During the last decades, surface waters have become browner throughout the Northern Hemisphere. Acid recovery from past acid rain, and increased precipitation, owing to climate change, are now contributing to this effect.
Over the past 30 years, Lammi Biological Station at the University of Helsinki has carried out water quality surveys in 20 lakes of the Evo Natura 2000 area in Southern Finland. Over that time period, water browning has increased in each of the studied lakes.
Water colour is not just a visual effect; it is an important factor determining the structure and function of aquatic food webs.
By combining the water colour data with aquatic invertebrate data that had been collected from 1989 to 2008 in five of the Evo area lakes, researchers found that when the water gets browner, the abundance of aquatic invertebrates declines.
“Aquatic invertebrates are the main food source for many secondary predators such as fish and waterbirds. Diminishing invertebrate populations may have major consequences for boreal ecosystem functioning. We also have a study showing a decline in the number of breeding pairs of waterbird that feed on invertebrates in boreal lakes in the same period,” says Dr. Céline Arzel from the Department of Forest Sciences, Faculty of Agriculture and Forestry, University of Helsinki.
Both results call for further investigations of biodiversity loss and its drivers in boreal lakes, to understand how, and how much, the whole ecosystem, its functions and services are impacted by water browning.
This study was made possible thanks to the financial support of the Finnish Cultural Foundation Häme Regional Fund allocated to Céline Arzel.
Arzel C., Nummi P., Arvola L., Pöysä H., Davranche A., Rask M., Olin M., Holopainen S., Viitala R., Einola E., Manninen-Johansen S. 2020. Invertebrates are declining in boreal aquatic habitat: The effect of brownification? Science of the Total Environment, Volume 724, 138199. DOI: https://doi.org/10.1016/j.scitotenv.2020.138199
Johan Elmberg, Céline Arzel, Gunnar Gunnarsson, Sari Holopainen, Petri Nummi, Hannu Pöysä, Kjell Sjöberg. 2019. Population change in breeding boreal waterbirds in a 25‐year perspective: What characterises winners and losers? Freshwater Biology 9 October 2019.