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 postindustrial
era in lakes spanning temperate to Arctic regions.