Distribution of the intertidal macrofauna of the Colorado River Delta, northern Gulf of California, Mexico

Surveys were conducted in 1999 and 2000 to document the composition and distribution of the shelly (mollusk, echinoderm and brachiopod) fauna of the intertidal zone of the Colorado River Delta, northern Gulf of California, Mexico. Samples of the benthic fauna were taken along ten high-to-low intertidal transects, during two seasons to assess seasonal variation in the fauna. Twenty-six species were identified among 1954 individuals in 112 samples. The fauna was dominated by the epifaunal gastropod Nassarius moestus, infaunal bivalves, infaunal echinoids and the lingulid brachiopod Glottidia palmeri. Faunal density was generally greater toward the lower part of the intertidal zone. Species richness did not vary significantly among transects, although Shannon diversity and equitability were greater in the southern transects, those distant from the river mouth. The distinctive species composition, diversity and equitability of the southern transects may be the result of regional variation in salinity and substrate. Current faunal densities of 3-7 ind m^-2 are much lower than the estimates before upstream dams and water diversions affected the habitats of the Colorado River Delta. The post-dam decrease in density is largely the result of the decline in the population of the mactrid bivalve Mulinia coloradoensis.