What Sediments Can Tell Us About Lake Health
The Environmental Change and Aquatic Biomonitoring (ECAB) Laboratory has started a new project investigating dissolved oxygen (DO) in lakes across New Brunswick, some of which experience blue-green algal blooms during late summer. Dissolved oxygen is crucial to many aquatic organisms and a fundamental measure of lake health. Algal blooms, specifically their breakdown by microbes, utilizes DO. The decomposition of algal-derived organic matter can thus result in low DO in the cold, deep waters of a lake.
The field team, composed of undergraduate researchers, MSc students, and Dr. Kurek have collected lake sediment cores and limnological measures (e.g. water chemistry, DO, water temperature). We have traveled to study lakes across New Brunswick, from lakes near the southeastern Maine (USA) border, to those near Quebec’s Gaspesie region, and to lakes near the Maritime province’s highest mountain, and even one meromictic lake near our campus in Sackville. A total of 20 lakes, 10 with reported algal blooms according to NB’s Office of the Chief Medical Officer of Health, and 10 reference lakes that are remote and have had no algal blooms, were selected for study.
What do lake sediments tell us about oxygen levels? Good question! It isn’t so much about the sediment itself, but more about what is in it. We are interested in the small invertebrates that live in the sediment. Specifically, we are studying the larval remains of non-biting midges, commonly known as chironomids. Chironomids are well-known bioindicators of water quality and overall lake health. We can use the abundance and diversity of chironomids to make inferences about their surrounding environment, specifically whether DO was higher or lower in the past compared to recent years. The head capsules of chironomids preserve well in sediments and can be used to identify the species living in the lake. As sediment accumulates slowly in lakes over time, lake health can be assessed based on the bioindicators found.
Like most organisms in a lake, chironomids require oxygen. Each species has different DO requirements. Some species may flourish in high DO levels whereas others may be able to tolerate very low levels of DO. ECAB’s goal is to examine the chironomid remains in the lake sediments and use this bioindicator data to understand long-term trends in lake health and the trajectory of DO. We will compare the chironomids in the top sediment layers (which consists of the most recent deposits) to those in the sediment 40 cm down core. Typically, these older sediments date to at least 1900, which is over a century ago! These sediment-based measures allow us to reconstruct past oxygen levels in a lake. There is no other way to generate data such as these given that most lakes are not monitored and certainly were not in the distant past prior to major human impacts to the environment.
Combined with other limnological data collected when we visit each lake, these sediment measures will help us to determine trends in lake health in aquatic systems that experience algal blooms and those that are healthy and maintain decent water quality. Funding for this research is provided by New Brunswick’s Environmental Trust Fund and supports recent efforts of the province to better manage water resources, including New Brunswick’s 2500 lakes and ponds.