Clarifying the Role of Wildfires and Permafrost Thaw in Brownification in a Shallow Boreal Lake Using Paleolimnological Analyses of Diatom Assemblage Change

Abstract

Permafrost thaw in discontinuous permafrost peatlands has been linked to enhanced export of chromophoric dissolved organic matter (cDOM) into lakes, reducing water clarity through ‘lake brownification’. In the Dehcho region (Northwest Territories, Canada), diatom (siliceous algae, Class Bacillariophyceae) assemblages in small, shallow lakes have been shown to be structured along a cDOM gradient, indicating that lake brownification can be inferred from subfossil diatoms in lake sediment cores. This thesis presents the results of a diatom-based paleolimnological study of a small, shallow Dehcho lake covering the last ~300 years. Results indicate a shift beginning circa ~1920s from epiphytic and large-sized diatom taxa characteristic of low cDOM lakes (Denticula kuetzingii, Navicula) towards increased abundance of Pseudostaurosira brevistriata and Stauroforma exiguiformis associated with higher cDOM. The timing of this shift coincided with an anomalous sediment geochemical signature potentially indicative of a drought or wildfire that likely initiated an accelerated loss of permafrost.