Quantifying the effects of hydrological changes on long-term water quality trends in temperate reservoirs: insights from a multi-scale, paleolimnological study

Declining water quality in reservoirs is of growing concern in many regions, yet there is still little understanding of long-term water quality trends in these systems. Across the landscape, reservoirs have diverse origins, functions, and operational strategies. In temperate environments, winter water-level drawdown is a common operational practice in reservoirs but the long-term impacts of this hydrological modification has not been extensively studied. We paired a comparative, pre-dam-to-contemporary study (i.e. a top-bottom design) of 12 reservoirs with a detailed paleolimnological study of a focal lake to generate quantitative insights into the relative effect of hydrological changes vs. landscape and climatic drivers on water quality. The focal reservoir, Grand Lac Saint-Francois, is of relatively similar morphometry, geography, and limnology to our other sites, and has experienced annual winter water-level drawdown of similar to 5m since it was dammed approximately 100years ago. Based on our top-bottom analysis, we did not find strong correlations between long-term changes in water quality (i.e. diatom-inferred TP estimates) and winter water-level drawdown amplitudes. Instead, reservoir morphometry and watershed characteristics (i.e. geography, maximum depth, and cropland areas) appeared to be stronger drivers of trends across the region. From the detailed paleolimnological analysis, we found that sedimentary pigments and DI-TP concentrations significantly increased over the last century based on Mann-Kendall trend analyses. Breakpoint analyses showed that changes in biological-proxy trends, as well as the sedimentology (i.e. lithology and accumulations rates), coincided with dam construction and the onset of water level regulation. However, given the high variability in metrics and the extent of water level monitoring records, we were unable to quantitatively associate the impacts of drawdown with water quality trends at Grand Lac Saint-Francois. Conversely, we did find that watershed nutrient surpluses from livestock farming, and warming temperatures were significant explanatory variables of water quality metrics.