INTEGRATING HYDROGEOLOGY AND AQUATIC ECOLOGY: EVIDENCE THROUGH THE STRUCTURE OF BENTHIC MACROINVERTEBRATE COMMUNITIES AND WATER GEOCHEMISTRY IN THE OWENS VALLEY, CA

The southern Great Basin is one of the most arid regions in the Northern Hemisphere. It encompasses a series of elongate, north-south oriented endorheic basins that have been isolated from adjacent drainages since the late Tertiary. Since the late Miocene/ early Pliocene, tectonic development and climatic fluctuations have been the major drivers of aquatic habitat isolation and aquatic organism speciation in the southern Great Basin. As the climate dried and the hydrological connectivity between basins ceased, isolated springs became the most vital and remnant aquatic habitats in the region. Springs occur throughout the region, and some have persisted for millennia and support minimally vagile aquatic life whose ancestors arrived during the pluvial periods. As with other aquatic systems, physicochemical characteristics of spring environments strongly influence the structure of benthic communities in these systems.

The geologic heterogeneity and subsequent diversity in groundwater geochemistry in Owens Valley provides a wide range of physicochemical conditions to study the integration of hydrogeology and aquatic community structure. In this study, we test the hypothesis that the structure of benthic macroinvertebrate (BMI) communities is a function of the geochemistry of the spring environment. Freshwater BMIs are sensitive to physicochemical characteristics of their environment, which depends heavily on the surrounding geology. We examined these relationships by sampling BMIs and quantifying the isotope hydrology and geochemistry of springs that are undisturbed by human or natural factors. Weathering-derived geochemistry indicators such as Ca/ Na, Mg/ K, and ⁸⁷Sr/⁸⁶Sr ratios show that local variations in geology exert a primary control on the chemical composition of spring waters in the Owens Valley. The differences in BMI communities between sampled springs can be attributed to groundwater-rock interactions and the geochemistry of springs. Additionally, results suggest that the structure of BMI communities in mountain front springs can be predicted by geochemistry and vice versa.