9 ± 0.6 mg 1-1 reflecting good aeration throughout the sampling périod. This suggests good mixture and a highly dynamic hydrography. Temperature showed no drastic variations (28.0 ± 1.7 OC), possibly because of the constant mixing with shallow water. The highest larval counts were for November and early January and inelude families common to reefs and estuaries (BaJistidae, Lutjanidae) and to coastal areas (Centropomidae, Gerreidae, Haemulidae, Carangidae, Engraulidae, Hemiramphidae, and representative Pleuronectiforrnes) existing in a common area.