Climate crisis - Water in lakes is turning brown: cold-water fish are suffering
Climate change is also having a major impact on life in lakes. Fish adapted to colder water areas can be affected in two ways, as a research team has discovered: directly through the warming of the water and indirectly through the increasing turbidity of the water, known as browning.
Many lakes are becoming browner as a result of global warming because more organic material is being produced in them and carried in from the surrounding soil - for example during heavy rainfall. This phenomenon can already be clearly observed in countries such as Sweden and also in some regions of Germany.
Browning can significantly reduce the oxygen content of the cooler deep water in particular, as the team led by Stephen Jane from Cornell University in Ithaca explains in the scientific journal "PNAS". Darker water absorbs more heat, which further increases the climate change effect on the surface of the lakes. As a result, stratification between the warmer surface water and the cooler deep water is increased and mixing decreases. This in turn can lead to a lack of oxygen at the bottom of the water, as oxygen is mainly produced in the upper layers - especially as the browning allows less sunlight to reach the depths, which algae need to produce oxygen, as the researchers explain.
Long-term values analyzed
Between 1994 and 2012, the scientists recorded the development of the surface temperature, the oxygen content in the deep water and the dissolved organic carbon in 28 lakes in the Adirondack region in the north-east of New York State. Surface temperatures and carbon levels increased, while the oxygen content in the deep water decreased.
In a supplementary analysis, the consequences of browning for the habitat of brook trout (Salvelinus fontinalis) were investigated using temperature and oxygen data from 15 of the lakes from 2021. In late summer in particular, the habitat suitable for the fish shrank significantly in a large proportion of the water bodies. Like trout and salmon, brook trout belong to the group of salmonids that need cool, oxygen-rich water to live.
Loss of habitat
When comparing the results with historical records for 1467 Adirondack lakes, the researchers found that the char have suffered a significant loss of suitable habitat in most lakes since the 1980s.
The research team concludes from the results that the browning caused by global warming poses an existential threat to populations of cold-water fish in temperate lakes. Deep areas of lakes are only a refuge for such species in the course of climate change if they remain oxygen-rich. However, this is questionable in many cases due to browning. There is therefore a threat of mass extinctions and the disappearance of species from more and more lakes.
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Source: www.stern.de
