Abstract
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 (40 ± 16 mg NO3-N m−2 and 0.58 ± 0.13 mg TP-P m−2; TP for total phosphorous) would correspond to yearly depositions lower than 119 ± 71 mg NO3-N m−2 yr−1 and higher than 1.71 ± 0.91 mg TP-P m−2 yr−1. 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.
Highlights
Worldwide nitrogen cycle perturbation is the second most important global environmental concern, just after massive extinction of species and even more important than global warming (Rockström et al, 2009; Steffen et al, 2015)
The Ergaki Natural Park in the Western Sayan Mountains was reckoned to be an area above background levels but of low atmospheric nutrient deposition
While nitrogen values were comparable to the lowest records in other mountain areas elsewhere in the world, phosphorus deposition was likely at the very lowest range ever measured on terrestrial ecosystems before
Summary
Worldwide nitrogen cycle perturbation is the second most important global environmental concern, just after massive extinction of species and even more important than global warming (Rockström et al, 2009; Steffen et al, 2015). The anthropogenic mobilization of formerly inaccessible nitrogen compartments has more than doubled natural nitrogenase-mediated inputs of reactive nitrogen forms into the global nitrogen cycle (Vitousek et al, 1997). Massive fossil fuel combustion since the industrial revolution, chemical fixation of atmospheric diatomic nitrogen to produce fertilizers since the Second World War and the wide extension of leguminous crops are the most important human sources of nitrogen cycle perturbation (Vitousek et al, 1997). A substantive part of this anthropogenic reactive nitrogen is spread, air-transported and deposited all over the world with a diverse impact on different ecosystems. A series of studies all over Sweden and abroad showed atmospheric nitrogen deposition turned unproductive lake phytoplankton from natural nitrogen to in-
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