Disruptions and re-establishment of the calcium-bicarbonate equilibrium in freshwaters

During recent decades, increasing anthropogenic activities have affected natural ionic composition, including the strongest and most common relationship between ionic concentrations in the majority of natural global freshwaters, i.e., the Ca²⁺-ANC (acid neutralizing capacity) equilibrium. Using long-term monitoring data and MAGIC modelling, we evaluated effects of major present environmental stressors (synthetic fertilizers, liming, acidic deposition, forest disturbances, and climate change) on the Ca²⁺-ANC equilibrium. We evaluated the effects for three different types of terrestrial ecosystems, a circumneutral lowland agricultural catchment, two acid sensitive mountain forest catchments differing in forest health, and one acid sensitive alpine catchment. All catchments are in a region with the world-largest changes in fertilizing rates and acidic deposition in the 20^th century, with increasing impacts until the late 1980s, and their subsequent abrupt, dramatic decreases. These strong changes resulted in a substantial disruption, followed by continuing re-establishment of the Ca²⁺-ANC relationship in all study waters. The shape of the disruption and the following re-establishment of its new value were dependent on the intensity, duration, and combination of stressors, as well as on catchment characteristics (bedrock composition, soil amount and composition, vegetation status, and hydrology). We conclude that a new equilibrium may deviate from its natural value due to the (1) legacy of fertilizing, acidic deposition and liming, affecting the soil Ca²⁺ pools, (2) forest disturbances and management practices, and (3) climate change.