Sensitivity of freshwaters to browning in response to future climate change

Many boreal waters are currently becoming browner with effects on biodiversity, fish production, biogeochemical processes and drinking water quality. The question arises whether and at which speed this browning will continue under future climate change. To answer the question we predicted the absorbance (a₄₂₀) in 6347 lakes and streams of the boreal region under future climate change. For the prediction we modified a numerical model for a₄₂₀ spatial variation which we tested on a temporal scale by simulating a₄₂₀ inter-annual variation in 48 out of the 6347 Swedish waters. We observed that inter-annual a₄₂₀ variation is strongly driven by precipitation that controls the water flushing through the landscape. Using the predicted worst case climate scenario for Sweden until 2030, i.e., a 32 % precipitation increase, and assuming a 10 % increase in imports of colored substances into headwaters but no change in land-cover, we predict that a₄₂₀ in the 6347 lakes and streams will, in the worst case, increase by factors between 1.1 and 7.6 with a median of 1.3. This increase implies that a₄₂₀ will rise from the present 0.1–86 m⁻¹ (median: 7.3 m⁻¹) in the 6347 waters to 0.1–154 m⁻¹ (median: 10.1 m⁻¹), which can cause problems for the preparation of drinking water in a variety of waters. Our model approach clearly demonstrates that a homogenous precipitation increase results in very heterogeneous a₄₂₀ changes, where lakes with a long-term mean landscape water retention time between 1 and 3 years are particularly vulnerable to climate change induced browning. Since these lake types are quite often used as drinking water resources, preparedness is needed for such waters.