PATTERNS OF NITROGEN TRANSPORT IN STREAMS OF THE LAKE TAHOE BASIN, CALIFORNIA-NEVADA Robert N. Coats Hydroikos Associates 1 Piombo Place San Rafael, CA 94901 USA coats@hydroikos.com Human intervention in the global nitrogen cycle has increased dramatically in the last half-century. The effects of excess nitrogen from fertilizer and sewage on aquatic ecosystems and groundwater supplies have long been recognized. More recently, nitrogen saturation--an excess of available nitrogen over the biotic and abiotic retention capacity of terrestrial ecosystems-- has led to increased attention to nitrogen cycling and to the linkages between terrestrial and aquatic ecosystems. At Lake Tahoe, the accumulation of nitrogen, over half of it originating in direct atmospheric deposition to the lake, has been sufficient to shift the primary limiting nutrient in the lake from nitrogen to phosphorus. Because of the importance of human-induced changes to the global nitrogen cycle, the results of water quality monitoring at Tahoe are of interest far beyond the boundaries of the Tahoe basin. The data base of the Lake Tahoe Interagency Monitoring Program (LTIMP) provides a unique opportunity to characterize the spatial and temporal patterns of nitrogen transport in subalpine streams. We hope to answer the following questions: 1) What is the relative importance of nitrate-N, ammonium-N and organic N in stream loads, and how do the concentrations of different forms vary with season and discharge? 2) What are the major sources of the different forms of nitrogen in basin streams? 3) What are the major hydrological and biogeochemical controls on the flux the different forms of nitrogen to the lake? 4) How does the biological availability of particulate and dissolved organic nitrogen relate to its origin? Although the LTIMP data may not provide definitive answers to all of these questions, they do allow us to frame the questions heuristically. As a first step in this analysis, we calculated discharge-weighted mean concentrations for nitrate-nitrogen, ammonium nitrogen, and dissolved and particulate organic nitrogen for 10 watersheds. We also calculated total loads of nitrate-N, ammonium-N and organic N (dissolved plus particulate), for the 10 LTIMP watersheds, over the period 1989-1998. The figures below show the total nitrogen loads, averaged over the ten years for each watershed, and averaged over area for each year. Both the concentration and total loads of nitrogen in basin streams are dominated by organic nitrogen. Nitrate-nitrogen accounts for about 7 percent of the nitrogen load and ammonium-nitrogen for only about 1.5 percent. On average, about 55 percent of the organic nitrogen is dissolved, although this fraction varies widely among streams. Dissolved organic nitrogen (DON), like nitrate, is highest at high discharge early in the runoff season. It typically drops during late snowmelt, but increases again during the summer low-flow period, probably due to instream biological activity. Intense summer rainstorms, especially on the east side of the basin, are responsible for the highest peaks in organic nitrogen. The dominance of organic nitrogen in basin streams contrasts sharply with streams in the eastern U.S., where nitrate is the most important form in transport. In spite of the basin’s air quality problems, it has not yet approached the levels of nitrogen deposition that are characteristic of much of eastern North America. The biological availability of the organic nitrogen in the streams and lake is an important open question. Probably the availability changes with season, hydrologic conditions, and the flow-paths of water through the forest floor and soil. At the basin and decadal scales, the temporal and spatial variation in total nitrogen load is explained largely by variation in annual runoff. The relationship between annual runoff and total annual nitrogen load is good enough that annual runoff alone could now be used to estimate total annual nitrogen load to the lake from the 10 LTIMP streams. Future work on nitrogen in Tahoe basin streams might focus on 1) relationships between watershed characteristics (soils, vegetation, hydrogeology and land use) and nitrogen yield; 2) concentrations of particulate organic nitrogen, dissolved organic nitrogen, and dissolved inorganic nitrogen in relation to hydrologic flowpaths and runoff events; 3) the sources and biological availability of dissolved organic nitrogen in streamwater; 4) methodologies for calculating total load; and 5) the contribution of urban runoff to the nitrogen load of the lake.