The influence of nitrogen and dust deposition on alpine aquatic biogeochemistry

Human modifications of the landscape including agriculture, grazing, and industry have altered the chemistry of the atmosphere changing the global mobility of key macronutrients. Elevated amounts of nitrogen and dust in the atmosphere are of particular interest because they are limiting nutrients in freshwater ecosystems. Upland lakes are ideal locations for evaluating spatial and temporal changes in deposition chemistry. The surface water composition of high elevation lakes in catchments with little to no vegetation should reflect contributions from catchment geology (Ca, Mg, Na, K) and atmospheric nutrient deposition (N,P). Alpine watersheds typically have slow weathering rates due to cold temperatures, short snow-free seasons, little to no vegetation cover, short hydraulic residence times, and small catchments. Consequently alpine lakes are characteristically nutrient poor and are sensitive to variations in atmospheric deposition. Due to their inherent oligotrophy, aquatic ecosystems in high elevation lakes should be among the first lakes to show a response to increased dissolved, mineral, and organic deposition. Because these lakes are remote, they are typically not impacted by other human activities. We collected wet and dry atmospheric deposition data across depositional gradients of nitrogen and dust in the southwestern USA and compared the results to lake water chemistries. Several results stand out. 1) Spatial and temporal variations show strong correlations between atmospheric deposition and lake chemistry, illustrating the sensitivity of alpine lakes to remote human disturbance. 2) Shifts were not exclusive to alpine lakes alone; though effects are stronger at higher elevations, 3) Recovery after alleviation of deposition may be rapid, though in many locations deposition will continue to increase. These results are important because the absolute and relative amount of nutrient and base-cation availability can have strong influences on water quality and aquatic species composition.