Relative role of decreasing precipitation sulfate and climate on recent lake recovery

In the last 2 decades, acidic sulfur (S) depositions and SO(4) concentration in surface water significantly decreased in many regions of the world, while at the same time temperatures have been increasing, particularly in the decade 1995-2005, which was one of the warmest ever recorded in the Northern Hemisphere. Understanding the potentially antagonistic or additive effects of decreasing S depositions and increasing temperatures on lake chemistry is essential to adequately evaluate the effectiveness of reduction programs for S emissions. By assessing the rate of change in climate variables, precipitation SO(4) and lake chemistry for 47 temperate and boreal catchments, we found that changes in climate, particularly higher annual temperatures, were more often correlated to lake's pH and alkalinity than was the rate of decreasing S depositions between 1989 and 2005. It suggests that the recent improvement in the acid-base status of the studied lakes that cover a large area of northeastern North America as well as a large array of acid-base conditions cannot be attributed solely to a decrease in SO(4) precipitation, but also to climatic variations. Not taking this information into account could lead to an overestimation of the benefit of lower SO(4) deposition on aquatic ecosystems recovery.