Acid rain — a strong external driver that has suppressed water colour variability between lakes

Increasing water colour observed in lakes in the northern hemisphere is frequently explained by several factors, including the decrease in acid deposition, climate change and recently increased concentrations of Fe. As the anthropogenic sulphate deposition levels off, pressure from an external lake quality driver with regional coverage declines. To investigate the impact of acid rain reduction on lake colour variability, we examined 25 lakes in a lake- district of southeastern Norway by analyzing atmospheric deposition, climate and water chemistry data from 1983 to 2012. We observed a marked shift in lake colour after the wet year 2000, probably triggered by a flush of water that has lifted the base line for lake colour to a higher level. Lakes had synchronous temporal trends of many water quality variables, such as conductivity and several major ions. Our data suggest that this is a response to reduced acid deposition. In contrast, lake colour and colour related variables such as Fe and TOC, showed moderate to low coherence. We propose that declined pressure from a strong external driver promotes the importance of climate variability and local catchment specific processes, giving rise to increased colour variability between lakes with time.