Environmental factors influencing phytoplankton communities in Lake Diefenbaker, Saskatchewan, Canada

Lake Diefenbaker (LD) is a large reservoir located in the Canadian Prairies. Approximately 98% of the inflow is from the South Saskatchewan River (SSR). The composition and ecology of the phytoplankton community along the length of LD has not been reported since the 1980s. This is a potential problem for a reservoir that serves as an important source of water for indirect domestic consumption, irrigation, and recreation. Therefore, we examined epilimnetic whole water samples along the length of LD during the open-water season (June to October) in 2011 and 2012. We estimated the trophic status from chlorophyll a (TSIchl a) and examined the phytoplankton community in relation to environmental factors. We observed high flow events that carried large nutrient loads and associated turbidity into LD from the SSR. Despite the high flow events, mean TSIchl a placed LD as a mesotrophic system with a highly diverse phytoplankton community (72 genera). The increased nutrient and non-algal turbidity associated with high flow events may be related to the dominance of the cryptophytes and the bacillariophytes (both constituted ~ 89% of the total phytoplankton biomass). Inflow and water temperature explained 41% of the variation in cryptophyte biomass, whereas mixing depth and particulate nitrogen to particulate phosphorus molar ratios explained 38% of the variation in bacillariophyte biomass. Despite the low cyanobacterial biomass (< 5%), we observed some potential toxin and bloom-forming genera that may threaten the water quality of LD if conditions become favorable during low flow.