What causes a healthy lake to tip toward a harmful algal bloom? A study co-authored by Faculty of Science Professor Huaiping Zhu points to dissolved oxygen as a potentially overlooked piece of the puzzle.
Healthy lakes are essential to ecosystems and communities. They provide habitat for wildlife, support fisheries and recreation, help sustain local economies and, in many cases, serve as sources of drinking water.
Those benefits, however, are under threat from expanding clusters of algae that can spread across freshwater systems, block sunlight from reaching aquatic plants, disrupt oxygen levels and produce toxins that can affect wildlife, pets and people.
"Harmful algal blooms are an increasing concern in freshwater ecosystems around the world, including the Great Lakes," says Zhu, a professor in 91ÑÇÉ«'s Department of Mathematics and Statistics and director of the Laboratory of Mathematical Parallel Systems (LAMPS).
As these outbreaks become more common, scientists are studying what causes healthy lakes to become prone to bloom. Improving that understanding could help researchers and policymakers develop more effective ways to predict, prevent or manage these events.
Scientists have long studied how nutrients, temperature and other environmental conditions contribute to bloom events. co-authored by Zhu suggests one overlooked driver may be dissolved oxygen, the oxygen naturally present in lake water that aquatic plants and animals rely on.
While dissolved oxygen is often used as a measure of water quality, Zhu felt it deserved a closer look because of the role it plays in aquatic environments. In particular, it influences the behaviour of zooplankton, tiny animals that feed on microscopic algae known as phytoplankton. When oxygen levels are low, zooplankton become less effective at feeding on algae. As that natural control weakens, algae can grow more rapidly, increasing the likelihood of a bloom.
Zhu, along with former LAMPS postdoctoral candidate Juan Li and Nanjing Normal University Professor Yongzhong Song, wanted to better understand that relationship by developing and using a mathematical model.
"Our work wanted to fill that gap by explicitly connecting dissolved oxygen, phytoplankton and zooplankton in one dynamical model," he says.
The researchers tested different environmental scenarios to explore what happens when oxygen levels rise or fall and identify conditions that could tip a lake from a relatively stable state toward an algal bloom.
Oxygen levels can be influenced by a range of factors, including temperature, water conditions and biological activity. Climate change can also affect levels by warming waters and altering ecosystems.
The researchers found that when oxygen availability falls, the effects can spread through the ecosystem. A loss of this natural balance can cause lakes to become increasingly favourable for algae outbreaks.
The model also suggests why algal blooms can sometimes prove difficult to reverse once they appear. As oxygen levels decline and zooplankton become less effective at controlling algae, the balance of the ecosystem can shift leading to conditions that favour algal growth, making it harder for a lake to return to its previous state.
"The underlying system can cross a critical threshold and shift rapidly into a different ecological state," Zhu says.
While the research is theoretical, Zhu says the findings could explain how blooms develop and how they might be prevented. By identifying the conditions that make freshwater bodies of water more vulnerable to algae, researchers can identify which factors deserve the most attention before an outbreak occurs.
The study suggests that maintaining healthy oxygen levels, alongside efforts to reduce nutrient pollution, could improve the natural grazing activity of zooplankton and strengthen a lake's ability to keep algae growth under control. Over time, that knowledge could inform more targeted approaches to lake management and ecosystem protection.
Zhu emphasizes, however, that dissolved oxygen is only one piece of a much larger puzzle.
"Natural lake ecosystems are much more complicated than any single mathematical model," he says. "Our study provides one step toward understanding these interactions, but there is considerable opportunity to make the models more realistic by incorporating additional environmental factors and field data."
Future research could incorporate variables such as temperature, light, nutrient availability and water movement to build a more complete picture of how freshwater systems respond to environmental change. Zhu's team is also advancing related work through initiatives such as the Ontario Climate Data Portal, which supports research into climate impacts and environmental change.
"We hope this work contributes to a better mechanistic understanding of harmful algal blooms and ultimately supports more effective management," he says.