Water quality of a small headwater lake reflects long-term variations in deposition, climate and in-lake processes

Small headwater lakes reflect the changes in atmospheric pollution, and are also indicators of climate change. This study examined water chemistry changes in a pristine small lake in southern Finland during the period 1990-2009. Recovery from acidification was indicated, while no major changes in trace metal concentrations were found. In the course of the study, total phosphorus concentration was fluctuating in deteriorated oxygen conditions in the hypolimnion. However, total and organic nitrogen remained stable, but inorganic nitrogen concentration increased during the period, and was more dependent on the hydrology and in-lake processes than nitrogen deposition. Dissolved organic and inorganic carbon concentrations also increased, presumably due to decreasing acidity and changes in runoff, and increased alkalinity and decreased phytoplankton production, respectively. The results emphasise the importance of integrated intensive long-term monitoring of physical, chemical and biological variables for detecting impacts of changing environmental conditions.