Abstract
A Holocene lake sediment record spanning the past 7300 years from Wishart Lake in the Turkey Lakes Watershed in the Hemi-Boreal of central Ontario, Canada, was used to evaluate the potential drivers of long-term change in diatom assemblages at this site. An analysis of diatom assemblages found that benthic and epiphytic taxa dominated the mid-Holocene (7300–4000 cal yr BP), indicating shallow, oligotrophic, circum-neutral conditions, with macrophytes present. A significant shift in diatom assemblages towards more planktonic species (mainly Cyclotella sensu lato, but also several species of Aulacoseira, and Tabellaria flocculosa) occurred ~4000 cal yr BP. This change likely reflects an increase in lake level, coincident with the onset of a more strongly positive moisture balance following the drier climates of the middle Holocene, established by numerous regional paleoclimate records. Pollen-inferred regional changes in vegetation around 4000 yrs BP, including an increase in Betula and other mesic taxa, may have also promoted changes in diatom assemblages through watershed processes mediated by the chemistry of runoff. A more recent significant change in limnological conditions is marked by further increases in Cyclotella sensu lato beginning in the late 19th century, synchronous with the Ambrosia pollen rise and increases in sediment bulk density, signaling regional and local land clearance at the time of Euro-Canadian settlement (1880 AD). In contrast to the mid-Holocene increase in planktonic diatoms, the modern increase in Cyclotella sensu lato likely indicates a response to land use and vegetation change, and erosion from the watershed, rather than a further increase in water level. The results from Wishart Lake illustrate the close connection between paleoclimate change, regional vegetation, watershed processes, and diatom assemblages and also provides insight into the controls on abundance of Cyclotella sensu lato, a diatom taxonomic group which has shown significant increases and complex dynamics in the post-industrial era in lakes spanning temperate to Arctic regions.
Highlights
Aquatic ecosystems in the 21st century are threatened by numerous anthropogenic disturbances, such as climate warming, land use change, and atmospheric deposition; interactions between these stressors may lead to complex and unpredictable effects [1, 2]
To evaluate the relationship between forest succession, paleoclimate and diatom assemblages, we present a paleolimnological diatom record for Wishart Lake, in the Turkey Lakes Watershed, which has been a monitoring site in northwestern Ontario since 1980
As a further test of the 210Pb dates, we considered Ambrosia pollen counts from two Wishart Lake cores taken in 1980 AD, Core 86 and
Summary
Aquatic ecosystems in the 21st century are threatened by numerous anthropogenic disturbances, such as climate warming, land use change, and atmospheric deposition; interactions between these stressors may lead to complex and unpredictable effects [1, 2]. Climate warming in temperate and Arctic lakes affects ice phenology, lengthens the ice-free season and can produce more stable and deeper thermal stratification, thereby affecting habitat available to aquatic biota [4,5,6]. These processes have been hypothesized to explain major recent changes in freshwater diatom assemblages in regions with seasonal climates, notably, increases in diatoms in the Cyclotella sensu lato group, and often concomitant decreases in tychoplanktonic Aulacoseira spp. and in benthic diatom taxa [5]. Tracking and explaining fluctuations in abundances of Cyclotella sensu lato diatoms in the pre-industrial Holocene is necessary to better explain the recent responses of this important group of bio-indicators
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