Geochemical and microbial factors driving crustacean assemblages in adjacent aquifer units within the same aquifer

Abstract. Aquifers harbor unique and highly adapted species, contributing to critical ecological processes and services. Understanding the key factors driving invertebrate assemblages in aquifers is a challenging task that, traditionally, has primarily been achieved in karst. This study aimed to uncover the factors influencing the composition and functionality of groundwater crustaceans (dimensional range from 0.036 to 1 mm) in a volcanic aquifer in central Italy. The aquifer consisted of three adjacent aquifer units (AUs) showing different geochemistry (i.e., sulfate-depleted, K-rich and, alkaline earth). We adopted a multidisciplinary approach, integrating hydrogeology, geology, microbiology, and ecology to determine whether the environmental differences that we highlighted in the three AUs were reflected in the biological assemblages. We unveiled significant differences in both the taxonomic and functional composition of groundwater crustaceans across the three AUs, and these patterns remained consistent throughout the survey period. Notably, the sulfate-depleted AU lacked groundwater-obligate species, burrowers, and stenothermal and moderately stenothermal species. The K-rich and alkaline-earth AUs had different species; however, these species exhibited similar functions related to locomotion, diet, and feeding habit. Stenothermal and moderately stenothermal crustacean species were only found in the K-rich AU, which lacked epigean species. Our findings suggest that major ions (SO42-, Ca2+, NO3-, and K+), trace elements (B, Al, V, Se, and Ba), microbial factors, and carbohydrate catabolic profiles might be the main descriptors of groundwater-obligate species abundances in the volcanic aquifer. Our findings revealed a correlation between the abundances of groundwater-obligate crustaceans and low-nucleic-acid (LNA) cells, suggesting a potential selective feeding behavior of groundwater invertebrate species on the aquatic microbial community. Our research emphasizes the need to consider diverse hydrogeological contexts within individual aquifers. Potential avenues for future research should further consider food web dynamics in groundwater communities and their impact on carbon and nutrient cycling.