HERE YESTERDAY—GONE TODAY: PRELIMINARY INVESTIGATION OF ANTHROPOLOGICALLY INDUCED LITTORAL COMMUNITY CHANGE IN THE COLORADO RIVER DELTA

The northern Gulf of California is home to diverse littoral communities, commonly dominated by bivalve mollusks. In the Colorado River Delta region, the Holocene invertebrate assemblage was dominated by the bivalve mollusk Mulinia coloradoensis. However, 20^th-century diversion of the Colorado River for hydroelectric power generation and agricultural irrigation, especially in the USA, significantly impacted near shore marine communities in the region. As a result, venerid bivalves (e.g., Chionista fluctifraga), are now the most common large intertidal bivalve, albeit at much lower abundances than in pre-dam communities. Furthermore, recent intertidal surveys suggest that significant changes in community composition and structure accompanied decreased benthic productivity. Here we present preliminary results of a study designed to investigate benthic community changes associated with anthropologically induced environmental change. We collected three samples separated in space and time. The first, “Acres of Clams,” was collected from a submodern chenier. This is the most landward deposit examined in this study. Based on previous C-14 dating, the timing of chenier formation was likely less than 1500 ybp. This thanatocoenosis was derived from pre-dam intertidal communities. The second sample, “Active Chenier,” was collected from the modern beach ridge, which began forming nearly 100 yeas ago when the Colorado River was initially diverted. This deposit is derived from specimens that lived on the tide flat prior to river diversion, as well as younger specimens that lived after dam emplacement. The final sample, “Modern Tide Flat,” was collected from retrograding, coarse-grained bars. To date, nearly 80000 specimens have been counted and identified (AOC: n = 36968; 57 species; AC: n = 24025; 60; MTF: n = 16142; 49 species). Rarefaction analysis suggests similar species richness among the samples. Despite taxon count ranks, the MTF has the highest Shannon-Wiener index and AC the lowest. Non-metric multidimensional scaling of bootstrapped samples suggests community structure differs among the samples. Principal components analysis suggests community structure differences likely reflect physiological and ecological changes associated with an altered salinity regime following river diversion.