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Abstract: The fate and future of the Colorado Delta clam: Paleoecological evidence for endangerment, paleoecological guidline for restoration.
Paleoecological evidence can be used to identify the cause of species endangerment. Before dams and diversions on the Colorado River, the mactrid bivalve Mulinia coloradoensis was the most abundant mollusk inhabiting the tidal flats of the Colorado Delta. More thank 90% of the ~ 2 trillion shells that make up the islands of the delta are from this species. Recent surveys of the delta's living fauna encountered only twelve specimens of M. coloradoensis - less than 1% of the shelly individuals found.
The del 18-O values in the shells of pre-dam M. coloradoensis are significantly more negative than del 18-O values in the live-collected shells of the Chione cotezi, the most common bivalve living on the delta today. This indicates that M. coloradoensis lived in waters much less saline than found today and strongly implicates the diversion of the Colorado River flow as the cause of the population crash of the species.
Paleoecological evidence can also be used to prescribe restoration efforts. Although complete resumption of the Colorado River's flow to its delta would likely restore the population of this species, this simple prescription is politically unrealistic: all of the Colorado River's water is now diverted for agricultural or domestic uses in the U.S. and Mexico. Partial restoration might be feasable if upstream water users could agree on an "allocation for nature". What percentage of the river's flow would be needed to restore only a part of the population of Mulinia coloradoensis? We examined the del 18-O values in the shells from Las Isletas, 50 km south of the river's mouth, where a viable population existed despite near-complete mixing of river water and normal Gulf water. Assuming that this population was a source rather than a sink, the del 18-O values can be used to calculate the necessary proportion of river water and the required flow in acre-feet per year.
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