ABSTRACT: Silence of the Clams: Paleoecological evidence for population decline in the Colorado Delta

RODRIGUEZ, Carlie A., DETTMAN, David, FLESSA, Karl W.,
Department of Geosciences, University of Arizona, Tucson, Arizona, 85721

Geological Society of America 1998 Annual Meeting, Toronto, Canada.

The Colorado River is the most important source of water for the arid southwest. In fact, the entire flow of the river is now diverted and dammed for human uses before it reaches the delta in the Gulf of California. This has changed the environments and biotas of the delta. Whereas the delta once received large amounts of freshwater, now it receives none. Many organisms living on the delta depended on this inflow for creating suitable environmental conditions. Those conditions have now changed.

The bivalve mollusk Mulinia coloradoensis was once the most abundant species of mollusk that lived in the Colorado River Delta area. It was so abundant that the Delta's beaches are made up of its shells. However, now it has declined, possibly to the point of extinction. Comparison of oxygen isotope ratios in the shells of dead Mulinia coloradoensis and the living marine bivalve Chione fluctifraga in the Colorado Delta today, show that the two species differ in their isotopic values. M. coloradoensis has oxygen isotope values that are much lighter than the isotopic values from live C. fluctifraga. Shells from live-collected C. fluctifraga have a mean stable oxygen isotope value of -1.35. Shells of dead M. coloradoensis have a mean oxygen isotope value of -2.00. This suggests that Mulinia was living in water that was lower in salinity than the water that Chione lives in today. It is likely that M. coloradoensis was tolerant only of the lower salinity conditions created by river water reaching the sea. Now that freshwater no longer reaches the Delta, this area may not be suitable for this mollusk to survive, and its population is decreasing dramatically.

We can estimate the isotopic composition of the Colorado River before diversion using freshwater clam shells collected in 1894. These data allow us to predict the minimum river input to the delta that would be necessary to restore the populations of Mulinia coloradoensis to pre-diversion levels.


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