Recent changes in production in oligotrophic Uinta Mountain lakes, Utah, identified using paleolimnology

We use multiple proxies from lake sediment records of six remote alpine lakes in the Uinta Mountains, Utah, to investigate primary production and potential drivers of changes in trophic status over the last two centuries. Chlorophyll a, chlorophyll a flux, and percentage of organic matter (determined by loss on ignition) increase beginning in the mid-20th century in five of the six alpine study lakes, indicating increasing production. These changes tend to coincide with decreasing sedimentary d15N and increasing abundance of the nitrophilous diatom Asterionella formosa. An earlier, more subtle change in diatom community composition and d15N indicates that human activities prior to 1950 had measurable effects on these lake ecosystems, although no corresponding increase in primary production was observed at this time. Multiple factors can lead to increased primary production at alpine sites, but the evidence presented shows that enhanced atmospheric deposition of nitrogen and phosphorus explain the recent, more prominent increase in production. Although previous research has suggested that alpine lakes are nitrogen limited, our results suggest nutrient limitation varies spatially and temporally, and is complex in these oligotrophic systems. This and other factors, such as catchment characteristics, will affect the sensitivity of a lake to atmospheric deposition–fertilizing effects. The changes observed show that remote lakes are vulnerable to long-distance transport of nutrients, and that the risk of eutrophication could be intensified by increased nutrient inputs from expanding mining, fossil fuel combustion, and agriculture and by rapid warming predicted for the southwest. In this study, we investigate the potential drivers of changing primary production since the 19th century in alpine lakes in the Uinta Mountains, Utah. Because phosphorus (P) and nitrogen (N) are the elements that most often limit freshwater production, changes in inputs of N or P can alter ecosystem processes, reduce biodiversity, and lead to surface-water eutrophication (Vitousek iet al. 1997). Although few symptoms of eutrophication have been reported for Uinta Mountain lakes, the rapid industrial, agricultural, and urban expansion occurring upwind of the Uinta Mountains along the Wasatch Front may put Uinta Mountain lakes at risk of enhanced deposition of nutrients, in particular fixed N species (NO{3 and NH z 4 ) and P, which are delivered mainly by precipitation and dust, respectively. In the last century, humans have drastically increased the amount of biologically available N (NO{3 and NH z 4 ) on a global scale, largely via synthetic fertilizer use, fossil fuel burning, intensive animal husbandry, and the cultivation of leguminous crops (Vitousek et al. 1997). Records maintained by the National Atmospheric Deposition Program National Trends Network (NADP NTN; http://nadp.sws. uiuc.edu/ntn/) for the U.S.A. show that on a national scale, areas with the greatest wet deposition of NO{3 and NH z 4 generally coincide with areas of high population density and intensive agriculture. In these areas, runoff from fields and urban areas, sewage, and industrial effluent tend to be much greater sources of N to aquatic systems than atmospheric deposition. In contrast, in remote alpine sites, atmospherically derived N makes up a larger proportion of anthropogenic N inputs, as many of the other sources are absent. High-resolution modeling of N deposition that combines NADP NTN records with United States Geological Survey (USGS) annual snowpack surveys and precipitation models reveals that remote high-elevation sites have some of the highest levels of atmospheric nitrate deposition in the western U.S. (Nanus et al. 2012). Elevated atmospheric N deposition resulting from N volatilized from farmer fields and feedlots or from fossil fuel combustion has been linked to subtle ecological changes in alpine, arctic, boreal, and temperate sites in the Northern Hemisphere (Holtgrieve et al. 2011). Although it is evident that recent values of d15N in lake sediments are decreasing in a variety of remote aquatic systems (Holtgrieve et al. 2011), there are still uncertainties regarding the influence of factors other than enhanced atmospheric N deposition on primary production in dilute alpine systems (Catalan et al. 2013). For example, P has been given relatively little attention in these environments, even though particulate P in dust can contribute substantial amounts to an alpine lake (Psenner 1999). Recent increases in P loading to Emerald Lake, Sierra Nevada Mountains, California, have been attributed to atmospheric deposition of organophosphate pesticide or aeolian transport of dust from the San Joaquin Valley (Sickman et al. 2003). Climate warming, local grazing, and fish stocking can influence primary production and result in changes in d15N; diagenesis of organic matter can also mimic some of the * Corresponding author: [email protected] Limnol. Oceanogr., 59(6), 2014, 1987–2001 E 2014, by the Association for the Sciences of Limnology and Oceanography, Inc. doi:10.4319/lo.2014.59.6.1987