Groundwater biodiversity and constraints to biological distribution

Groundwater hosts a high diversity of living forms including viruses, prokaryotes (bacteria and archaea), microeukaryotes (fungi and protozoans), and metazoans, along with invertebrates, salamanders, and fish. Groundwater communities are not only diverse in terms of composition but also in terms of functions triggered by multiple trophic and nontrophic interactions among organisms. Due to the absence of photosynthetic primary production in aquifers, the composition, abundance, and activity of heterotrophic prokaryotes and eukaryotes (with the exception of chemoautotrophic microorganisms), are constrained to various degrees by the low amount of surface-derived organic matter (OM) reaching groundwater. Groundwater metazoans additionally experience further constraints in their spatial distribution. From local to regional scales, the composition of groundwater metazoan communities in consolidated and unconsolidated rocks is largely determined by the size of voids, their interconnectedness, and their connectivity to the surface environment. The latter exerts a major control on thermal variability, availability of OM, and dissolved oxygen (DO). Reduced thermal variability of deeper subsurface environments may select for low thermal tolerance of species, which may, in turn, constrain their dispersal along spatial temperature gradients. Increased OM supply to groundwater enhances the complexity of food webs and diversity of organisms present, with DO depletion due to microbial aerobic respiration affecting the survival of metazoans. The constraint on biological distribution imposed by the interplay between OM and DO depends on scales of heterogeneity of the two variables. Studies modeling the distribution of species and communities rarely integrate species interactions despite evidence that competition for scarce resources and/or predation may play a major role in species distributions. The next step is to build on the understanding of biological distribution for evaluating the fate of biodiversity in response to anticipated changes in temperature, recharge rate, and organic carbon concentration in groundwater.