Abstract
Groundwater is not only one of the largest drinking water resources on our planet but also an ecosystem. While not easily accessible, those who reach the groundwater will find a unique and highly adapted biodiversity thriving in the perpetually dark environment. However, groundwater fauna remains largely understudied and is consequently forgotten in many policy and conservation agendas. Given the current global biodiversity decline, neglecting the ecosystem that provides us with one of the most important ecosystem services of all is concerning. Understanding how biodiversity patterns are shaped by past and current environmental drivers and how these patterns are affected by anthropogenic activities is of high interest in ecological research as well as to inform conservation and management strategies. In terms of biodiversity, Switzerland is an ideal study system because it covers a variety of biogeographically unique environments within a comparably small area. Its variable topography, the range in elevation and climates, and its position among major European drainage basins provides favorable conditions for a rich biodiversity. Additional spatial heterogeneity is created through historical biogeographic factors such as glaciation and the pronounced variability in land use types and intensities even across small spatial scales. These environmental factors are well-studied in aboveground systems, but they remain understudied in difficult-to access ecosystems such as the groundwater. Even in Switzerland, which is generally well-studied, subterranean biodiversity research has dramatically lagged behind and data has been extremely limited. Yet, belowground systems might show different biodiversity patterns than ecosystems aboveground, due to their energy limited and thermally buffered conditions. To fill the existing knowledge gap on groundwater fauna, I applied a citizen science approach, collecting data from hundreds of water providers across the country. The resulting dataset is arguably one of the globally most systematic and highest resolved datasets on groundwater fauna at the given scale, consisting of almost 4,000 samples from 900 sites. Focusing on groundwater amphipods (particularly the genus Niphargus), which are among the largest and most common groundwater-dwellers in Central Europe, I provided a first conclusive overview of their diversity and distribution across Switzerland. As a key result of my thesis, I identified a high contribution of rare and narrowly distributed species to overall groundwater amphipod diversity. A large proportion of these rare species are new to science and not formally described yet. While 19 species of groundwater amphipods were documented for Switzerland in the monograph published by Altermatt et al. (2019), we now know that this number should be at least 40, and that there is likely some more species yet to be discovered. My thesis also demonstrates that the species richness of groundwater amphipods by now considerably exceeds that of aboveground freshwater amphipods (40 groundwater vs. 21 surface water species). What factors shape groundwater biodiversity patterns? I investigated this question including multiple ecological and biogeographical perspectives. I linked individual species’ distributions to broad environmental variables such as aquifer characteristics, elevation, and biogeographic regions. Biodiversity above- as well as belowground is dynamic, shaped intrinsically by historic conditions. Using the Last Glacial Maximum (24,000 years ago) as a climatic extreme, I found a strong signal of the historic ice extent on the present-day biodiversity and distribution patterns of groundwater amphipods. Compared to most aboveground taxa, whose distribution patterns are shaped by post-glacial recolonization of Alpine areas, my results underline that dispersal-limited subterranean organisms might until today reflect local persistence patterns. While biodiversity and occurrence patterns of subterranean organisms are shaped over thousands of years, these patterns are currently challenged by anthropogenic activities, including pollution, groundwater exploitation, climate change, and increasing geothermal use. Despite using data from drinking water extraction sites that are expected to exhibit minimal aboveground impacts, I found signals of land use type and intensity not only in groundwater quality, but also in the occurrence of groundwater amphipods. Groundwater amphipods were more commonly found at forested sites compared to sites with higher crop or intensive pasture coverages. The spatial scale of these signals was identified to be between 400–1,000 m around the sampling site, exceeding the average extent of groundwater protection zones by 1.2 to 3 folds. These results highlight the potential to incorporate an ecosystem-perspective into groundwater management in order to enhance the effectiveness of conserving groundwater quality and biodiversity. A comprehensive understanding of the research design and its implications on the collected data are crucial for robust data analysis and interpretation. How do methodological choices and environmental complexities affect research results? In this thesis, I evaluated the repeated and systematic samples from the citizen science approach to better understand groundwater fauna detection. Obligate groundwater fauna occurrence patterns were found to be temporally consistent, in contrast to the seasonal variability observed in other macroinvertebrates (which are commonly found in water extracted from shallow aquifers). Yet, results on groundwater amphipods indicated that detection rates are generally low (< 0.1 for some species), leading to a high rate of false negatives, where amphipods might be present but go undetected during sampling. In addition, the high variability among sampling sites complicates a robust detection estimate. Considering such aspects is foundational for subsequent ecological research and when designing long-term monitoring programs on groundwater fauna. To sum up, due to intensified research on Swiss groundwater ecosystems during the past few years, the subterranean amphipod diversity is now by far exceeding the diversity of aboveground amphipods. For groundwater amphipods, 11 species remain to be formally described, of which 7 (e.g., Niphargus sp. Schrattenfluh and Niphargus sp. Faido) were newly discovered in this thesis. Some of the taxonomy needs to be resolved, particularly the Niphargus rhenorhodanensis lineages. Undoubtedly, some species have yet to be discovered. Nevertheless, in this thesis, I aim to demonstrate that a solid baseline knowledge of Switzerland’s groundwater amphipods has been established. I hope my findings will promote further research and encourage the integration of groundwater fauna into biodiversity conservation and groundwater management practices.