Our winters are becoming milder and, as a result, many lakes freeze over less and less often. However, this has far-reaching consequences, because the ice cover brings about fundamental changes for water and inhabitants – and most aquatic organisms have adapted to this. Scientists have now examined in more detail the consequences for the limnic ecosystems if there is no winter rest under the ice.
There are about 100 million lakes worldwide. Of these, most lie in the cool regions beyond the 45th parallel north, meaning these lakes freeze over fairly regularly in winter. The ice cover isolates the lakes from the surrounding landscape and atmosphere, thereby affecting fundamental physical, chemical, and biological processes in these waters. “The sea ice affects the transfer of energy, heat, light and material between the lakes and their surroundings, creating conditions that are dramatically different from those in open water,” explain Emily Cavaliere of the University of Saskatchewan and her colleagues.
Isolated world under the ice
The research team investigated in more detail the consequences of a fluctuating or absent winter ice cover on these processes within the framework of three case studies. “We know that lake ice cover will decrease, but so far we have lacked a conceptual framework to understand and predict the effects of such changes on the structure and function of ecosystems,” explains co-author Stella Berger from the Leibniz Institute for Freshwater Ecology and inland fisheries (IGB) in Berlin. The team’s sea-ice continuum concept allows the changes in an ice-covered lake and the effects to be described in more detail.
For example, the thickness and optical properties of ice and snow regulate the amount of solar radiation entering the lake while shielding it from the wind. As a result, the ice cover also regulates the mixing of the water and influences the vertical thermal and chemical gradients of the lake. This is important, for example, for the availability of oxygen in the water column. At the same time, the changed water temperature also has an impact: “In a lake without ice, the water warms up faster in spring, which can result in the development of heat-loving blue-green algae. As a result, the water quality can deteriorate,” says the researcher.
Effects into the following summer
Another factor is nutrients: “The supply, accumulation and conversion of nutrients and carbon in the winter creates the conditions for the primary producers in the spring,” explains Berger. The winter therefore decides on the food that is available in spring and summer for the algae and thus the basis of the food web. For example, the team found that high chlorophyll concentrations in winter are coupled with low chlorophyll levels in the following summer. Apparently, the higher productivity of the algae in a mild winter means that there is a lack of nutrients in the following months. “So if the metabolic activities in the lake are really boosted in winter due to the low ice cover, this can reduce the amount of food for living beings in summer,” says Berger.
All of this not only affects the primary producers in the lake, but also the entire habitat of the water body via the food chain. As the scientists explain, if the ice cover continues to decrease, there will be winners and losers at all levels of the food web. The latter mainly includes organisms whose seasonal activity is strongly adapted to the winter rest period of the water body and to the strong contrast between summer and winter. Because they occupy a rather specialized niche in the ecological structure of the limnic environment, the loss of ice will disadvantage these species in particular. This phenomenon can already be observed in fish, explains the research team.
For the future this means: As a result of climate change and the increasingly mild winters, the composition of the biocoenosis in the lakes will change – also in our latitudes. According to Cavaliere and her colleagues, the dropping out of some predominantly cold-loving species will lead to ecological cascade effects that will result in shifts in the entire spectrum of species.
Source: Leibniz Institute for Freshwater Ecology and Inland Fisheries (IGB); Article: Biogeosciences, doi: 10.1029/2020JG006165