Diatom Assemblages in Relation to Water-Depth Gradients in Eight Boreal Shield Lakes From Northwestern Ontario, Canada

Abstract

The uncertainty surrounding the impact of future changes in climate and water resources has created renewed interest on how lakes have responded to drought in the past. There is a need to determine potential future available water by understanding past changes in water levels; the underlying ecological characteristics of using diatoms as a proxy for lake-level reconstructions is the basis of this thesis. By integrating knowledge from past water-level fluctuation studies and theories, along with developing a better understanding of diatom ecology in lake systems, more effective techniques are being developed to improve water-depth reconstructions. Diatom assemblages were examined from eight lakes in northwestern Ontario collected in surface sediments along a depth gradient at ~1-m water-depth intervals. Three major zones, based on the composition of diatom assemblages in each lake were consistently identified in all lakes: i) a near-shore assemblage of Achnanthes (sensu lato) species and other benthic taxa (Nitzschia, Cymbella); ii) a mid-depth small Fragilaria (sensu lato)/ small Aulacoseira zone with various Navicula taxa, and iii) a deep-water planktonic zone. The depths at which transitions between these zones are located varied among lakes, and the depth of the transition between the planktonic and benthic zones was consistent with water chemistry variables (e.g. DOC, TP) that are related to light attenuation. Deeper pelagic to benthic transitions occurred in lakes with the lowest DOC and TP (i.e. generally more light attenuation in lakes with higher concentrations of TP or DOC). Other findings included a decrease in species evenness and numbers with depth, along with an increase in scaled chrysophyte relative to diatoms.