Spatial Variability in Sedimentary Algal Microfossils and Its Bearing on Diatom-Inferred pH Reconstructions

Abstract

Spatial variability of algal microfossils was investigated in 20–21 surface (0–0.25 cm) sediment samples from Hannah and Clearwater lakes, Ontario, together with the variability in 20 downcore diatom-inferred pH profiles from Hannah Lake. Strong habitat-dependent spatial variability in the distribution of microfossils was observed in lakes of simple morphology. The abundance of planktonic taxa was high in deepwater sediments in both the lakes, while benthic taxa were common only in the sediments of shallow waters of Clearwater Lake. It is hypothesized that greater circulation and presence of sufficient light at all depths allowed benthic diatoms to flourish even in samples from the deep areas of Hannah Lake. The use of pH indicator diatom assemblages rather than individual taxa greatly reduced the variability in computing diatom-inferred pH. The variations were negligible within the samples collected from greater water depths. Results suggest that a single paleo-pH profile will provide an indication of the direction in change in pH, but that absolute pH results can be obtained only by utilizing several cores from many depth strata. The ability of index α and index B to accurately predict recent pH shifts of Hannah Lake was impaired because of its very acidic nature (pH < 4.5) in the past.