A large population of crayfish (Pacifastacus Zeniuscdus) inhabits the littoral zone of Lake Tahoe. The effect of crayfish grazing on the primary productivity of the periphyton was investigated in field and laboratory enclosures by varying the ratio of crayfish to substrate. In the field, primary productivity was enhanced after GO days in a low density crayfish site and inhibited in a high density site. These results were duplicated in laboratory trials. Primary productivity of periphyton showed stimulation from grazer biomasses below 131 g m-' but was inhibited by overgrazing when crayfish biomasses exceeded 203 g 11i2. Grazing effects were also estimated on higher aquatic plant beds found in the littoral area of Tahoe by inclusion-exclusion experiments.

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