Assessment of the physical environment of epigean invertebrates in a unique habitat: the case of a karst sulfidic spring, Slovenia Janez Mulec, 1 * Andreea Oarga, 2,† Edmund K. Schiller, 3 Aurel Perşoiu, 4 Ladislav Holko 5 and Stanka Šebela 1 1 Research Centre of the Slovenian Academy of Sciences and Arts, Karst Research Institute, Titov trg 2, 6230 Postojna, Slovenia 2 University of Nova Gorica, Vipavska 13, 5000 Nova Gorica, Slovenia 3 Natural History Museum Vienna, Burgring 7, 1010 Wien, Austria 4 Stable Isotope Laboratory, Ştefan cel Mare University, Universităţii 13, 720229 Suceava, Romania 5 Institute of Hydrology, Slovak Academy of Sciences, Ondrašovská 16, 03105 Liptovský Mikuláš, Slovakia ABSTRACT A well-fissured and faulted karst area at the Žveplenica sulfidic spring created suitable conditions for epigean copepods. Their presence in sulfidic water is a rare phenomenon. To understand the conditions related to sulfidic habitats in the region, two additional springs were analysed: the Studenec karst spring and the Sovra artesian borehole, which contains dissolved sulfide. Water temperature, chemistry and the stable isotopes of oxygen and hydrogen show that the Sovra borehole water has a different origin and history to that of the Žveplenica and Studenec springs water, which is derived from local precipitation. The low slope of the δ 18 O–δ 2 H regression line for the Žveplenica spring indicates a mixing of local precipitation water with geothermal sulfidic water that may explain the presence of live copepods. These harpacticoid and cyclopoid copepods must have survived anoxia and high sulfide for some time. The mean transit time for Žveplenica spring cannot be defined precisely from the available data, although the degree of attenuation of the isotopic signal indicates mean transit times of about 1·0 to 1·5 years. Sites similar to Žveplenica spring should be carefully analysed because not only do they provide insights into biodiversity but they also represent an ecotone between the anoxic/oxic and high/low sulfide of these karst waters. Copepods in similar geological settings can be used as additional parameters to evaluate conditions in subsurface sulfidic habitats and their spatial connectivity and responses to hydrological events. Copyright © 2014 John Wiley & Sons, Ltd. KEY WORDS Copepoda; ecology; sulphidic water; stable isotopes Received 6 May 2014; Revised 17 November 2014; Accepted 18 November 2014 INTRODUCTION Water is the milieu for all biochemical reactions and a habitat for many organisms. The suitability of aquatic habitats for organisms is determined by the characteristics of the water and, more precisely, by the presence of dissolved and suspended material (Chapman, 1996). Some organisms have wide ecological tolerance to environmental variability, whereas others show very narrow tolerances. Organisms with broad tolerances typically have flexible habitat requirements and are usually much more common. Alter- natively, rare organisms may frequently occur in clumps, so their local density is not necessarily low (Pianka, 2011). Different taxonomic groups of animals have been identified in Žveplenica sulfidic spring, in Slovenia (Mulec et al., 2009). Cyclopoida and Harpacticoida (Crustacea: Copepoda) were the most abundant group collected, representing 84% of the total individuals. The remaining groups were Trichoptera larvae, oligochaets and gastropods. Approximately 275 l of water was discharged from the subsurface of this spring per individual copepod (Mulec et al., 2009). Three copepod species, Bryocamptus echinatus luenensis (Schmeil, 1894), Bryocamptus (Rheocamptus) zschokkei (Schmeil, 1893) and Paracyclops fimbriatus (Fischer, 1853), were identified in Žveplenica spring, with many ovigerous female, copepodits and nauplii observed (Oarga et al., 2010). The presence of copepods at different development stages in Žveplenica spring indicates that they reproduce and have an unusually high ecological tolerance to hydrogen sulfide, which has not been reported previously. The best known sulfidic environments for which stygobiotic species of copepods have been described are *Correspondence to: Janez Mulec, Research Centre of the Slovenian Academy of Sciences and Arts, Karst Research Institute, Titov trg 2, 6230 Postojna, Slovenia. E-mail: janez.mulec@guest.arnes.si † Present Address: Ştefan cel Mare University, Universit  aţii 13, 720229 Suceava, Romania ECOHYDROLOGY Ecohydrol. (2014) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/eco.1585 Copyright © 2014 John Wiley & Sons, Ltd.