| Summary: | Groundwater is a vital resource. It contains 97% of unfrozen water on the planet,
playing a key role in present and future water needs for humanity. However, our
knowledge about the ecosystem functioning is very poor, and groundwater
environments are increasingly exposed to anthropic impacts and climate change related processes. Novel biochemical (e.g. isotopic ecology) and genetic (e.g. eDNA)
techniques, widely employed in fresh surface water studies, have the potential to
unravel the complex dynamics shaping subsurface ecosystems, providing new
insights to the small but quickly growing field of groundwater ecology. Stygofauna,
together with microbes, are crucial actors in shaping and maintaining the organic
matter (OM) cycles in environments characterized by low energy and scarce carbon
availability. In order to understand groundwater ecological patterns, we investigate
calcrete stygofaunal shifts linked with contrasting rainfall periods (low rainfall (LR),
dry season; high rainfall (HR), wet season), through an interdisciplinary design
composed of hydrology, isotopic ecology and genetics. Our results indicate that the
inflow of rainfall under HR is responsible for increased nutrient concentrations in the
system and dissolved organic carbon (DOC) pulses from the surface. Both the
meiofaunal and stygofaunal communities’ benefit from these organic inflows, with
gamma and proteobacteria the biota that fuels carbon and nutrients to the higher
levels of the trophic web. The HR regime - and its subsequent terrestrial carbon
incorporation - triggers a cascade effect driven by microbes (OM processors) and
amphipods (biofilm grazers), which is finally transferred to the aquatic beetles (top
predators). Overall, and in line with other work in the same research area, the inflow
of rainfall triggered shifts towards more deterministic dynamics, revealing a complex
web of interactions in a seemingly simple environmental setting. This study provides
a preliminary untangling of the biochemical flows driven by rainfall in a calcrete
aquifer. More investigations involving multidisciplinary approaches on other
subsurface ecosystems, i.e. alluvial aquifers, will help to understand present
ecological patterns and predict future scenarios in groundwaters.
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