Science-Based Decision Making in the Lake Tahoe Watershed

Abstract

A close partnership between researchers, resources agencies, and community groups allows for the most effective ecological and economic approach to environmental protection and watershed management. One of the better documented examples of this type of cooperative effort comes from the Tahoe basin (CA-NV) where studies over the past four decades have shown that many factors including, land disturbance, habitat destruction, air pollution, soil erosion, roads, have interacted to degrade the Basin's air quality, terrestrial landscape, and streams, as well as the lake itself. For effective lake management, we need to know (1) what are the specific sources of sediment and nutrients to the lake and what are their respective contributions, (2) how much of a reduction in loading is necessary to achieve the desired lake condition (lake response), and (3) how will this reduction be achieved? In this paper we present examples of ongoing research and monitoring which are proving extremely useful in watershed management. Topics include: changes in lake clarity and the importance of long-term data; response of lake phytoplankton to nutrients and a initial budget for nitrogen and phosphorus; paleolimnological reconstruction of baseline conditions and ecosystem response to anthropogenic disturbance; a clarity model to assess lake response to management; and a sediment transport model to evaluate nonpoint source loading. Only be providing a fully integrated watershed approach can we hope to develop effective restoration/mitigation efforts so essential to providing future generations with a quality environment in the Tahoe basin. Background The role of our nation's universities in the evolving dialogue on watershed management is at a critical stage. Given the increased need to understand the (1) effects of pollution, (2) source(s), transport and fate of pollutants, and (3) continued public demand for clean air, land and water, these institutions are positioned to work closely with other stakeholders. A closer partnership between researchers, resources agencies, and community groups allows for a more effective ecological and more economic approach to environmental protection and watershed management. One of the better-documented examples of this type of cooperative effort comes from the Tahoe basin (CA-NV), where investigations by the Tahoe Research Group (TRG) at the University of California-Davis have provided clear evidence for the onset and progression of cultural eutrophication (Goldman 1988). Lake Tahoe is world-renowned for its clarity and water quality, however, continuous monitoring and research since the early 1960s has shown that algal growth is increasing at a rate greater than 5 percent per year. There has been a corresponding decline of clarity at an alarming rate of nearly one foot per year. This long-term trend in transparency is both statistically significant and is now perceivable to even the casual observer. Significant portions of the once pristine basin are urbanized. Studies by the TRG and others have shown that land disturbance, habitat destruction, air pollution, soil erosion, roads, etc. have all interacted to degrade air quality, terrestrial landscape, and streams, as well as the lake itself. We now know that once nutrients enter the lake they remain in the water, and can be recycled for decades (Jassby et al. 1995). As a consequence, the materials accumulate over time and contribute to Lake Tahoe's progressive decline. The ability of Lake Tahoe's large volume to dilute nutrient and sediment loading to levels where they have no significant affect on lake water quality has been lost.