Influence of coastal upwelling and river discharge on the phytoplankton community composition in the northwestern Gulf of Mexico

The regional circulation in the northwestern Gulf of Mexico during late spring-summer is modulated by upwelling-favorable winds that can cause coastal upwelling in the western region and by freshwater inputs from the Mississippi-Atchafalaya Rivers. Spatial variability and temporal dynamics of phytoplankton community composition were examined during two upwelling-favorable periods using data obtained with an Imaging FlowCytobot (IFCB) from two cruises on the Texas-Louisiana shelf in June 2013 and 2014 and from the Texas Observatory for Algal Succession Time series (TOAST) at Port Aransas (Texas). Phytoplankton spatial distributions were determined by the influence of upwelling and river discharged waters. In the 2013 cruise, upwelling was detected in a large portion of the western region and the phytoplankton assemblages were dominated by diatoms, mostly chain-forming taxa. As revealed by the TOAST time series, the upwelling onset caused a dramatic increase in diatom carbon biomass. In the areas not affected by upwelling, variation in the river plume distribution that resulted from the circulation and the different discharge magnitudes for each year influenced the spatial distributions of the phytoplankton community composition. Dinoflagellates and other flagellated taxa were notably dominant during the 2013 cruise, whereas both diatoms and flagellated groups dominated the assemblages during the 2014 cruise. High stratification promoted by freshwater input, notably higher during 2013 than 2014, likely favored the dominance of flagellated groups in 2013. This study provides evidence of the influence of coastal upwelling in the phytoplankton community of the northwestern Gulf of Mexico and contributes to the knowledge of the drivers of community composition in this high-productivity area.