Gold mining in northern Canada, while profitable, has left a legacy of arsenic (As) contamination, especially in lakes. Arcellinida, lacustrine benthic protozoans, have great As-biomonitoring potential that has yet to be exploited by lake restoration initiatives. This research aims to further develop Arcellinida as a robust tool for monitoring As contamination and remediation efficacy in impacted lakes. In Chapter 2 a novel Arcellinida sample processing protocol, using chemical deflocculation, was developed to determine optimal approaches for reducing residual organic matter in lake sediments with minimal Arcellinida test degradation. The results revealed that treatment with 5% potassium hydroxide led to the highest reduction in observed organic content and rate of test damage. In Chapter 3 results of a study where Arcellinida were examined from 22 near-surface sediment samples (top 1 cm) collected from similar environments in two eastern Canadian lakes was carried out to evaluate faunal assemblages homogeneity in these targeted environments, and to assess the sensitivity of Arcellinida assemblages to inter-annual environmental variability. The results of this study confirmed the faunal homogeneity of assemblages in limnologically similar environments and demonstrated the sensitivity of Arcellinida assemblages to environmental changes at inter-annual time scales. In Chapter 4 an inter-lake survey of 91 lakes in the Yellowknife area, Northwest Territories (NT), Canada, was conducted to quantify the response of arcellinidan to gold mining-derived As contamination and to statistically determine As-tolerance limits of different Arcellinida species. The results demonstrate the applicability of using Arcellinida to delineate the spatial extent of As contamination and the reliability of using As tolerance-limits-based Arcellinida groups to infer As contamination levels in lake sediments. In Chapter 5 the arcellinidan response to intra-lake changes in As contamination, derived from two former gold mines, was investigated in 40 near-surface sediment samples from two lakes in the low Arctic, central NT (Tundra Mine) and in a boreal setting, southern NT (Giant Mine). It was demonstrated that Arcellinida assemblages responded differently to the impact of As, likely due to different As mineralization in each lake, which was in turn a by-product of employing different ore-processing techniques at the former gold mines.