The mud at the bottom of a lake isn’t just mud, it contains within it clues that, once decoded, tell us how the environment has changed over time. Clues can be anything that sinks to the bottom of the lake and is deposited into the mud. Often, this includes things that were living (and died) or were produced within the lake (e.g., the remains of micro-organisms, organic or inorganic particles, etc.), but also things that blow, wash, or fall into the lake (e.g., tree pollen, pollutants, soils, etc.). Lake sediments (and the clues contained within them) build up continuously over time, creating a chronological record of the history of the lake and its watershed, with the oldest mud at the bottom of the record and progressively more recent mud at the surface.
Dr. Emily Stewart, a postdoctoral fellow in Professor Jennifer Korosi’s lab, uses sediment records to assess hundreds of years of environmental history to determine how lakes have responded to stressors like climate change or watershed development. As a Weston Family Postdoctoral Fellow in Northern Research, Emily’s current research focuses on understanding the ecological effects of permafrost thaw on lakes in the Canadian Arctic.
The Mackenzie Delta Region of the Northwest Territories is chock-full of lakes. Recent climate warming affects the perennially frozen ground, or permafrost, of the Arctic landscape, accelerating the occurrence and intensity of drastic ground disturbances known as retrogressive thaw slumps. These thaw slumps often occur along the shorelines of the many lakes in the region. The visuals are stunning – once-solid ground near the lake edge falls away, creating a large fan-shaped depression that can be many tens of metres wide and often 5 to 10 metres deep.
Where does all of that ground go? You guessed it – much of it slides into the lake. The inputs of mineral-rich permafrost soils cause drastic changes in the lake. After a slump occurs, we can compare the biological, chemical, and physical traits of lakes affected by thaw slumps to similar unaffected lakes to determine how the lake has likely changed with the disturbance. But what if we could track the changes from before they happened to after they have finished? This is the main aim of Emily’s postdoctoral work, using the natural environmental archives sitting in the bottom of lakes to do so.
In 2019, Emily collected sediment cores from lakes in the Mackenzie Delta Region. A sediment core is a collection of the intact natural archive from the lake bottom, where, essentially, a tube is pushed or hammered into the mud, plugged and pulled to the surface, much the same way you can trap water in a straw using your thumb. The sediment cores are sliced into layers, and these layers are dated using radioisotopes – an element that radioactively decays into a new element over time and that we measure in the sediments. Based on the known half-life, or the amount of time it takes half of the element to radioactively decay, we then back calculate the age of the sediments using these radioactive measurements. Various clues in the form of biological and chemical fossils and the sediment physical characteristics themselves can then be assessed to infer changes in the lake ecosystem. Emily focuses on identifying the bodily remains of aquatic insects that form a crucial component of the lake ecosystem. Shifts in these organisms can indicate key lake ecosystem changes.
Emily’s preliminary findings suggest that highly active retrogressive thaw slumps (i.e. those that release soils into the lake often and continually) appear to suppress the survival of these keystone aquatic insects. Discovering the subtler effects of thaw slumps, and even recovery of these affected aquatic communities once a slump stabilizes, is Emily’s ongoing aim. Overall, Emily’s research will help shed light on how lake ecosystem function shifts with longer and stronger thaw slump disturbances. Preserving lake ecosystem integrity and services in the North will hopefully begin with a better understanding of how these precious freshwaters are affected.
Emily Stewart, PhD, is an aquatic ecologist who specializes in assessing long-term environmental change of freshwaters, particularly in the context of anthropogenic stressors. Her previous work has ranged from documenting the long-term ecological trajectories of arctic lakes affected by historical human sewage inputs to tracking century-scale population changes of nesting waterbirds in the Great Lakes using pond sediments.