HYDROLOGICAL RECONSTRUCTION OF EXTINCT, THERMAL SPRING SYSTEMS USING HYDROBIID SNAIL PALEOECOLOGY

The thermal springs of the Cuatro Cienegas basin (Coahuila, Mexico) are home to a diverse fauna of endemic snails, fish, turtles, and freshwater stromatolites. However, water diversions and other anthropogenic impacts have lowered the water table substantially and are leading to the extinction of these unique spring systems. A preliminary study of living systems indicated a clear differentiation in habitat preferences between hydrobiid snail genera. Using paleoecological reconstruction based on the presence, abundance, and distribution of four genera of hydrobiid snails, we were able to reconstruct the extent and hydrologic flow conditions of the Garabatal thermal spring system, one of several threatened systems in the Cuatro Cienegas basin.

Spatially explicit distribution maps of the Garabatal system were created by sampling extinct pools and channels along transects and defining microhabitats based on relative snail abundances. Remnant pools, spring heads, and channels were also sampled for hydrobiids as reference points. This method allowed us to identify sites of low water flow, high water flow, presence of stomatolites, and springheads in the extinct pools. We found that the presence of Nymphophylus, preferring hard substrate, indicated high flow conditions and the presence of stromatolite ledges. Mexipyrgus was found in soft, muddy substrate and was correlated with low flow conditions. The presence of Mexithauma was correlated with the presence of stromatolites and a high abundance of Coahuilix served as a good indicator for the presence of springheads.

The more ephemeral nature of hydrobiid shells, versus the temporal stability of stromatolite mounds, allowed for the snails to serve as a bioindicator for discriminating between pools recently dried through irrigation and more ancient pool remnants. A map of hydrologic flow was constructed and compared favorably with previously documented pool distribution and flow conditions. Thus, the paleoecological reconstruction allowed us to assess the impact of water diversion on the Garabatal thermal spring system and is an important tool for planning habitat restoration in this impacted, arid region.