Ecological changes in two contrasting lakes associated with human activity and dust transport in western Wyoming

The atmospheric transport and deposition of aerosols has the potential to influence the chemistry and biology of oligotrophic alpine lakes. In recent decades, dust and nitrogen emissions to alpine ecosystems have increased across large areas of the western U.S., including Wyoming. Here, we use sediment geochemistry and Sr/Sr and Nd/Nd isotopes to examine historical dust deposition rates to alpine lakes in the southwestern region of the Wind River Range, Wyoming. We evaluate the biological response using diatom fossil assemblages and sediment pigment concentrations. Sediment core analyses indicated that prior to a recent rise in dust flux, phosphorus concentrations and species composition were similar to those found in other alpine lakes in the region. Concomitant with a 50 fold increase in dust flux to the sediments circa 1940, sediment proxies revealed a twoto threefold increase in normalized sediment phosphorus content, an increase in the diatom-inferred total dissolved phosphorus concentration from 4 to 9-12 lg L, a tenfold increase in diatom production, and a relative increase in cyanobacteria abundance. The increase in dust influx during the 20 century appears to be due in part to human factors and demonstrates the potential for dust and other atmospheric pollutants to significantly alter remote aquatic ecosystems. Human land-use including agriculture, grazing, and industry are altering regional biogeochemical cycles through elevated emissions of nitrogen compounds (Galloway et al. 2003) and dust to the atmosphere (Neff et al. 2008). Mountain ranges act as natural barriers to atmospherically transported material where deposition may occur as dry fall or dissolved in precipitation. Because alpine lakes are characteristically nutrient poor and derive the bulk of their nutrients from atmospheric sources, these lakes are very sensitive to variation in atmospheric deposition (Psenner 1999; PulidoVillena et al. 2006; Mladenov et al. 2009). Over the last two decades, dust emissions have increased in many regions of the western U.S., in some areas by up to 400% (Brahney et al. 2013). The effects of dust deposition on alpine lakes can be challenging to determine because dust is not routinely measured, and because nitrogen (N) and dust deposition may be co-occurring in a variety of western U.S. locations. Many mountain ranges in western states are downwind of urban, industrial, and agricultural sources of N as well as arid and semiarid regions that regularly produce dust storms (Fenn et al. 2003; Painter et al. 2007). The influence of nitrogen deposition on alpine lake ecology has been investigated in several regions of the U.S. and has been shown to cause large changes in ecological and biogeochemical processes (Baron et al. 2000; Elser et al. 2009). Relatively less is known about the contribution of nutrients and alkalinity from dust including the deposition of phosphorus (P) and calcium carbonates to aquatic ecosystems (Morales-Baquero et al. 2006; Pulido-Villena et al. 2006). In the mountain ranges of the interior western U.S. and in other mountain ranges near arid and semiarid regions, the erosion of desert and agricultural soils has the potential to bring nutrients, particularly phosphorus (within soils, organisms, and phosphate minerals), acid-neutralizing minerals (carbonates), and heavy metals to alpine lakes. Measurements of dust chemistry have shown that dust can transport appreciable amounts of phosphorus and carbonates to depositional *Correspondence: [email protected] Present address: Department of Ecosystem and Conservation Science, University of Montana, Missoula, Montana 59812, USA ‡Present Address: Department of Earth and Environmental Sciences, University of British Columbia, Okanagan. British Columbia, VIV 1V7, Canada, 1-250-807-8207 1 LIMNOLOGY and OCEANOGRAPHY Limnol. Oceanogr. 00, 2015, 00–00 VC 2015 Association for the Sciences of Limnology and Oceanography doi: 10.1002/lno.10050