Shallow Waters, Deep Troubles

Researchers have found that shallow lakes may not only be more sensitive to human-caused nutrient pollution, but harder to restore compared to their deeper cousins.

Asian Scientist Magazine (Jul. 15, 2022)–You’ve likely seen it before—a pond covered with floating green carpets, or a lake swirling with murky green clouds. These are tell-tale signs of eutrophication: a condition where the water is so concentrated with nutrients, there’s been a rapid population explosion or bloom of phytoplankton—bacteria and microscopic algae—as they gobble up the excess food.

Why is this a bad thing? Because when phytoplankton dominate a body of water, other inhabitants often suffer in the process. Blooms can block sunlight from reaching underwater plants, choke out aquatic animals by creating low-oxygen “dead zones” in the water, and release foul-smelling toxins that make water undrinkable for humans.

Now, a research collaboration between scientists in China and Canada has found that water depth could be a major factor in determining how vulnerable a lake is to human-caused eutrophication—and how difficult it might be to fix the situation.

In a recently published study of 1,151 lakes across the United States and 27 European countries, a team of researchers who study inland aquatic ecosystems from the Chinese Academy of Sciences (CAS), China, and the University of Regina, Canada, found that shallow lakes (up to 13.8 metres deep) were far more likely to reach a eutrophic state than deeper lakes. The study was published in Water Research.

Using heatmaps and statistical tools the team hunted for specific features in land use and land cover that would likely to lead to eutrophication. They researchers found that multiple interconnected factors contributed to a eutrophic tipping point; for example, shallow lakes are often in lowland regions with reduced natural forest cover, which happen to be sites of intense human activities like agriculture, industry, and urban development. Fertilizer, factory waste, and other nutrient-rich pollutants drain into and concentrate in such lakes, sending nutrient levels skyrocketing.

Shallow lakes also tend to have less water in general, making it easier for nutrients to concentrate. Even when two lakes have similar water volume, algal blooms more likely show up in the shallower one. That’s because excess nutrients—instead of settling at the bottom of the lake, too deep for phytoplankton to consume—often get stirred up by disturbances in the water, by weather or human activity.

These factors may also explain why shallow lakes are more resistant to restoration efforts, according to study leader Professor Boqiang Qin of CAS’s Nanjing Institute of Geography and Limnology (NIGLAS).

“Shallow lakes in agricultural or populated regions may be particularly susceptible to eutrophication, [which] may be not an occasional occurrence,” Qin added.

According to co-author Dr. Jian Zhou, also of NIGLAS, their findings “help set realistic goals and adjust community expectations” when it comes to protecting or remediating shallow lakes from eutrophication.

“It may be a challenge to convince stakeholders to continue to invest in nutrient reductions without evidence of rapid improvement, but it is necessary for long-term water quality improvement,” Zhou told Asian Scientist Magazine in an email interview.

Zhou added that the team’s findings apply beyond to the continental regions the team studied. “For example, the distribution of eutrophic lakes in China displays many similar features including lowland locations, high agricultural land use, and symptoms of recently-accelerated eutrophication; such as in the middle and lower reaches of the Yangtze River basin,” said Zhou.

Moving forward, Zhou and colleagues plan to investigate the mechanics of how water depth drives lake eutrophication, as well as how it affects nutrient cycles, phytoplankton growth and community structure within lake ecosystems.

Source: Chinese Academy of Sciences; Photo: Shutterstock

The article can be found at: Zhou et al. (2022) Anthropogenic eutrophication of shallow lakes: Is it occasional?

Rachel Soon is a science communicator with a passion for delving into the living world's workings, from microbes to ecosystems, and helping others do the same. She graduated from Imperial College London, UK, and holds an MA in Science Education from Teachers College, Columbia University, USA.

Related Stories from Asian Scientist