The Use of 23S Ribotyping to Detect Harmful and Nuisance Phytoplankton in a Large, Subtropical Reservoir During an Extended Drought Period
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Inland subtropical water bodies are highly susceptible to freshwater harmful algal blooms (HABs). Still, there remains a lack of studies on this subject and the conditions encouraging blooms in this climate. Central Texas, USA, went through an extended drought from 2011-2015 – a phenomenon common in the subtropics. Lake Buchanan, a large inland reservoir, experienced rapid shifts in the phytoplankton community during this period as the lake transitioned to more eutrophic conditions, and serves as an excellent model for subtropical lakes due to its location and size. Samples were taken bimonthly and included measuring water quality parameters, nutrients and phytoplankton, along with the identification of living and preserved phytoplankton to assess the impacts of the transition. The phytoplankton community was evaluated by cell counts and DNA barcoding using 23S ribotyping to verify the presence and abundance of different strains. Abiotic and biotic factors were evaluated to determine which variables contributed to the formation of HABs. DNA sequencing analysis confirmed the presence of known bloom-forming cyanobacteria. Overall, this study shows that the saxitoxin-producers Planktothrix, Aphanizomenon, and Cylindrospermopsis thrived in drought conditions (p = < 0.001) whereas Limnothrix and Pseudanabaena did not. The diatoms Fragilaria and Lindavia increased in terms of community dominance after the end of the drought. Following the drought period, Aphanizomenon ovalisporum, Phormidium tenue, and Planktothrix sp. were present along with additional potentially harmful yet rarely studied species. These results suggest that drought-induced eutrophication lead to the dominance of harmful cyanobacteria in Lake Buchanan. Thus, subtropical reservoirs should be monitored closely during extended drought periods, as the risks associated with eutrophication and HABs are predicted to be higher.