Assessing and conserving groundwater biodiversity: an introduction

1. This paper is an introduction to and overview of a special issue on groundwater ecology with a focus on biodiversity assessment and conservation. The subterranean environment harbours unique biological communities of remarkable diversity, comprising both microorganisms and micro-, meio- and macrofauna. This biodiversity of ground waters remains poorly known compared to that of freshwater surface habitats. 2. The papers in this special issue summarise the outcome of a large-scale European survey, the PASCALIS project (Protocol for the ASsessment and Conservation of Aquatic Life In the Subsurface), complemented by results from a few other comprehensive studies on groundwater diversity. The papers fall into five broad groups. The first group reviews the biology and ecology of poorly investigated but important subterranean taxa such as Bacteria and Archaea, Oligochaeta and Copepoda. Five papers deal with groundwater biodiversity patterns in the six selected PASCALIS regions. Issues addressed range from broad-scale diversity patterns to partitioning of species richness, cryptic diversity, sampling methodology, and the use of surrogate taxonomic groups for assessing overall diversity. Four papers analyse biodiversity patterns in single European regions, the French Jura, the Walloon region of Belgium, the Lessinian Mountains of Italy and south-western Germany. Two additional papers address the issue of site selection for the conservation and protection of groundwater species, and the fifth group of three papers expands two central PASCALIS themes - sampling adequacy and the role of dispersal. Finally, a concluding synthesis paper summarises advances in understanding groundwater biodiversity patters and conservation and points out important challenges for future research. 3. Collectively, the information presented in this special issue represents the first broad-scale examination of aquatic subterranean diversity based on a rigorously standardised sampling protocol, the first examination of stygofauna patterns in both karstic and non-karstic aquifers, a first step toward integrating traditional and molecular taxonomic studies of groundwater biodiversity, and a start toward elaborating strategies for groundwater reserve selection.