The effects of climate variability on the structure of the phytoplankton community in Tumaco Bay, Colombia

Spatiotemporal variability in the diatom and dinoflagellate community structure and chlorophyll a (chl a) concentrations in Tumaco Bay during the 1993–2005 period was related clearly to seasonal and interannual variability in environmental conditions due to the migration of the Inter-tropical Convergence Zone (ITCZ) and the influence of El Niño Southern Oscillation (ENSO) events. A total of 134 species of diatoms that belong to 57 genera, and 78 species of dinoflagellates that belong to 25 genera were identified during the survey. The diatom community was the dominant group in the waters of the bay, being the most abundant with the greatest number of species observed. The most important species was the centric diatom Skeletonema costatum (Cleve 1878).;It was found that the migration of the ITCZ on the region caused a strong annual cycle. With the migration of the ITCZ to the south, Northeasterly Trade Winds dominated the area from about December to April producing oceanic and coastal upwelling. Colder sea surface temperatures (SST), the rising of the thermocline and halocline to the surface, increased nutrients and abundant rainfall were detected. This time of the year was described as the rainy season (RS)/cold phase. Opposite conditions were observed the rest of the year, defined as the dry season (DS)/warm phase. Southeasterly Trade Winds dominated at this time of the year due to the migration of the ITCZ back to the north. This seasonal cycle was affected by interannual variability due to ENSO. Five El Niño and four La Niña events were identified. In general, La Niña episodes were characterized by a shallow thermocline, cold SST, higher salinities, and the highest concentrations of nutrients. Opposite characteristics were observed during El Niño events. However, during the highest precipitations observed in every El Niño event, nutrient concentrations increased significantly.;Important environmental correlates of community structure were identified using Canonical Correspondence Analysis (CCA). Strong gradients in temperature, salinity, nutrients and Secchi depth (water clarity) correlated predominantly with diatom and dinoflagellate community structure. Seasonal variability in the communities provided some important findings: (1) increased species richness and abundance of both groups were correlated with increased nutrients during the cold phase/RS; (2) diatom blooms occurred during the cold phase; (3) dinoflagellate blooms occurred at the beginning of the warm phase; (4) decreased species richness and abundance and lack of blooms characterized the DS; (5) abundances were higher nearshore than at the offshore part of the bay; and (6) chl a concentrations increased during the cold phase.;The influence of ENSO in the communities also provided some important observations: (1) species richness and abundance of diatoms increased during La Niña; (2) species richness and abundance of dinoflagellates increased during El Niño when precipitation and nutrients were high; (3) harmful species and harmful algal blooms (HAB) increased during ENSO events; (4) communities recovered quickly after El Niño events; and (5) chl a concentrations decreased during El Niño and increased during Normal and La Niña conditions.