Atmospheric nitrogen deposition to the Lake Tahoe Basin, California/Nevada.

摘要

Decreases in the clarity of Lake Tahoe have been attributed in part to atmospheric nitrogen deposition, however no estimates of dry deposition of N-containing gases to the semi-arid Lake Tahoe Basin have been performed. This research shows that the possible range of dry fluxes, calculated from preliminary nitric acid (HNO₃) and ammonia (NH₃) measurements, were much higher (1.2 to 8.6 kg N ha−1 yr−1 ) for Lake Tahoe's watershed than for the surface of Lake Tahoe itself (0.58 to 1.2 kg N ha−1 yr−1), largely due to the high affinity of HNO₃ for the leaf surfaces to which it deposits. Laboratory gas exchange experiments on conifers native to the Lake Tahoe Basin found that accumulation of HNO₃ on leaf surfaces reduces this affinity. Accounting for this phenomenon in a “bigleaf” inferential model at a representative site in the Tahoe Basin reduced estimated HNO₃ flux by 30%. Scaling these improved estimates to the entire Tahoe Basin required coupling the inferential model with a GIS framework and simultaneous measurements of the major N-containing gases occurring within the Tahoe Basin. Both local and regional sources contributed to the resulting fluxes (0.7 to 2.1 kg N ha−1 yr−1), which varied nonlinearly with forest canopy cover across the landscape. Watershed retention estimates based on these inputs are as a result both lower (average = 50%) and more variable (10 to 90%) than previously considered.; These results demonstrate how Tahoe's forested watersheds capture local and regional pollutants more efficiently than the lake itself and imply that watershed processes control more of the Lake Tahoe N budget than previously considered. The results also indicate that other ‘pristine’ semi-arid forests in the West may be receiving substantial amounts of N via dry deposition from regional sources. Local-scale assessments that integrate model data and land-cover within a GIS framework will be necessary to determine the magnitude of these inputs, and their potential for causing eutrophication.