Compound-specific stable isotopes of organic compounds from lake sediments track recent environmental changes in an alpine ecosystem, Rocky Mountain National Park, Colorado

Compound-specific nitrogen, carbon, and hydrogen isotope records from sediments of Sky Pond, an alpine lake in Rocky Mountain National Park (Colorado, United States of America), were used to evaluate factors contributing to changes in diatom assemblages and bulk organic nitrogen isotope records identified in lake sediments across Colorado, Wyoming, and southern Montana. Nitrogen isotopic records of purified algal chlorins indicate a substantial shift in nitrogen cycling in the region over the past ,60 yr. Temporal changes in the growth characteristics of algae, captured in carbon isotope records in and around Sky Pond, as well as a 260% excursion in the hydrogen isotope composition of algal-derived palmitic acid, are coincident with changes in nitrogen cycling. The confluence of these trends is attributed to an increase in biologically available nitrogenous compounds caused by an expansion of anthropogenic influences and temporal changes in catchment hydrology and nutrient delivery associated with meltwater dynamics. Dissolved inorganic nitrogen (DIN) from atmospheric deposition is an important part of the nutrient budget in unproductive lakes (Bergström et al. 2005). Variations in the concentration of nitrogenous compounds (N) in atmospheric inputs (including nitrate and ammonium), as well as changes in the amount of precipitation received, will alter N availability. Globally, agricultural practices and fossil fuel combustion have profoundly increased the biological availability of N compounds (Galloway 1998). Across Colorado’s Front Range, the growth of cities and industrialized agriculture coincide with the passage from historical states of N limitation in lacustrine environments to frequent N saturation (Williams et al. 1996). Lake sediments from mountainous Colorado, Wyoming, and southern Montana record rapid increases in the abundance of mesotrophic diatoms over the past ,60 yr (Baron et al. 2000; Wolfe et al. 2003; Saros et al. 2005) concurrent with a 1 Corresponding author ([email protected]).