Atmospheric fluxes are an important component of the global cycle of many elements. These fluxes are often caused by long-range transport of elements from distant emission sources to receptor areas, where they are scavenged by precipitation and deposited on land or ocean. In each environment, the atmospherically deposited elements may have an important effect on biogeochemical cycling in the existing ecosystems.

Atmospheric long-range transport takes place in the free atmosphere above the mixing boundary layer, which generally has a depth of 1,000-1,500 m. The chemistry of the free atmosphere can be thus considered to reflect the composition of the global atmosphere better than the boundary layer below it. High mountains protrude above the boundary layer, intercepting the global flux of elements. Therefore, high elevation stations are one of the best settings to monitor the long-range (i.e. regional and global) transport of chemical elements. In addition, high mountains are usually free of direct local impacts that could mask global background signals.

In the lake Redon station we have been monitoring the deposition of trace elements since 2004. Here we analyse the time series of data in order to determine the variation in deposition over the past decade, and to examine in detail the influence of climate and the changes in anthropogenic emissions on the trends detected.

Time trend of DIN in the Pyrenean lake district. Box-plots showing the decreasing evolution of the distribution of DIN concentration in water from lakes in the central Pyrenees from 1987 to 2011. The box-plots show medians (line within the box), 25th and 75th percentiles (sides of the box), 10th and 90th percentiles (error bars) and 5th and 95th percentiles (circles). Data are from four synoptic surveys carried out in the years indicated on the graph; the number of lakes included (n) is also indicated.

Humans have dramatically disturbed the earth's N cycle to the point that this alteration is considered one of the most serious threats to the global ecosystem. Part of the excess N circulates through the atmosphere and is deposited with precipitation, reaching areas that would otherwise be free of direct human impact. Atmospherically deposited N has an acidifying effect on aquatic ecosystems, and most studies of N pollution have focused on this aspect.

 However, recent research has also addressed the global impact of atmospherically deposited N as a nutrient in lakes. Recent findings indicate that increased atmospheric deposition of reactive nitrogen of human origin has caused changes in the pattern of ecological nutrient limitation in lakes in the northern hemisphere. An increase in the stoichiometric ratio of nitrogen (N) to phosphorus (P), and hence a shift from pristine N limitation to human-induced P limitation of phytoplankton growth, seems to have been driven by deposition of atmospheric N. These findings challenge the classical paradigm of lake productivity being naturally limited by P availability. However, deposition of atmospheric P may also be highly relevant.