Algae and water fleas in lakes: Light color influences food webs

Phytoplankton is an important component of the food-web and is predated by a wide variety of aquatic organisms, such as water fleas, copepods and fish. These microscopic algae also play a crucial role for Earth's climate, as they absorb significant amounts of carbon dioxide (CO2) and produce oxygen in lakes and oceans worldwide. Phytoplankton obtains the energy needed for these processes from sunlight that penetrates through the water column. The light spectrum, which encompasses all the colours of the rainbow – from violet to indigo, blue, green, yellow, orange, and red – plays a crucial role as well. Researchers have now discovered that these light colours influence not only phytoplankton, but also the predator-prey relationships in the lake. This was shown by a recent study conducted at the Institute for Chemistry and Biology of the Marine Environment (ICBM), University of Oldenburg, in collaboration with the University of Greifswald. It was published October 13^th 2025 in the Journal of Ecology.

Sensitivity of lake food webs

“So far, research on the ecological processes in water bodies has focused primarily on the amount of light,” says study author Sebastian Neun from the Plankton Ecology Group at ICBM. “We showed that the colour of light has a direct influence on phytoplankton, which in turn affects nutrient cycling in the lake ecosystem.”

According to the scientists, future research should take the light spectrum into greater consideration. “The amount of microalgae in many lakes is increasing, making them increasingly green,” explains study author and planktologist Dr. Maren Striebel. “Light conditions underwater will change even more in the future, and this will influence the sensitive relationship between microalgae and other more complex aquatic organisms such as crustaceans and fish.” High nutrient content of wastewater and agricultural runoff is promoting the growth of algae, especially in combination with higher temperatures, and leading to algal blooms in water bodies.

Lake near Wilhelmshaven becomes a research laboratory

In May 2022, the research team conducted an experiment in Lake Schortens, a lake near Wilhelmshaven in Lower Saxony. The scientists filled bottles with lake water containing phytoplankton, wrapped them with red, blue, and green light filter foils and placed them at different positions underwater.

Over a period of two weeks the team monitored how the microalgae developed under different light conditions and different nutrient levels. Phytoplankton was then fed to water fleas to draw conclusions about how different light and nutrient conditions influence the nutritional quality of phytoplankton for their consumers.

The result: the less light that is available to the microalgae underwater, the more important the colour of light became for their growth. Different phytoplankton species also reacted differently to light colour and nutrient content by altering the composition of fatty acids. This in turn influenced the growth of the water fleas, suggesting that the colours of light also influence lake food webs. “We can conclude that the spectrum of underwater light has a much greater influence than previously assumed,” says Neun.

The team around Neun and Striebel is already investigating the special role of different light colours for phytoplankton in a three-year research project. Once again, the focus is on Lake Schortens. Currently, the response of various phytoplankton species isolated from the lake is being tested in Plankton Ecology laboratories in Wilhelmshaven under different light conditions.