Algal blooms have turned many lakes into toxic soups. Because of algae, lakes are no longer suitable for drinking water or fishing. In addition, algal blooms force authorities to close down bathing water. But no lake is alike: while algae emerge in one lake, other lakes seem to remain clean. A team of Dutch and Chinese scientists conducted research into algal blooms in more than 19,000 Chinese lakes.
Nature is full of variety. Just look outside: every tree looks different. Even if they are all oak or beech, one tree is slightly smaller, wider or more skewed than the other tree. This variation also exists underwater: each lake is unique, one is deep, the other large or shallow. “The world has 1.4 million lakes, and there is not a single lake on earth that has its similitude. Due to this great variety management of lakes is difficult”, explains Annette Janssen, researcher of Wageningen University & Research. “Where some lakes are vulnerable to algal blooms due to nutrient pollution, other lakes seem rather resilient.”
Scientists from the Netherlands, together with scientists from China, have mapped out the properties of more than 19,000 Chinese lakes. Their recent publication in July 2021 in Water research shows that the most vulnerable lakes are relatively close to the sea. “This is also the place where most pollution occurs”, says Janssen. The fact that the most vulnerable lakes are also the most polluted is an unfortunate combination of factors, but it also helps to better understand how we can best combat algae, according to the Dutch and Chinese scientists. “Now that we know where the most vulnerable lakes are, we can better estimate where measures need to be taken to keep or turn the lakes clean.”
The research also shows that the vulnerability of lakes to algal blooms depends on the properties of the lake such as depth or temperature. Shallow lakes can have a lot of water plants which put resistance to algal blooms, while in deeper lakes such amounts of plants are rare. Temperature is also important: algae thrive on high temperatures and are therefore much more common in the warmer lakes on earth. Finally, also the lake sediment is important: if the sediment of a lake has a lot of clay, it can act as a kind of battery: sometimes sediments are charged with nutrients such as phosphate, which can be released later when conditions change promoting future algae growth.
Due to the different properties of lakes, two lakes can react very differently to similar levels of pollution. Take, for example, Lake Baiyangdian, which is 170 km south of Beijing and receives more than twice as much nutrient pollution than Lake Taihu, about 140 km west of Shanghai. Despite the greater pollution in Baiyangdian, the problems with algae growth in Taihu are larger. “When I visited Lake Taihu, the water looked like thick pea soup, the algae had grown so dense”, says Janssen. “But, at the Baiyangdian Lake, I only saw water plants.” This is because Taihu has different properties than Baiyangdian, making Taihu more vulnerable to algae. For example, the Taihu has a large surface area. Lakes with a large surface area are more prone to wind which is detrimental to water plants. The absence of plants provides room for growth for algae.
The study of the properties of lakes by Dutch and Chinese scientists has provided many insights. “But we are not there yet”, warns Janssen. “Future changes in land use, climate and economy could alter vulnerability.” So more research is needed to see how the lakes will respond to these changes.