Investigating the response of groundwater ecosystems to water level fluctuations

<p dir="ltr">Anthropogenic activities have significantly disrupted the Earth's hydrological cycle, resulting in various negative impacts on freshwater ecosystems. A major global concern is the depletion of groundwater reserves, driven by the growing demand for freshwater to meet the needs of industry, agriculture, and society. Groundwater accounts for 97% of the accessible freshwater on Earth, serving as a crucial resource, particularly in arid and semi-arid regions where surface waters are often intermittent. Groundwater sustains a range of groundwater-dependent ecosystems (GDEs), including terrestrial, aquatic, and subterranean systems, and provides vital ecosystem services that support the ecological health of these ecosystems and the industries reliant on this resource. As a result, over-abstraction of groundwater poses a substantial threat to the ecosystems, industries and the societies that depend on groundwater reserves and the ecosystem services they provide.</p><p dir="ltr">In essence this thesis is a study of groundwater ecology focusing on the study of the biota within subterranean ecosystems. Key biota including prokaryotes, including bacteria and archaea, play crucial roles in nutrient cycling, organic matter decomposition, and energy flow within these systems. Fungi that contribute to the breakdown of organic material, facilitating nutrient availability and enhancing soil structure and stygobionts, specialized invertebrates adapted to life in groundwater, maintain an aquifers effective porosity and maintain microbial biofilm densities, remain the focus of this thesis. Recent advances in environmental DNA (eDNA) techniques have revolutionized the understanding of groundwater ecology, allowing researchers to detect and monitor these organisms more effectively without the need for direct sampling. eDNA has enabled more comprehensive insights into the biodiversity and ecological dynamics of groundwater ecosystems, and is a technique heavily relied on to determine the presence and relative abundance of groundwater taxa throughout this thesis.</p><p dir="ltr">The over-abstraction of groundwater (resulting in groundwater drawdown or the lowering of the groundwater table) particularly threatens the integrity of subterranean ecosystems by reducing the availability of saturated habitat for groundwater biota to inhabit and disrupting the hydrological connectivity between groundwater and surface aquatic and terrestrial systems. Changes to connectivity are expected to impact oxygen and nutrient dynamics within subterranean ecosystems with flow on effects impacting the distribution and abundance of biota. Despite the known widespread impacts of groundwater over-abstraction on terrestrial environments e.g., land subsidence, loss of river base flow and terrestrial GDE decline, the impacts on subterranean biota within groundwater ecosystems have been rarely considered. Therefore, the aim of this thesis was to investigate how the environmental changes associated with groundwater abstraction impact the microbes and invertebrates inhabiting subterranean aquatic ecosystems. The central hypothesis was that changes in groundwater levels and the depth of the water table play a significant role in determining the distribution and abundance of groundwater biotic assemblages.</p><p dir="ltr">This thesis addressed the central hypothesis in two ways. First, it evaluates the natural distribution of groundwater biota (prokaryotes, fungi, and stygobionts - the collective name for obligate groundwater fauna, i.e., invertebrates that solely inhabit subterranean aquatic environments) with depth within three alluvial aquifers of south-eastern Australia. As well as depth, the relationships of environmental factors, such as sediment type, oxygen and nutrient availability, were also considered. The results of these studies showed that the distribution and abundance of groundwater biota significantly varied with depth, as hypothesised. Shallow aquifer environments typically supported diverse and taxa rich communities with a higher abundance of stygobiotic individuals. Concentrations of oxygen and nutrients had a weak, negative correlation (i.e., decreasing concentrations) with increasing depth. Stygobionts were more abundant in shallow than deep alluvial environments, highlighting the vulnerability of stygobiont communities to drawdown events due to their apparent preference for the higher oxygen and nutrient environments close to the water table.</p><p dir="ltr">Second, the thesis investigated the impact of groundwater level declines on groundwater biota in field and laboratory settings. This was achieved first through a 28-day, field-based groundwater pumping experiment, in which abiotic conditions and microbial communities were studied before, during and after the onset of groundwater abstraction and subsequent drawdown in a shallow alluvial aquifer. While groundwater drawdown was achieved during the 28-day pump test, large precipitation and subsequent flood events resulted in groundwater recharge also occurring during the study period. These groundwater recharge events inhibited the ability to observe drawdown across multiple monitoring periods. The results revealed that variation in groundwater chemistry and prokaryotic communities between bores (a narrow deep hole drilled into the ground that allows access to groundwater), was greater than variation within individual bores throughout the experiment. This suggests that biological communities were responding to site specific changes in water levels and groundwater chemistry, rather than reflecting a broader pattern of change throughout the aquifer at the local scale.</p><p dir="ltr">The response of stygobionts to declining water levels was also examined in laboratory experiments. Laboratory column experiments were conducted to assess the response of stygobiont copepods and amphipods to declining water levels in various sediment types and with different rates of drawdown. All stygobiont taxa examined showed limited capacity to move with the receding water levels, thus became stranded in the unsaturated sediments. Larger taxa, faster drawdown rates, and smaller sedimentary environments all increased the likelihood of an individual being stranded. The results of this study suggest that many of the stygobionts observed in field studies may have limited capacity to migrate with declining water levels and would be at risk from drawdown events.</p><p dir="ltr">The new knowledge from this thesis provides empirical evidence of the likely and actual impacts of changes in groundwater levels to the microbes and invertebrates inhabiting subterranean aquatic ecosystems. The central hypothesis that the depth of the water table and changes in groundwater levels significantly influence the distribution and abundance of groundwater biotic assemblages were upheld. These findings highlight the significant risk that groundwater drawdown poses to groundwater ecosystems, and the need for further research to inform more effective management strategies and determine sustainable abstraction limits to ensure the protection of groundwater ecosystem integrity. Groundwater abstraction is essential for society and informed management strategies are required to balance societal and environmental water needs and protect and preserve the essential ecosystem services that groundwaters provide and on which societies and interconnected ecosystems depend.</p>