Unlocking History: How Studying Lake Beds Reveals Hidden Environmental Records

Canada is home to more lakes than any other country, showcasing a wide range of sizes, shapes, and water qualities. These lakes hold remnants of their past, recorded in the sediments that settle on their floors.

When lakes experience issues like algal blooms or excessive weeds, it can be hard to tell if these changes are natural or caused by human actions. To figure this out, we need to understand a lake’s history, especially how it functioned before large human populations moved in.

In the field of paleolimnology, scientists study lake sediments to track changes over time. By examining the organic and inorganic materials that build up in these sediments, we gain insights into how lakes have evolved.

One key to this research is tiny organisms called diatoms. These single-celled algae have unique, glass-like cell walls that preserve well in sediments. Because each species has a distinct shape, they tell us a lot about the environment in which they lived. Some diatoms float in the water, while others cling to rocks or plants on the lakebed.

Diatoms also adapt to their surroundings—like differing nutrient levels or water acidity—making them reliable indicators of past conditions in lakes. While some materials break down over time, the remains of chlorophyll a, the main pigment in plants, still offer valuable information about historical lake productivity when measured in sediments.

By studying changes in diatom types along with chlorophyll a levels, researchers can show how the base of the lake food web has shifted through the centuries.

Our team studied diatoms and chlorophyll a in over 200 Canadian lakes through the LakePulse project. We took sediment samples from the surface and the deepest layers—these represent modern and pre-industrial conditions. The findings revealed two main trends related to human impact and climate change.

First, lakes surrounded by agriculture or urban areas showed the most significant changes. Diatom populations shifted to species that tolerate higher nutrients and salinity. This was especially evident in the Prairies, where farming has intensified, affecting shallow lakes prone to runoff.

The second trend was an increase in planktic diatoms as warmer temperatures extend the summer stratification period in lakes. During the summer, layers of warm water sit above colder water, and many lakes now experience this stratification for longer periods due to climate change.

Research indicates that planktic diatoms thrive in these warm, layered conditions. The rise in their numbers across most Canadian lakes suggests a clear impact of climate change, regardless of human activities. Additionally, higher levels of sedimentary chlorophyll a point to increased productivity, especially as ice-free periods lengthen.

The changes occurring in Canadian lakes due to climate change and human development are concerning. Warmer temperatures and nutrient overload can lead to harmful algal blooms. When these algae die and decompose, they consume oxygen, depleting it in the water and creating problems for cold-water species like lake trout, which depend on oxygen-rich environments to survive.

Using paleolimnology, we can understand how ecosystems have shifted over time. This knowledge is crucial for preserving the health of Canada’s freshwater resources for future generations.

Katherine Griffiths of Champlain College Saint-Lambert co-authored this article.