Researchers draw attention to a growing problem in many of the world’s clear lakes: thread algae are growing on the shore. The carpets made of green hairs threaten the ecosystem and can cause problems in water use. The causes of the increased growth remain unclear: nutrient inputs alone cannot be responsible – apparently other factors play a role, such as the loss of algae-eating aquatic animals, climate change and invasive species. The problematic phenomenon should now be investigated more closely, the scientists say.
They come in many different shapes, colors and water areas: Algae are one of the natural forms of life in bodies of water and form important elements in the material cycle. They only pose a problem when their growth gets out of hand or when species spread where they were not previously common. This is exactly what is now becoming apparent in the case of thread algae. This is not a single species – many different species are grouped together under this term because of their thread-like or cotton-wool-like appearance. They are actually more typical for ponds or bodies of water with relatively high nutrient contents. But as the international team of scientists reports, there are now signs of an increased spread of these algae forms in many of the more clean and clear lakes of the world.
Worldwide phenomenon
“We are very astonished that thread algae blooms are now appearing in the shallow bank zone in lakes, about which we had hardly had to worry so far,” says co-author Sabine Hilt from the Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB). Some large lakes such as Lake Tahoe (USA) and Lake Baikal (Russia) and even remote waters in mountain regions are affected. “In Germany, too, there are problems with the mass development of thread algae on some lakes,” says Hilt. The green algae carpets are unsightly for bathers and can give off a fishy odor. Above all, however, they can endanger the communities on the sea floor and change the food web. In addition, toxins from cyanobacteria can accumulate in the biomass. However, there is still a considerable need for research into the possible effects of thread algae blooms, the scientists emphasize.
But what is behind the green trend in the world’s clear lakes? At least in part, inputs of nutrients could contribute to growth – a process known as eutrophication. Because thread algae have a higher nutrient requirement than the smaller, slow-growing species that they replace in the clear lakes. It is known that nutrient intake has increased in some formerly nutrient-poor lakes in recent years. One possible reason for this is nitrogen and phosphorus inputs from wastewater. In addition, forest fires can also cause more nutrients from the surrounding area to reach lakes. “However, the current models for eutrophication of lakes cannot fully explain the greening of the riparian zones,” the researchers write.
Complex cause?
Apparently, according to them, other factors also play a role – such as the decline in algae eaters: It seems possible that in some bodies of water the entry of pesticides or other stress factors lead to the death of the microorganisms in the water, which normally eat thread algae. In addition, climate change also seems to play a role in the career of thread algae. Co-author Sudeep Chandra from the University of Nevada in Reno draws attention to evidence that thread algae occur more frequently in the clear mountain lakes in the western United States because the water temperatures rise there. In Lake Tahoe, the increasing occurrence in the shallow shore zones also appears to be related to the shorter snow cover, says Chandra.
According to the researchers, changes in the composition of the species could also be involved in the phenomenon: for example, the alien zebra mussel has spread widely in the Great Lakes in North America for around 30 years. As the scientists explain, the shellfish feed on plankton – and then make the nutrients bound in them available for the thread algae. The spread of this species may have favored the mass development of thread algae in the riparian zones there.
“Our overview study shows that a variety of environmental factors can influence the phenomenon,” says Hilt. So there is now a need for research. “We also still know far too little about where and to what extent thread algae spread strongly. “Remote sensing methods could now be used,” says the scientist. “In addition, lay researchers – citizen scientists – could help to elucidate the phenomenon by sharing their observations via cell phone,” says Hilt.
Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries, specialist article: BioScience, doi: 10.1093 / biosci / biab049