The influence of mosquito control recirculation ditches on plant biomass,production and composition in two San Francisco Bay salt marshes

Vegetation of two San Francisco Bay, California, U.S.A. tidal marshes was examined to determine the effects of recirculation ditches designed to eliminate mosquito-breeding. Salicornia virginica L. biomass and production in Petaluma Marsh and plant species composition in Suisun Marsh were measured with respect to distance from ditches and natural channels. In Petaluma Marsh, both annual above-ground production estimates and infrared aerial photographs indicated that S. virginica growth rates were higher near ditches than in the open marsh. In the floristically diverse, less saline Suisun Marsh, there was a displacement of the more salt tolerant S. virginica by the less tolerant Juncus balticus Willd. and a significantly greater number of species near the ditches. Results in both marshes are correlated with low groundwater salinities near ditches and suggest that tidal circulation within ditches locally ameliorates extremes in soil conditions.