The role of land-use and land-cover change to browning of surface waters

Browning – that inland waters are becoming increasingly brown – is widely observed in northern regions, caused by increased leaking of dissolved organic carbon from the catchment. Browning alters the ecosystem function, e.g. impaired light penetration reduces primary production, which in turn leads to reduced fish production. Another consequence is that it gets more challenging and expensive to produce drinking water.
Many studies investigating underlying drivers of browning emphasise the recovery of acidification as a major factor, the reasoning being that soil organic carbon is less soluble and mobile at high acidity. This suggests that inland waters were unnaturally clear during the acidification period, and that current browning is a return to a more natural state common before acidification. However, results from our group are at odds with this theory. Uniquely long time series show that inland waters in southern Sweden were much clearer before acidification than they are today and that water color did not decline when acid deposition rose.
The results instead suggest that a major transition in land-use and land-cover, including passive and active afforestation of abandoned agricultural land, ceased forest grazing and introduction of spruce dominated forest management, is the primary driver behind browning.
The hypothesis of this project is that afforestation favoring coniferous species has led to a buildup of organic soil layers, which leak dissolved organic carbon to surface waters, and that this is a major cause of browning. The project builds on reconstruction of historical land-cover in individual lake catchments by interpretation of aerial photographs from the 1940s and onwards. By combining information on land-cover change and historical records of water color (up to 90 yrs), we can test if browning is linked to afforestation. If we find support for this, the results could contribute to recommendations for future forest management to mitigate browning.