LA COMUNIDAD FITOPLANCTÓNICA DURANTE EVENTOS DE SURGENCIA Y NO SURGENCIA, EN LA ZONA COSTERA DEL DEPARTAMENTO DEL MAGDALENA, CARIBE COLOMBIANO

The phytoplankton community structural attributes were evaluated in Granate and Gaira inlets and Taganga Bay in order to establish the community´s response to changes in oceanographic dynamics modulated by upwelling and non-upwelling processes throughout the coastal waters of the Magdalena region, Colombian Caribbean, between February and June of 2008, during the main dry and short rainy seasons. Average wind speed divided in the research period in two seasons: February-April (upwelling: S) and June-July (non upwelling: NS) of 2008. In each period (upwelling and non-upwelling), 51-47 genera were identified distributed in 29-28 families, corresponding 78.23-86.19 % to centric diatoms, 16.68-9.47 % to pennate diatoms, 4.57-3.78 %) to cyanobacteria and about 0.50-0.55 %) to dinoflagellates, flagellates, and chlorophyta. The most abundant genera were Chaetoceros (colonial) and Skeletonema (colonial) with a relative abundance of 47-19.57 % and 8.13-60.93 %, respectively, and densities greater than 2000 cel/L. Chlorophyll a concentrations showed low values in this study from 0.72 ± 0.47 ng/m3 during S until 0.15 ± 0.15 mg/m3 during NS. The photosynthetic activity index (IAF1) of 6.34 ± 5.34 indicates either high pigment quality or low zooplankton grazing, representing an optimal phytoplankton physiological state during periods of S. In contrast, the IAF1 low average value of 0.52 ± 0.84 at BS ubducates a wide decline in the phytoplankton community's physiological state. A temporal segregation pattern was defined by cell densities from 22142.23 ± 15236.72 cel/L, for S and NS periods, respectively. Tempperature, salinity, and nitrate concentration (0-60 m depth), were the variables that represented the most seasonal changes, and affected the phytoplankton community structure.