2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 134-11
Presentation Time: 11:30 AM


DIETL, Gregory P., Paleontological Research Institution, 1259 Trumansburg Road, Ithaca, NY 14850, SMITH, Jansen A., Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14850, ZAMORA, Hector A., Department of Geosciences, University of Arizona, Tucson, AZ 85721, DURHAM, Stephen R., Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY 14853 and FLESSA, Karl W., Department of Geosciences, University of Arizona, 1040 E. 4th St, Room 208, Tucson, AZ 85721

The over-allocation of the Colorado River’s water has led to the ecological devastation of its delta. In the past, the Colorado River annually delivered over fifteen million acre-feet (~1.85x1010 m3) of water to its delta but in recent years that number has been zero. Recently, environmental pulse flows equal to approximately 1% of the natural annual river discharge were negotiated by the United States and Mexico to be released over a five-year trial period.

In the week prior to the first pulse, which occurred on March 23, 2014, we traveled to the Colorado River Delta (CRD) to collect a pre-pulse flow baseline for the benthic molluscan community. We expected low live-dead agreement in species composition given the consensus that the damming of the Colorado River significantly changed the molluscan community. Preliminary analysis of our data, from 91 live-dead samples from two localities, using a plot of taxonomic similarity (Jaccard-Chao; JC) versus rank-order abundance (Spearman’s Rho; Rho), indicated, however, that there is high concordance between live and death assemblages; that is, all data points fall within the upper right quadrant of the JC-Rho plot. It is possible that the death assemblage has been altered in the decades since the Colorado River was dammed and that its pre-dam signal has been overprinted by recent inputs. However, we also collected 15 bulk samples from nearby beach-ridge accumulations (cheniers) known to be from the pre-dam era. In a follow-up analysis, comparing the chenier assemblage with the live-dead dataset, all points fell once again in the upper right quadrant of the JC-Rho plot.

Although live-dead fidelity in species composition was high, Hurlbert’s Probability of Interpecific Encounter (PIE) showed a large difference between live (PIE = 0.667), dead (PIE = 0.457), and chenier (PIE = 0.259) samples, suggesting that the abundance of species in the live assemblage is more even than the death assemblage, and that both are more even than the pre-dam chenier assemblage. These results are consistent with the consensus that the once dominant bivalve, Mulinia coloradoensis, has declined in recent years as a result of increasing salinities in the CRD. As a baseline metric for assessing the restoration effects of the pulse-flows, PIE is more sensitive than the JC-Rho plot in detecting ecological changes.