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Abstract
Groundwater environments interact with and support subterranean biota as well as superficial aquatic and terrestrial ecosystems. However, knowledge of subterranean energy flows remains incomplete. Cross-boundary investigations are needed to better understand the trophic structures of groundwater ecosystems and their reliance on carbon inputs from aboveground. In this study we used carbon and nitrogen stable isotope analyses combined with radiocarbon fingerprints to characterise organic flows in groundwater ecosystems. We coupled these data with DNA metabarcoding of the gut contents of stygofauna to further elucidate organic matter (OM) sources and shifts in diet preferences. Samples were collected from the arid zone Sturt Meadows calcrete aquifer under low rainfall (LR) and high rainfall (HR) conditions. Bayesian modelling of Δ14C, δ13C, and δ15N data indicated that primary consumers (copepods) incorporated mainly particulate organic carbon (POC) under LR but during HR shifted to root derived material (either exudates or direct root grazing). By contrast, diets of secondary consumers (amphipods) were dominated by root material under both LR and HR. Our DNA metabarcoding-based results indicate that amphipods relied primarily on root inputs from perennial trees (likely Eucalyptus and Callitris) during the dry season (LR). Under HR, diets of both amphipods and copepods also included organic material derived from a broad range of more shallow rooted shrubs, and ephemeral herbs and grasses. Our findings illustrate the complexity of functional linkages between groundwater biota and surface terrestrial ecosystems in environments where aboveground productivity, diversity and OM flux to groundwater are intimately linked to often episodic rainfall.
Highlights
Groundwater is the largest active freshwater resource on Earth, and a key component of the global hydrological cycle as well as a matrix for unique and diverse subterranean ecosystems (Gibert et al, 1990; Gleeson et al, 2016)
This study, to our knowledge, provides the first evidence based on isotopic modelling combined with gut content analysis that surface derived plant-material plays a critical role in sustaining stygofauna of shallow groundwater ecosystems of the arid zone (Figure 3)
We found that roots and other plant material constitute a key organic source for the subterranean ecosystem at Sturt Meadows, in an arid and highly dynamic hydrological environment characterised by oligotrophic conditions and truncated food webs due to the lack of autochthonous primary producers (Saccò et al, 2021b)
Summary
Groundwater is the largest active freshwater resource on Earth, and a key component of the global hydrological cycle as well as a matrix for unique and diverse subterranean ecosystems (Gibert et al, 1990; Gleeson et al, 2016). Groundwater sustains surficial (aboveground) aquatic and terrestrial ecosystems. Unveiling Stygofaunal Plant Consumption biomes is exemplified by groundwater-dependent ecosystems (GDEs), which rely on either the surface expression or subsurface presence of groundwater to sustain biodiversity, growth and productivity (Gou et al, 2015). GDEs encompass diverse environments and associated biotic communities, including riparian and floodplain vegetation, wetlands and estuaries, hyporheic ecosystems, springs, aquifers and caves (Murray et al, 2003). While it is often assumed that subterranean fauna in shallow groundwaters are dependent on carbon and nutrient inputs from this vegetation (Venarsky et al, 2022), there have been remarkably few studies that have determined whether particular plant species are more or less important for sustaining subterranean aquatic fauna, or how these may vary seasonally
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