ASSESSING ECOLOGICAL IMPACT ASSESSMENT - LESSONS FROM MONO LAKE, CALIFORNIA

Because of its high salinity and alkalinity, Mono Lake, in eastern Cal ifornia (USA), is a relatively simple ecosystem. It has become the foc us of an environmental controversy over the effects of 50 yr of divers ions of water from tributary streams to supply water to Los Angeles. D iversions lowered the lake level, increased the salinity, changed the availability of aquatic habitats, and altered the configuration of the shoreline and of islands that support breeding colonies of gulls. We consider (1) how two independent panels of experts synthesized scienti fic information on the lake ecosystem to assess the environmental cons equences of these changes, and (2) how the findings of these groups in fluenced policy decisions and how well subsequent changes in the lake matched expectations. Despite differences in composition and approach, the two panels reached generally similar conclusions. These conclusio ns have been a major component of legal activities and the development of management plans for the lake and basin ecosystem. Both panels con cluded that, because of the simplicity of the lake ecosystem, ecologic al consequences of changes in lake level and salinity associated with continuing diversions were likely to be unusually clear-cut. At certai n lake levels these changes would be expected to alter algal and inver tebrate populations and the populations of aquatic birds that feed upo n them or to disrupt breeding activities in gull colonies. Projections about when critical lake levels might be reached, however, have not b een met. This is largely because stream flows into the lake have been altered from recent historic patterns by the cessation of water divers ions due to governmental and legal actions (prompted in part by the pa nels' findings) and by a prolonged drought. These events illustrate th e difficulty of projecting a timetable for environmental changes, even in simple and well-studied ecosystems.