Assessing environmental stressors on a commercial walleye fishery from a large northern ecosystem (Tathlina Lake) using water chemistry and paleolimnology

Tathlina Lake (Northwest Territories, Canada) is a large, shallow ecosystem in the rapidly warming northern boreal forest. This lake is of considerable cultural and economic significance as it supports a commercially important walleye (Sander vitreus) fishery that has experienced large fluctuations since the 1940s, the causes of which are poorly understood. Here we used paleolimnology to describe long-term environmental changes in the lake that may have contributed to recent collapses in walleye populations. The sub-fossil remains of diatoms and chironomids were used to assess changes in turbidity, nutrients, and oxygen levels, all of which are important to walleye. Minimal changes have occurred in diatom assemblages from the early 1920s to present, suggesting that turbidity and nutrients have not changed markedly in the lake. Hypoxia-tolerant chironomid taxa were found throughout the sediment record, and our modern water chemistry showed that oxygen levels were supersaturated in the summer, but close to the lower tolerance limit of walleye in winter. An increase in sedimentary chlorophyll-a since ~ 1940 suggests Tathlina Lake is affected by recent climate warming. Our findings indicate that walleye populations are likely regularly exposed to hypoxic winter conditions, which may increase the sensitivity of the population to other interacting stressors that occur with recent climate warming. Long-term records of environmental change in large, shallow northern lakes are rare, and paleolimnology provides a framework to reconstruct missing monitoring data, especially in lakes that are economically and culturally important to northern communities.