Seasonal and interannual variability in phytoplankton species composition in the Cariaco Basin, Venezuela

Phytoplankton species composition and its sources of seasonal and interannual variability in the Cariaco Basin off the Venezuelan coast were studied using 6.5 years of monthly data from the CArbon Retention In A Colored Ocean (CARIACO) Time-Series station (10.50°N, 64.67°W). Dimensionless and dimensional parameterizations of Margalef's diagram (1978) and Smayda and Reynolds' Intaglio (2001) were generated (habitat diagrams) to explain the seasonal distribution of the phytoplankton community assembly as a function of the physical forcing and nutrient supply. High seasonal, interannual, and vertical variability in the water column was observed due to wind induced upwelling, eddy circulation, and ENSO effects in the Caribbean Sea basin; these processes were the major source of variability in phytoplankton community structure during this study. The presence, abundance, and dominance of the phytoplankton groups observed varied with the incidence of upwelling and upwelling relaxation processes. Diatoms and nanoflagellates were the most abundant and dominant groups with opposite abundance patterns during the seasonal cycle. Nanoflagellates were present throughout the year as the phytoplankton background population. While both large and small cells were observed during periods of intense upwelling, the former were more abundant. However, when upwelling was less intense, smaller cells were numerically dominant. High primary productivity (PP) was related to diatom abundance. Lower PP was observed in years with less intense upwelling periods and low diatom abundance. Oligotrophic-highly stratified conditions intensified during ENSO years and promoted the growth of the picophytoplankton fraction during upwelling relaxation, as inferred by pigment data. Adaptations of the habitat diagrams showed shifts in abundances of different taxonomic groups in the community structure and species composition in time. Results were consistent with the theoretical framework of Margalef (1978) and Smayda and Reynolds (2001), but with nuances that have important implications for describing the ecological diversity of this tropical basin.