Seasonal dynamics of phytoplankton in two warm temperate reservoirs: association of taxonomic composition with temperature

Variations in phytoplankton composition were observed over 2.7 years in two warm temperate reservoirs in north central Texas, Joe Pool Lake (JPL) and Eagle Mountain Lake (EML), and analysed in relation to temperature, light, dissolved nutrients, seston nutrient ratios and hydrological variables. The dominant variations in both phytoplankton composition and environmental variables were seasonal and strongly related to the annual temperature cycle. Phytoplankton composition was annually periodic, with filamentous cyanobacteria, pennate diatoms and chlorophytes dominant in summer, while other chlorophytes, centric diatoms, cryptophytes, dinoflagellates and other flagellates either dominated in other seasons or remained abundant year round. The seasonal dynamics of several major taxa could be represented by a niche response model based on temperature alone, although seasonal variations in many environmental variables likely contributed to variation in phytoplankton composition. Such seasonal variations dominated these data, but interannual variation in phytoplankton composition and environmental variables also appeared to result from dry weather late in the study.