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A paleolimnological assessment of the impact of nitrogen deposition to alpine lakes in Rocky Mountain National Park.
WOLFE, ALEXANDER 1, Van GORP, ALISON1, BARON, JILL2, 1 2
ABSTRACT- It is well documented that atmospheric deposition of fixed nitrogen from anthropogenic sources is a major ecological concern for regions adjacent to highly populated and industrialized centers, such as New England and western Europe. On the other hand, far less is known regarding the impacts of more moderate rates of deposition to sensitive alpine catchments in the western USA. Lake sediment cores provide reliable archives of recent biological and biogeochemical changes to these systems, with the advantage of being able to place observations of recent change in the context of natural variability, inferred from parallel studies of sediments pre-dating human influences. Cores from five lakes in Rocky Mountain National Park, high in the Colorado Front Range (>3,200 m asl), record near-synchronous changes including expansions of mesotrophic diatoms and depletions of sediment nitrogen isotopic signatures. The amplitude of recent shifts in sediment proxies is greater in lakes east of the Continental Divide than in those situated west of the Divide, implying that excess nitrogen originates from agricultural and industrial sources in the South Platte River valley, including the Denver-Ft. Collins urban axis. Although the up-slope winds needed to transport nitrogenous compounds westward are a minor component of regional atmospheric circulation, they are nonetheless potent vectors for atmospheric deposition. Paleolimnology reveals how these inputs are inducing directional biological and biogeochemical changes to these seemingly pristine natural areas.
KEY WORDS: diatoms, nitrogen, alpine, lakes