Sebastian Neun, Institute for Chemistry and Biology of the Marine Environment (ICBM) at the University of Oldenburg, Germany, discusses his article: Light spectrum matters: Interactive effects of light and nutrients on phytoplankton communities and trophic transfer
Where is the rainbow?
All lakes are characterised by a tightly linked network of food chains, and phytoplankton forms the basis of almost all of these, even though most species are so small that they are invisible to the human eye. To provide this service, phytoplankton requires the sufficient availability of nutrients and light for photosynthesis. However, a special feature underwater is that, unlike air, water strongly influences the availability of light. Roughly half of the light reaching the water surface is attenuated in the top few centimetres. In addition, the visible spectrum of solar light is composed of different light colours: violet, blue, green, yellow, orange, and red (the typical colours of the rainbow). Not all the colours can penetrate that deep because there are so many substances in the water which interact with the incoming radiation by absorbing or scattering the different wavebands.
What is at the end of the rainbow?
Phytoplankton possess a variety of light-absorbing molecules (pigments) to harvest solar energy for photosynthesis. The ability to absorb the different light colours varies greatly from species to species. Therefore, not only the intensity but also the spectrum of light is important for phytoplankton, although comparatively little is known about the role of light spectrum in ecological processes.
In our study, we investigated the ecological significance of the light spectrum and how it influences the community structure and biochemical composition of phytoplankton. Since the uptake of light and nutrients in phytoplankton are loosely coupled, the ratio of fixed carbon to nutrients in their cells can vary greatly and depends critically on the environmental conditions, as does the production of more complex molecules. Zooplankton, especially water fleas (Daphnia), need a balanced diet and are very sensitive to a lack of nutrients and fatty acids because both are mandatory for their growth. But how do the abiotic factors of the environment like nutrient supply, light intensity, and light colour influence this interaction between phytoplankton and zooplankton? In a field experiment, we filled bottles with the natural phytoplankton community, shaded them with different light filters, and placed them at different water depths. We thus manipulated the available light intensity, spectrum, and amount of nutrients for the phytoplankton.
We found that essentially all these factors influenced the community structure and biochemical composition of phytoplankton. The colour of light became particularly relevant when light intensity (and not nutrients) limited phytoplankton growth. Under such conditions, phytoplankton communities are very sensitive to the available light spectrum and change their species composition depending on the colour of the light. Changes in community composition further affected the nutritional quality of the phytoplankton for their primary consumers, resulting in altered growth of herbivorous Daphnia. We observed both differences in the nutrient content and fatty acid composition of phytoplankton, induced by the interactive effects of nutrients and light, including its wavelength spectrum.
Take away
We showed that not only light intensity, but also the light spectrum, plays a significant role for plankton in freshwater. Our results suggest that light intensity, light spectrum, and nutrients interactively act at the base of aquatic food webs by altering the species composition and biochemical composition of phytoplankton, particularly in scenarios where light availability is constrained. These effects propagated along the food chain and impacted the performance of herbivorous zooplankton. Thus, we demonstrate that the light spectrum is a significant component of the abiotic environment that influences both plankton dynamics and trophic transfer in aquatic ecosystems.
