Dynamic phytoplankton community structure in a subtropical reservoir during an extended drought, Central Texas, USA.

Subtropical lakes remain understudied by comparison to lakes in temperate and tropical regions when considering environmental factors impacting cyanobacterial harmful algal bloom (CyanoHAB) propagation and success. Central Texas, USA, exhibited an extended, severe drought from 2010 to 2015. Phytoplankton data from Lake Buchanan, a large reservoir on the Colorado River, displayed rapid shifts in the community during this period as the reservoir transitioned to more eutrophic conditions. This study serves to assess the likelihood of CyanoHABs considering this recent transition in trophic status and determine factors promoting success of potentially harmful species. Routine monitoring was performed every other month, including measurements of environmental parameters, nutrients, as well as the identification of phytoplankton using DNA barcoding and cell counts. DNA sequencing analysis confirmed the presence of multiple known bloom-forming cyanobacteria. Potential producers of saxitoxins Planktothrix, Aphanizomenon/Chrysosporum, and Raphidiopsis thrived in drought conditions, whereas Limnothrix and Pseudanabaena performed better during non-drought conditions. Following the drought period, Chrysosporum ovalisporum, Phormidium tenue, and Planktothrix sp. were still present along with other potentially harmful species; however, the community had shifted to predominantly diatoms comprised of Fragilaria and Lindavia. These results suggest the drought conditions, i.e., increased conductivity, temperature, and nutrient loads, likely influenced the success of potentially toxic cyanobacteria. The success of these taxa contributed to eutrophication and subsequent decline in diversity. Subtropical lakes should therefore be monitored more closely during extended drought periods, as the risks associated with eutrophication and CyanoHABs are predicted to be significantly higher. [ABSTRACT FROM AUTHOR]