Freshwater browning, the gradual darkening of lakes and wetlands due to rising concentrations of dissolved organic matter, is a widespread change across boreal regions. A doctoral thesis defended at the University of Helsinki examines how this process shapes the animals that depend on freshwater habitats, from aquatic invertebrates to the Smooth newt.
Browning research has so far focused heavily on lakes and rivers and on planktonic organisms. Dr. Clarisse Blanchet's thesis takes a different approach, combining long-term environmental monitoring, spatial gradients across habitat types, and a multispecies approach to study from ecosystem engineering to communities and individuals. The work draws on data collected in the Evo forest area in southern Finland, where Lammi Biological Station has maintained long-term monitoring of local lakes.
Across lakes, beaver wetlands, and temporary wetlands in the Evo area, increasing watercolor was associated with lower invertebrate taxonomic richness. At the same time, total invertebrate abundance showed a slight increase, suggesting that community composition shifts toward tolerant or generalist taxa rather than declining uniformly.
Habitat type proved to be an equally strong driver of community structure. Beaver wetlands supported the highest invertebrate richness of any habitat studied. Long-term monitoring of two connected lakes in the Evo area also showed that local disturbances, such as beaver flooding, can drive browning at least as strongly as broader regional factors like climate and acid recovery. Beaver wetlands thus provide favorable conditions to invertebrates, despite the negative effect of browning on invertebrate richness. Temporary wetlands, despite high water color variability, harbored the highest total invertebrate abundance. The contrast between these habitats suggests that a diverse network of freshwater environments may partly offset the negative effects of browning at the landscape scale.
In the fourth part of the thesis, Blanchet examined body coloration in Smooth newts captured across sites with varying water color. The results showed that UV brightness decreased across all body parts in darker waters, a pattern consistent with a camouflage response: in water where UV light is strongly absorbed, a body that reflects less UV is less visible to predators.
Male and female newts responded differently. In females, UV brightness decreased more strongly across the belly and tail. In males, the reduction was weaker in those same body parts, which carry signals used in courtship. The finding suggests that males face a trade-off between reducing their visibility to predators and maintaining the visual signals that matter for reproduction. As browning is expected to intensify across many newt breeding habitats, the informational value of UV-based signals may progressively decline.
Blanchet's connection to the research station goes back to the start of her academic career. She began as an intern at Lammi Biological Station in 2019, and the station's long-term datasets and infrastructure have been central to her work throughout the thesis. Lammi is part of the nationwide network of university research stations, which provides researchers with sustained field infrastructure and ecological monitoring across Finland's diverse environments.