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 warming of the water and indirectly through increasing cloudiness of the water, known as browning. Many lakes are becoming browner as a result of global warming because more organic material is created in them and is carried in from surrounding soil – for example during heavy rain. The 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, especially in cooler deep water, as Stephen Jane’s team from Cornell University in Ithaca explains in the specialist magazine “PNAS”. Darker water absorbs more heat, which increases the climate change effect on the surface of the lakes. As a result, the formation of layers between the warmer surface water and the cooler deep water increases and mixing decreases. This in turn can lead to a lack of oxygen at the bottom of the water, as oxygen is mainly formed in the upper layers – especially since 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 surface temperature, oxygen content in deep water and dissolved organic carbon in 28 lakes in the Adirondack region in northeastern New York state. Surface temperatures and the amount of carbon increased, while the oxygen content in the deep water decreased.
A complementary analysis explored the consequences of browning on brook trout (Salvelinus fontinalis) habitat using temperature and oxygen data from 15 of the lakes from 2021. Especially in late summer, the habitat suitable for the fish shrank significantly in a large part of the waters. Like trout and salmon, brook trout belong to the group of salmonids that need cool, oxygen-rich water to live.
Loss of habitat
Comparing the results with historical records for 1,467 Adirondack lakes, the researchers found that charr have suffered a significant loss of suitable habitat in most lakes since the 1980s.
The research team concludes from the results that browning caused by global warming represents an existential threat to populations of cold-water fish in temperate lakes. In the wake of climate change, deep areas of lakes only provide a refuge for such species if they remain oxygen-rich. However, this is questionable in many cases due to browning. There is therefore a risk of mass extinctions and the disappearance of species from more and more lakes.
