Response of phytoplankton community composition to physicochemical and meteorological factors under different hydrological conditions in Lake Diefenbaker.

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.

Understanding the factors associated with long-term reconstructed turbidity in Lake Diefenbaker from Landsat-imagery.

Turbidity affects a variety of aquatic ecosystem processes. Turbidity events are dominated by suspended sediment in many systems. High levels of suspended sediment in lakes can occur during periods of high inflows from turbid tributaries or suspension of sediment from lake beds. This study reconstructed historic turbidity levels using Landsat-imagery on Lake Diefenbaker (LD), a large river-reservoir constructed in the late 1960's on the naturally turbid South Saskatchewan River (SSR). We examined the factors that were associated with it. Reconstructed turbidity levels, from Landsat-images, were similar to actual turbidity. The SSR flow and wind speed explained 64%, 54% and 69% of the variability in estimated turbidity levels at the riverine zone, the transition zone and the entire reservoir, respectively. The decrease in estimated turbidity from June to October and down the length of the reservoir is likely associated with the decline in the SSR flow and the settling of suspended sediments. The relationship between estimated turbidity and wind speed may be associated with the re-suspension of bottom sediment at the upper reach of LD. Wind speed and direction were related to estimated turbidity at the lacustrine zone (r2adj = 0.19, P < 0.05), which may be attributed to the persistence of sediments. We observed high turbidity in 2002 that exceeded other estimates of turbidity. Since 2002 was preceded by a prolonged drought, the high estimate turbidity may be related to an increase in sediment loads from the SSR flow and an increase in shoreline erosion from a rise in LD's water level. Hence, extreme events (drought and flooding) are associated with high turbidity in LD. As the Canadian Prairies continues to undergo climate change, lakes located in this region are predicted to experience more frequent extreme events. These extreme events will cause further deterioration of water quality.