Regulation of the nitrogen biogeochemistry of mountain lakes by subsidies of terrestrial dissolved organic matter and the implications for climate studies

Stable isotopes of nitrogen (N) were analyzed in modern sediments of mountain lakes, dissolved organic matter (DOM), and sediment cores spanning the past 12,000 yr to test the hypothesis that spatial and temporal (100-1000 yr) variation in the N content of mountain lakes is regulated by influx of allochthonous DOM. Analysis of spatial patterns in an elevation gradient of 75 mountain lakes revealed that most N was associated with DOM rather than inorganic N, particularly in subalpine lakes (< 1700 m above sea level). Similarly, analysis of N isotope ratios (delta N-15) from 22 lakes showed that whole sediments of subalpine sites were significantly more depleted (0.74% +/- 1.58 parts per thousand) than were those of alpine lakes above 2200 m (3.04 parts per thousand +/- 1.21 parts per thousand), consistent with the depleted delta N-15 of isolated DOM (similar to 1.3 parts per thousand). Sedimentary delta N-15 values of Crowfoot Lake, presently near tree line, also varied greatly during the past 12,000 yr, with enriched values (similar to parts per thousand) during the alpine phases of the lake's history and depleted values (similar to 1 parts per thousand) during the intervening subalpine phase (ca. 10,050- 4160 C-14 yr before present) when DOM was abundant. In contrast, sedimentary delta N-15 values remained constant (similar to 2.5 parts per thousand) at Snowflake Lake, an alpine reference site that never experienced a DOM- rich subalpine phase. These analyses suggest that climate regulates N influx and lake biogeochemistry by changing the subsidies of terrestrial DOM, and warn that future climate change may initially reduce N influx on a decadal scale by reducing hydrologic transfer before increasing N subsidies on a centennial scale by increasing terrestrial production of DOM.