INTEGRATING HYDROGEOLOGY AND AQUATIC ECOLOGY IN DESERT SPRINGS: THE INFLUENCES OF PHYSICAL HABITAT, GEOCHEMISTRY, AND GROUNDWATER RESIDENCE TIME

The distinctiveness of springs as aquatic ecosystems is well known for mesic regions where most create first order streams that contribute to larger drainages. They are distinct aquatic in their narrow range of environmental variability and the absence of channel forming, hydraulic processes. These environmental characteristics also define desert springs, but most in these regions are isolated from other systems, and many frequently dry. The effect of human activity on physical characteristics of their environment is well known, but relationships between geochemistry, groundwater residence time, and aquatic ecology of desert springs are unknown.

Relationships between the above factors was examined in 39 springs near Death Valley, CA. Structural equation modeling (SEM) was used as an a priori tool to simultaneously evaluate many causal pathways in a single analysis. Our SEM analysis included six water quality (Temp, TDS, EC, pH, etc.), six geochemical (¹⁴C residence time, CaCO₃, HCO₃^-, Ca⁺²/Mg⁺², etc.) 12 physical habitat, and 11 biological variables (richness, springsnail abundance, mtDNA age of springsnail and fish divergence from ancestors, number of crenobiontic taxa, etc.). The analysis found that biology was significantly influenced by water chemistry and quality, and that the influence of physical habitat was not significant. This is contrary to lentic and lotic systems where physical habitat has a significant influence on fish and macroinvertebrate life. Additionally, there was a positive, significant correlation between residence time (89 ybp - 28,960 ybp) and the number of crenobiontic taxa in a spring (n= 1– 7). There was also a strong relationship between residence time and the age of lineal divergence of crenobiontic gastropods and fish (range = 0.52 Mya – 5.76 Mya). Taxa occupying younger springs had more recently diverged from ancestors than taxa in older springs.

Relationships between geochemistry, benthic macroinvertebrate (BMI), and microbial communities in Owens Valley, CA springs were also examined. There were similar patterns to BMI and microbial communities where springs with high Ca⁺²/Na⁺ ratios clustered similarly and different from springs with high Mg²⁺, SO₄^2-, Sr⁺², temperature, and EC.