In most lakes, there are millions of small creatures that generally remain hidden from our eyes. What they have in common is that they are suspended in water and move with the current. That is why, altogether, they are called plankton, which in Greek means “the one that wanders around.” Not only is there an incredible diversity of sizes and shapes among the plankton, but also of ways of life.

Relationships provide stability

Plant plankton (phytoplankton), which include green algae and diatoms, use the sun as an energy source and produce the substances they need to grow with the help of sunlight, CO2 and water. These primary producers form the basis of food webs in water bodies. The first to benefit from them are the animal plankton (zooplankton), which includes, for example, small rotifers and ciliates or water fleas that graze on algae. These creatures, in turn, provide the food base for predatory zooplankton species – which are subsequently eaten by larger predators such as fish.

But the interplay is not limited to eating and being eaten. Species also interact with each other, for example, by competing for sources of food or when they can thrive better under the protection of another species. All of these multiple interactions not only regulate the food web, but also provide stability to the entire aquatic ecosystem.

Warming reduces degree of connection

In spite of the great importance of this highly complex plankton network, little research has been done on how it responds to two of the most important anthropogenic threats – climate change and nutrient inputs caused by over-fertilisation. We know that higher water temperatures and changing phosphate concentrations affect the abundance and diversity of plankton communities in our lakes. However, it is still largely unknown how this affects interactions between species.

The aquatic research institute Eawag has now succeeded for the first time in providing substantiated statements on this subject. The results were recently published in the journal Nature Climate Change. Ewa Merz, ecologist and first author of the study, summarises: “We have found that the warming of lakes, as we have observed in recent decades, reduces interactions in the plankton network. There are fewer interactions and they are weaker. This decline is particularly pronounced when lakes simultaneously have high phosphate levels.” If the nutrient content in a body of water like Lake Zurich increases even slightly, this could already have dramatic consequences for the entire network in a warming world and destabilise the ecosystem. According to Merz, this could threaten not only a loss of species, but also a decline in ecosystem services – such as lower water quality due to increasing cyanobacterial blooms or a decline in fish populations due to changes in the food web.