Annual-scale assessment of mid-20th century anthropogenic impacts on the algal ecology of Crawford Lake, Ontario, Canada

Abstract

Meromictic Crawford Lake, located in SW Ontario, Canada is characterized by varved sediments, making it suitable for high-resolution paleoecological studies. Freeze cores, the only coring method available that reliably preserves the fragile laminations representative of seasonal deposition in the lake, were used to document siliceous diatom and chrysophyte community structure at an annual resolution from 1930-1990CE. Stratigraphically constrained cluster analysis identified major assemblage changes that are believed to have been caused by local, regional and possibly global anthropogenic impacts. The assemblage changes within the siliceous algae are attributed to regional weather and increased industrial emissions and related effects of acid deposition on the lake's catchment associated with the Great Acceleration -the massive economic, industrial, and demographic expansion beginning in the mid-20th century. Observed increases in spheroidal carbonaceous particles (SCPs) in varved lake sediment dating to the early 1950s record rapidly expanding steel production without emission controls around 30 km upwind of the lake. The findings reported here reflect major changes in earth systems that the Anthropocene Working Group recommends for a proposed epoch to be termed the Anthropocene, providing support for the laminated sediments from Crawford Lake as a potential Global boundary Stratotype Section and Point (GSSP).

Keywords: Acid Deposition; Algae; Anthropocene; Climate Change; Diatoms; Dissolved Organic Carbon (DOC); Scaled Chrysophytes; Varves.

Figure 1. Site location.

(A) Map depicting Crawford Lake in relation to Toronto within Ontario, Canada. (B) Situation of Crawford Lake relative to local geological formations. (C, D) Location of Crawford Lake locally. (E) Core location and general bathymetry within Crawford Lake. Bathymetry is adapted with permission from Boyko (1973).

Figure 2. Spheroidal carbonaceous particle (SCP) concentration data.

Relationship between the population of Hamilton, Ontario (‘Steeltown’, 303 m upwind from Crawford Lake) and the concentration of spheroidal carbonaceous particles (SCPs) in freeze core CRA19-2FT-B2 from ∼22 m water depth. (A) Population of the nearby town of Hamilton (number of people × 1,000). (B) Spheroidal carbonaceous particle (SCP) concentration in number of particles per gram of dry mass. (C) Composition of SCPs by size fraction percentage. Modified from McCarthy et al. (2023).

Figure 3. Relative abundance of common chrysophyte taxa within Crawford Lake by varve-inferred year (CE).

Percentage of scales belonging to Synura by year depicted. SCP concentrations in number per gram of sediment dry mass (gDM⁻¹) and plotted according to median sample year; from McCarthy et al. (2023). Zones created by broken-stick model validated CONISS delineations based on counted chrysophyte scale data.

Figure 4. Chrysophyte NMDS.

NMDS biplot based on square-root transformed chrysophyte scale count data (k = 3, stress = 0.061009). CONISS delineated zones marked with colours and symbols as depicted in the legend.

Figure 5. Relative abundance of common diatom taxa within Crawford Lake by varve-inferred year (CE).

SCP concentrations in number per gram of sediment dry mass (gDM⁻¹) and plotted according to median sample year; from McCarthy et al. (2023). Zones created by broken-stick model validated CONISS delineations based on counted diatom valve data.

Figure 6. Diatom NMDS.

NMDS biplot based on square-root transformed diatom valve count data (k = 3, stress = 0.09402618). CONISS delineated zones marked with colours and symbols as depicted in the legend. Taxonomic names of diatoms matched to abbreviations within this figure are available in table form within Supplement 6.