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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].sup.15]N) from 22 lakes showed that whole sediments of subalpine sites were significantly more depleted (0.74[per thousand] [+ or -] 1.58[per thousand]) than were those of alpine lakes above 2200 m (3.04[per thousand] [+ or -] 1.21[per thousand]), consistent with the depleted [[delta].sup.15]N of isolated DOM (~1.3[per thousand]). Sedimentary [[delta].sup.15]N values of Crowfoot Lake, presently near tree line, also varied greatly during the past 12,000 yr, with enriched values (~4[per thousand]) during the alpine phases of the lake's history and depleted values (~1[per thousand]) during the intervening subalpine phase (ca. 10,050-4160 [sup.14]C yr before present) when DOM was abundant. In contrast, sedimentary [[delta].sup.15]N values remained constant (~2.5[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.

We used multiple linear regressions across a Northeast Pacific region 56-lake set to examine the influence of sockeye salmon spawner densities and limnologlcal, climatic, and watershed characteristics on sediment stable nitrogen isotope ([delta][sup.15]N) and C : N signatures, geochemical proxies used in paleolimnological reconstructions of prehistoric salmon abundances. Across all sites (n = 56), sedimentary C : N was primarily related to variables reflecting allochthonous organic matter and nutrient fluxes, with 67% of the variance explained by watershed vegetation type, watershed area, mean slope, and salmon spawner densities. In a subset of sites with nutrient data (n = 41), 63% of variance in C : N was explained by precipitation and watershed vegetation type. Sediment [delta][sup.15]N was most strongly related to salmon spawner densities, but C : N and watershed area : lake area ratios explained significant residual variance (total 55%). In sites with nutrient data, salmon spawner densities, vegetation type, and spring nitrate explained 62% of the variance in [delta][sup.15]N. Sediment C : N and [delta][sup.15]N values exhibited distinct interregional variation, typically varying inversely. Regional [delta][sup.15]N-salmon density relationships (regression slopes) varied strongly with total annual precipitation ([r.sup.2] = 0.89, p = 0.016, n = 5), suggesting watershed organic matter and nutrient loading vary predictably in relation to regional biogeoclimatic conditions. Our findings demonstrate that paleolimnological analyses are useful for quantitative reconstruction of past salmon densities; however, inferences regarding past salmon populations must consider the factors regulating influxes of nitrogen from watershed sources.