RESUMO
Changes in hydro-meteorological conditions due to warming climate and the operation of reservoirs may support algal blooms. Lake Diefenbaker is a large reservoir on the Canadian Prairies. Annual flow volume from its major tributary, the South Saskatchewan River (SSR), varies with precipitation and temperature in the Rocky Mountains. Furthermore, plans are underway to increase water abstraction from Lake Diefenbaker for irrigation. Therefore, we used a nine-year dataset that comprised a drought year (1984), four consecutive high flow years (2011 to 2014), and four subsequent low flow years (2015 to 2018) to investigate how these changes could affect the major phytoplankton groups and cyanobacterial community. Diatoms (38.5%) were the most abundant phytoplankton, followed by cryptomonads (28.9%) under low and high flow years. Diatoms were associated with greater mixing in late spring and fall, whereas the cryptomonads were related to the high nutrients from spring flow. Cyanobacteria (79.3%) contributed the greatest to the total phytoplankton biomass under drought; we hypothesized that the high abundance of cyanobacteria during drought was associated with thermocline deepening and subsequent internal loading of nutrients. Microcystis, a potential bloom-forming and toxin-producing genus, was dominant during the drought and correlated with reduced water level, increased air temperature, and moderate wind speed. Although its biomass was low, another potential bloom-forming and toxin-producing genus, Aphanizomenon, was present in low and high flow years. Aphanizomenon was correlated with decreased SSR flow and increased particulate carbon to particulate phosphorus ratios, which may be related to their ability to cope with P limitation. These results highlight that Lake Diefenbaker and other similar reservoirs are vulnerable to an increase in potential toxic cyanobacteria species with future expectations of climate warming and water abstraction.