Light and nutrient co-limitation of phytoplankton communities in a large reservoir: Lake Diefenbaker, Saskatchewan, Canada

The reliance on reservoirs to meet global water demands is increasing, as is the need to develop an understanding of factors influencing water quality in these understudied water bodies. This study assessed how the interaction between light and nutrients influences phytoplankton dynamics in a large, heterogeneous prairie reservoir; Lake Diefenbaker, Saskatchewan. A multiple indicator assessment of factors (light, phosphorus [P], and nitrogen [N]) influencing phytoplankton biomass, physiology, and gross primary productivity (GPP) was conducted. Light deficiency was assessed through the application of an indicator threshold for the mean daily mixed layer irradiance as well as the ratio of this irradiance to the light saturation parameter derived from photosynthesis-irradiance curves. Short-term physiological assays and long-term compositional nutrient status indicators were applied to assess the in situ nutrient deficiency of the epilimnetic phytoplankton communities. We observed regional differences in light conditions and nutrient chemistry related to incoming flow and nutrient load from the South Saskatchewan River. Light deficiency was detected 59% of the times measured during the open-water season of 2013. Both GPP and quantum efficiency of photosystem II (ϕPSII) were relatively high (108 ± 101 mmol O2 m− 2 day− 1 and 0.57 ± 0.07, respectively) across all regions and months. During conditions of light sufficiency, there was no evidence of N deficiency, and P was the limiting nutrient to phytoplankton communities 90% of the times measured. Nutrient use efficiency and GPP rates were significantly higher under light sufficient conditions. The asynchrony of light and nutrient supply influences production and biomass accrual in this reservoir.