Arthropod Community Response to Mosquito Control Recirculation Ditches in San Francisco Bay Salt Marshes

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Abstract
The impact of mosquito control recirculation ditches on Pacific Coast salt marsh ecology is largely unknown. This study compares the arthropod community structure near mosquito control ditches and a natural channel with that in the open marsh. Biweekly D-Vac samples in two San Francisco Bay marshes were taken November 1977 to 1978. In the pickleweed-dominated Petaluma Marsh, arthropod diversity in the dry season was higher near both the ditches and natural channel than in the open marsh. This corresponded to gradients of increasing water table height, groundwater salinity, and soil surface salinity. Conversely, in the wet season, diversity was lower near the two newest ditches than the open marsh, whereas diversity near the natural channel and the oldest ditch was similar to the open marsh. These differences are best explained by higher plant structural diversity near the natural channel which offers refuge from high winter tides. There was no significant difference in arthropod biomass near a ditch and in the open marsh. In the floristically diverse Suisun Marsh, there were no significant differences between ditched, natural channel, and open marsh arthropod diversities during either season. Results suggest an eventual convergence of arthropod community structure along ditches and natural channels.