Dangerous unit mix for blue-green algae in lakes
The composition of blue-green algae in the lakes at the edge of the Alps has been becoming more and more uniform for almost 100 years. The main beneficiaries of climate warming and temporary nutrient oversupply are species that can adapt very rapidly to change and are potentially toxic. This is the conclusion reached by a team of researchers led by Eawag thanks to the analysis of DNA from sediment cores.
Construction algae, or technically correct cyanobacteria, are adaptable organisms. They belong to the lowest level of the food web in the lake. As recently as about 100 years ago, each lake was home to its own characteristic variety of them. A study published today by researchers from Switzerland and France in the journal Nature - Ecology and Evolution now shows that these differences are becoming smaller and smaller from lake to lake - from Lake Constance to Lake Geneva, from Lake Hallwil to Lake Maggiore.
Sediment cores from 10 lakes
The scientists took sediment cores from 10 lakes, analyzed the genetic material (DNA) of the blue-green algae in the datable sediments of the last 100 years and statistically evaluated the data. In this way, they were able to track how the number of genetically differentiable "species" has increased in some lakes, but how the composition is becoming increasingly uniform across all lakes. The proportion of rare species and species groups found only in a few lakes has been declining since 1950, while the proportion of common species has quadrupled.
Climate change and overfertilization as main reasons
The study reveals two main reasons for this trend: rising temperatures and the overfertilization of many lakes in the 1960s and 1970s. The higher temperatures lead to longer-lasting phases in which the lakes are not mixed because warm, lighter surface water does not sink to the depths. In Lake Zurich, for example, the Burgundy blood alga Planktothrix rubescens has benefited from this. It is not popular with water suppliers because it can form red algal carpets - hence the name - and is potentially toxic. "In general, the species that seem to be among the beneficiaries are those that actively influence their vertical position in the lake and can live with less light," says Marie-Eve Monchamp, lead author of the study, "and it's among these species that many are toxic." In addition to Burgundy blood algae, they include Microcystis species and the invasive species Dolichospermum lemmermannii. The latter was previously known mainly from lakes north of the Alps, but is now spreading to the south.
Hardly any turnaround despite environmental changes
It is interesting to note that the trend toward uniformity continues despite the decline in high phosphorus levels from the mid-1970s onward. The authors attribute this, among other things, to the fact that environmental conditions generally vary less and less from lake to lake. Temperature differences between lakes south and north of the Alps have also become smaller 1/2 Eawag: The Water Research Institute of the ETH Domain. Finally, concentrations of nitrogen, the second important nutrient, have remained virtually unchanged since the 1970s.
This study was conducted as part of the project "The impact of cyanobacterial blooms triggered by nutrient pollution on aquatic environments in the context of climate change", which was supported by the Swiss National Science Foundation SNF (No. 142165) .
