Winter atmospheric nutrient and pollutant deposition on Western Sayan Mountain lakes (Siberia)

Abstract

The world map of anthropogenic atmospheric nitrogen deposition and its effects on natural ecosystems is not described with equal precision everywhere. In this paper, we report atmospheric nutrient, sulfate and spheroidal carbonaceous particle (SCP) deposition rates, based on snowpack analyses of a formerly unexplored Siberian mountain region. Then, we discuss their potential effects on lake phytoplankton biomass limitation. We estimate that the nutrient depositions observed in the late-season snowpack (4016 mgNO3-Nm2 and 0.580.13 mg TP-Pm2; TP for total phosphorous) would correspond to yearly depositions lower than 11971 mgNO3-Nm2 yr1 and higher than 1.710.91 mg TP-Pm2 yr1. These yearly deposition estimates would approximately fit the predictions of global deposition models and correspond to the very low nutrient deposition range, although they are still higher than world background values. In spite of the fact that such a low atmospheric nitrogen deposition rate would be enough to induce nitrogen limitation in unproductive mountain lakes, phosphorus deposition was also extremely low, and the resulting lake water N: P ratio was unaffected by atmospheric nutrient deposition. In the end, the studied lakes' phytoplankton appeared to be split between phosphorus and nitrogen limitation. We conclude that these pristine lakes are fragile sensitive systems exposed to the predicted climate warming, increased winter precipitation, enhanced forest fires and shifts in anthropogenic nitrogen emissions that could finally couple their water chemistry to that of atmospheric nutrient deposition and unlock temperature-inhibited responses of phytoplankton to nutrient shifts.