A diatom-based paleolimnological investigation of historical water-quality and ecological changes in the Lake of the Woods, Ontario

A two-part paleolimnological study was conducted to examine changes in historical and modern water-quality in Lake of the Woods (LoW) in response to multiple stressors, such as climate change and shoreline residential development. Changes in diatom assemblages were analyzed in the modern and pre-industrial sediment intervals of 17 study sites, and in high-resolution 210Pb-dated sedimentary records from three northwestern bays (Clearwater, Poplar, and White Partridge bays). Patterns in diatom assemblage changes revealed pronounced and synchronous shifts over the last ca. 150 years. The most notable shift in the diatom community structure (∼1970 AD) was characterized by an overall shift towards a higher relative abundance of small, centric Cyclotella taxa and planktonic, pennate diatoms (e.g., Asterionella formosa, Fragilaria crotonensis), and a corresponding lower relative abundance of heavily silicified Aulacoseira taxa and small benthic Fragilaria taxa and Achnanthes taxa. Lakewater total phosphorus (TP) concentrations were inferred from sedimentary diatom assemblages. DI-TP reconstructions revealed either no change or a decline in DI-TP since pre-industrial (pre-1850) times at majority (88%) of the top-bottom study sites, and no distinct directional change over the past ca. 150 years at the northwestern bays. Therefore, we concluded that TP was not an important driver of the floristic changes we observed. Chl-alpha trends indicate that primary production increased during the last ca. 100 years, likely tracking increases in microbial blooms. Changes in diatom assemblage composition and primary productivity have occurred during a period of substantial warming for this region of northwestern Ontario. Strong correlations (r>0.50, p<0.005) between diatom compositional changes, chl-alpha trends, and local air temperature records and lake-ice phenology suggest that climate-induced changes in lakewater properties may have been key factors driving the observed changes. From these data, we conclude that climate warming, rather than changing shoreline development and TP changes, has had the most pronounced effect on algal communities in the LoW. Estimates of pre-impact lake trophic status can aid in setting realistic mitigation targets for lakes impacted by multiple stressors. Therefore, paleolimnological studies comparing pre- and post-disturbance algal assemblages, such as the investigations we have conducted, are of interest from a lake management perspective.