Decadal regulation of phytoplankton abundance and water clarity in a large continental reservoir by climatic, hydrologic and trophic processes

Large continental reservoirs are impacted by multiple stressors including climate change, watershed development, food-web alteration, water extraction, hydroelectric-power generation, and industrial aquaculture. Such complexity makes it difficult to identify the hierarchical relationships among regulatory processes, forecast effects of environmental change, or develop adaptive management strategies. Here we present a regression-based analysis of 19 years of limnological change in a representative main-stem reservoir within central North America to suggest that phytoplankton abundance and water clarity are regulated by hierarchical but independent mechanisms. The Qu'Appelle arm of Lake Diefenbaker, Canada, was monitored during summers (May-August) of 1995-2013 to evaluate the unique and interactive effects of continental climate systems, regional meteorology, river hydrology and limnological characteristics on phytoplankton abundance and water clarity. Regression models explained 48-52% of historical variation despite the absence of pronounced temporal patterns in monthly or summer phytoplankton abundance and water clarity. Phytoplankton abundance (mainly diatoms, flagellates) was correlated positively with soluble reactive phosphorus concentrations and inversely with the density of large herbivores, factors which were themselves correlated to variation in chemical conditions (oxygen, dissolved inorganic carbon) and dissolved organic carbon content, respectively. In contrast, water clarity varied directly as a function of climate systems (producing warm, dry winters) and inversely with river flow. We conclude that anticipated climate change (warmer, less runoff) will improve water clarity in lacustrine regions of large prairie reservoirs on decadal scales by reducing inorganic turbidity, while nutrient fluxes associated with economic development may independently regulate algal abundance.