Lake Plußsee (in the district of Plön, Schleswig-Holstein) is 29 meters deep and roughly the size of 20 football fields. It is home to numerous plant and animal plankton species that serve as food sources for larger organisms. My colleagues from the KlimaCampus and I are currently working to investigate how climate change has affected the lake’s animal plankton. Our analysis of the statistical data shows that both the water temperature and oxygen concentration changed significantly between 1969 and 2006 – with consequences for the small organisms.
In this regard, water circulation is extremely important. In summer and winter the lake water forms different water layers of varying temperatures and densities. Given this stable structure, nutrients and oxygen can barely be exchanged, and by the end of this stratification period the amount of nutrients in the upper layer is greatly reduced, as is the oxygen level at the bottom of the lake. This forces some species of the animal plankton to move to other depths, where they then have to compete with other species. It is only when the water is again mixed that the vital nutrients and oxygen are again distributed throughout the entire lake.
This complete mixing of the lake water normally happens in the spring and fall. As the temperatures at different depths grow closer together, the stark differences in density also fade away. During this phase the plant plankton can thrive: nutrients can be found once again, and there is enough light for photosynthesis. And, when the microalgae bloom, there is also ample food available for the animal plankton, allowing a variety of species to flourish.
Warmer temperatures extend the stratification period
In the wake of climate change, the air temperatures in spring and winter have risen, as a result of which the stratification period sets in earlier and the uppermost water layer is sooner and more intensively warmed. For the timeframe of our study, the recorded data shows that the average temperature in the upper water layer in April has risen by roughly three degrees; in summer, there is a similar rise.
Since the increasingly warmer surface layer enhanced the stability of stratification, the mixing phase in the fall now tends to start later in the year, and the period characterized by a stable water column, which also means competition for nutrients and a lack of oxygen, is now roughly three weeks longer.
And the plankton species are adapting: Warm-water species profit from these conditions and now enjoy a longer active phase. In contrast, cold-water species are increasingly being forced to withdraw to deeper and cooler water layers. Or they are outcompeted by other species that are better able to adapt to the changed conditions. Further, in the fall those species that can get by with less food are at a distinct advantage.
In short, while some species of plankton benefit from these changes, others are disadvantaged, which in turn is introducing a new dynamic to the structuring of the species composition. As plankton forms the basis of the ecological pyramid, this may very well affect the diets of fish and other larger organisms that depend on them.