Post-Acid Rain Recovery with Increasing Calcium Concentrations - Assessing relationships between trends in base cations relative to trends in sulphate, organic anions and bicarbonate

Summary

This study assesses long-term trends in surface water chemistry using data from the Norwegian national monitoring and the ICP Waters programs that originally focused on the effects of reduced sulphuric acid deposition. As was anticipated, in acid-sensitive regions heavily effected by acid rain, the widespread and substantial decline in sulphate concentrations has led to a corresponding decrease in calcium concentrations. Conversely, in regions less impacted by acidification, calcium concentrations are increasing in a large number of sites, largely in response to rising bicarbonate levels. At these sites, the increase in bicarbonate contributes to a higher acid neutralizing capacity (ANC), which more than offsets the decline in sulphate. The most likely drivers of this increase in bicarbonate include a warming climate, which promotes longer growing seasons and increased biomass production, thereby enhancing soil respiration and CO2 exchange (i.e., CO₂ pump). This process increases weathering rates and the release of base cations such as calcium. As the decline in acid deposition has stabilized and surface water pH increases above 5.6 in formely strongly acidified areas, similar increases in calcium concentrations may eventually be observed there as well. However, statistically separating the effects of acid rain recovery from those of climate change on water chemistry remains a significant challenge. The observed calcium trends may have ecological implications. Interestingly, in regions most severely impacted by acidification, no significant correlation was found between the declining calcium concentrations and biodiversity indices. In contrast, in less-affected regions, strong negative correlations were observed - contradicting conceptual expectations and highlighting the need for further investigation into how aquatic ecosystems responds to changing chemical conditions.