New paper on browning of lake waters

A new paper on lake water browning has just been published. Increasing lake water color has been observed in the Northern Hemisphere during recent decades. Browner water has pronounced effects on both the freshwater ecology and society, as it may reduce light transmission, alter aquatic species composition, enhance anoxia and increase the cost of drinking water treatment.  Explanation to increased lake color include less acid rain, as well as changes in climate and land use.

There is often a correlation between dissolved organic carbon (DOC) and color in lakes, and absorbance (both in the visible and UV-range) of terrestrial organic matter is often used as a proxy for DOC. However, there is now increasing evidence that lake color has increased more than DOC, implying that that other factors than DOC contribute to the browning of lakes.

This long-term study from a forested area close to Oslo (Østmarka), SE-Norway, shows that lake color, total organic carbon (TOC), iron and a specific UV-index for organic carbon (SUVA254) have increased with time (1983-2017). In other words, the lakes have become browner, with more aromatic organic matter and a higher content of iron. Regression models showed that TOC and iron together explained 89 % of lake color.

Yihua Xiao doing field work in Lake Solbergvann, Østmarka, SE-Norway in 2018 during her PostDoc stay at NMBU
Yihua Xiao doing field work in Lake Solbergvann, Østmarka, SE-Norway in 2018 during her PostDoc stay at NMBU. Photo: Gunnhild Riise

The color contribution from iron has, however, large variations, ranging from 3-40 %, with an average of 12 % for the 24 studied lakes. Much of the variation is related to transport of colloidal complexes of iron and dissolved organic matter (DOM), lake water retention time and pH. During high flow conditions, the transport of the colloidal complexes of iron and DOM seem to increase, enhancing the contribution of iron to lake color. Increases in hydrological extremes in a future climate may, therefore, enhance the variability in iron, where the impact of iron on lake color will be low in dry and high in wet periods.

Xiao, Y. and Riise, G (2021) Coupling between increased lake color and iron in boreal lakes. Science of the Total Environment. 767. https://doi.org/10.1016/j.scitotenv.2021.145104

Feature photo: Gunnhild Riise