Search

Entre 1999 y 2000 se determinaron la composición y distribución de la macrofauna bentónica (moluscos, equinodermos y braquiópodos) en la zona intermareal del Delta del Río Colorado, al norte del Golfo de California, México. Los muestreos se obtuvieron a lo largo de diez transectos de la zona mesomareal desde la parte superior a inferior, en dos épocas del año para determinar la variación estacional de la fauna. Fueron identificadas 26 especies en 112 muestras con un total de 1954 individuos. Dominó el gasterópodo epifaunal Nassarius moestus, bivalvos y equinoideos infaunales y el braquiópodo linguloide Glottidia palmeri. La densidad de la fauna fue mayor hacia la parte inferior de la zona intermareal. La riqueza de las especies no varió significativamente entre transectos, pero la diversidad (índice de Shannon) y equitatividad fueron mayores en los transectos del sur, los más alejados de la boca del río. La diversidad y equitatividad de los transectos del sur pueden explicarse por la variación en la región de la salinidad y tipo del sustrato. Actualmente la densidad de la fauna es de 3 a 7 ind m–2, mucho menor a la reportada antes de la construcción de las presas y que afectó los hábitat en el Delta del Río Colorado, principalmente por la disminución en la población del bivalvo máctrido Mulinia coloradoensis. 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.

The northeastern coast of Baja California (Mexico) has extensive surface mollusk-shell deposits in exposed, semiprotected or protected sandy beaches. Waves, tides, currents and sediment transport influence the shell concentrations in the intertidal zone and control the morphology of these beaches. In order to evaluate the taphonomic processes involved in the formation of shell deposits in an exposed beach, we studied temporal changes in thanatocenoses of bivalves along 200 m of beach at spring tides in March, May and November 1993, and March and November 1994. Species were identified and their size, orientation (concave-up vs concave-down) and azimuth orientation of the anterior-posterior axis were measured. The assemblage was dominated by Glycymeris gigantea and included 15 common species. The shells showed a variable azimuth orientation, but with a trend parallel to the coastline, predominance of downward concavity and a polymodal size distribution with modes in 1 and 3 cm, in a size range of 0.3 to 8.5 cm. The taphonomic characteristics of these shell deposits may be useful in coastal paleoceanographic reconstructions.

Time-space variations in macrobenthonic biocenosis are described in relation to the morphology of the beach profile and textural characteristics of the sediment at El Pelícano Beach, South of San Felipe, Baja California, Mexico. Sampling was carried out in March, May, September and November 1993 along a beach profile, at fixed stations from the higher high tide to the lower low tide. Abundance data matrices were analyzed by applying a Q-mode cluster analysis, in order to establish community structural changes and compare them with changes in the beach profíle and sediment grain size. In general, the number of species was consistently low and varied with the textural characteristics of the sediment and the morphology of the beach profile. Only the gastropod Nassarius iodes was dominant in all the samplings and beach protile levels, because of its tendency to follow the tide in order to remain in the swash zone where feeding conditions are optimum. Community structural changes at El Pelícano Beach indicate that the ecological and paleoecological interpretation of comparable coastal environments is complex, because of the biotic and biotic time-space variability.