Exploring the browning of two small headwater boreal lakes over 25 years: the role of beaver floods, climate, acid recovery and forestry

Many boreal freshwaters have become browner in the past decades, underscoring the need for long-term assessments of brownification drivers. Here, we examined long-term changes in water color (mg Pt/L), beaver activity, climate, acidifying ions concentration in precipitation, and forestry in two small boreal lakes and their catchment in 1994–2018; one lake being occupied by beavers (L. Majajärvi) and one never colonized (L. Horkkajärvi). During the study period, average annual air temperature increased (+0.049 °C per year) while average annual acidifying ion concentrations declined (-0.286 μeq SO 4 2- /L and -0.046 μeq NO 3 - /L per year); annual precipitation showed no trend. Both lakes exhibited browning over time (+3.30-3.42 mg Pt/L per year). Precipitation positively influenced water color in both lakes, while annual SO 4 2- concentration had a negative effect. In L. Majajärvi, water color was browner during beaver floods and afterflood phases, with precipitation and beaver activity explaining a similar share of water color variability, thus underlining their similar role in mobilizing DOC from the riparian zone. In contrast, the effects of annual SO 4 2- concentration on water color will likely subside as other factors will drive lake browning. Forestry and air temperature showed no direct effect, potentially masked by other factors. Our results highlight beavers as ecosystem engineers able to drive water browning locally. This study demonstrates the multifaceted and complex nature of water browning and emphasizes the need for broader-scale assessments integrating both local and global drivers. • Since 1994 annual average air temperature increased while acidifying ions’ concentration declined in Evo • The water color of two small lakes increased by ca. 3.4 mg Pt/L per year during 1994-2018 • Water color increased in both lakes with higher rainfall and lower SO 4 2- concentration • The beaver lake was browner during periods of beaver activities • Both local and global factors are jointly driving the browning of small lakes