CAVE AND KARST SCIENCE Vol. 36, No. 3, 2009 (published in 2010) Transactions of the British Cave Research Association © British Cave Research Association 2010 ISSN 1356-191X Ecology of the hyporheic zone: a review. Octavian PACIOGLU Roehampton University, School of Human and Life Sciences, Centre for Research in Ecology, London, UK. e-mail: o.pacioglu@roehampton.ac.uk Abstract: It is widely recognized that the hyporheic zone is a crucial component of river ecosystems, structurally and functionally. This article provides a contemporary review outlining the historical development of research in the field and an overview of the current state of knowledge regarding the structure and functioning of hyporheic invertebrate assemblages. The paper makes a call for a more holistic approach to the study of the hyporheic zone and the invertebrate communities that inhabit it. Keywords: hyporheic, stygobionts, chalk, karst, nitrate, organic pollution (Received 11 November 2009; Accepted 06 June 2010.) 69 A short history of hyporheic ecosystem investigations The hyporheic zone comprises the saturated sediments beneath and adjacent (extending some distance onto the floodplain in many instances) to streams and rivers. Although many scientists have attempted to define the hyporheic zone, Brunke and Gonser (1997) describe it as an ecotone that can be delimited by the rest of the adjacent environments because it combines the characteristics of both surface and groundwater habitats, but also develops its own biological and environmental gradients. The hyporheos is described as the invertebrate associations related to this habitat (Williams, 2003). The history of hyporheic research can be divided into three periods, following the development of ecology as a science: i) autecologic, ii) synecologic, and iii) present state of knowledge. The autecologic period (1955–1965) The term “hyporheic” was introduced in the literature in 1955 by Orghidan. It became recognized internationally and was adopted by other European researchers, despite opposition from the founder of phreatobiology, who defined the hyporheic zone as “the upper part of the phreatic” (Motas, 1962). Chappuis (1950) and White (1960) upheld this latter view in their description of the phreatic zone, and Delamare-Debouteville (1960) considered the fauna beneath rivers as being the “real phreatic fauna” (Williams and Hynes, 1974). On the Glossary of terms used in the text (alphabetical): Term Definition Published source Amphibiont (or amphibite) A particular kind of stygophile whose life cycle necessitates the use of both surface and groundwater habitats (like some species of stoneflies). Gibert et al.,1994 Epigean The surface water ecosystems (rivers, lakes, ponds, etc.). Gibert et al.,1994 Epikarst The upper surface of karst, consisting of a network of intersecting fissures and cavities that collect and transport surface water and nutrients underground; epikarst depth can range from a few centimetres to tens of metres. Pipan and Culver, 2007 Eupsammon Community of species living in the area situated further away from the sandy river bed that has fluctuating levels of humidity, but is never submerged. Wiszniewski,1948 Hydropsammon Community of species living in the area of a sandy river bed that is permanently submerged. Wiszniewski,1948 Hygropsammon Community of species living in an area of a sandy river bed that is occasionally submerged (during spates or high flow). Wiszniewski,1948 Hypogean All the ecosystems and habitats related to groundwater influence (phreatic, hyporheic, hypotelminorheic, psammal). Gibert et al.,1994 Hyporheos Community of invertebrates occurring occasionally or permanently in the hyporheic zone. Williams, 2003 Hypotelminorheic A peculiar type of habitat formed by the groundwater occurring at the surface on a slope with different angles and retained by an impermeable layer (commonly but not inevitably clay). Mestrov, 1962 Phreatobionts Similar to stygobionts, but restricted to deep phreatic waters (e.g. not related to occurrences in caves); a legacy name that bears mainly a historical value rather then a current scientific validity. Motas, 1962 Psammolitoral Species living in sandy habitats that are lateral expansions from the main water body (in some cases overlapping with the Eupsammon). Schmid-Araya, 1998 Psammon Comprises a community of organisms living in the sandy shore areas of lakes/ rivers that are a transitional zone between aquatic and soil habitats. Schmid-Araya, 1998 Stygobiont The stygobionts are specialized/obligate subterranean forms, completing their life cycle entirely within the groundwater. Gibert et al., 1994 Stygophile Species that actively exploit the groundwater environment, but can also live and reproduce in surface water habitats. Gibert et al., 1994 Stygoxene Species that have no affinities with groundwater, but do occur accidentally in caves or alluvial sediments. Gibert et al., 1994 other hand, Orghidan (1959) and Schrwoebel (1961) championed the existence of the hyporheic area as a transition zone between rivers and groundwater. Both argued that the physico-chemical characteristics of the hyporheic water were influenced by the surface waters (Danielopol, 1982a), predating the present concepts of up and downwelling areas of riffles and its crucial influence on water circulation patterns within the hyporheic zone. This period was dominated by taxonomic descriptions of hundreds of new species, but with only a very crude recognition of different habitat types, such as the hyporheic zone (Orghidan, 1955), psammon (Motas, 1962) or the hypotelminorheic (Mestrov, 1962). The synecological period of phreatobiology and hyporheic investigations (1965–1985) During this period, the phreatic waters and the hyporheic zone were considered as two separate ecosystems of equal importance. Between 1967–1985, intensive investigations led to further discoveries of new species of Hydracarina, Oligochaeta, Tricladida and several other microcrustacean groups (e.g. Cyclopoida, Harpacticoida, Ostracoda) (Botoseneanu, 1977; Danielopol, 1976, 1980, 1982b; Magniez, 1976; Lescher-Moutoue, 1974; Negrea, 2007; Williams and Hynes, 1974; Godbout and Hynes, 1982; Henry, 1976; Marmonier and Dole-Olivier, 1986; Rouch, 1988; Tetart, 1974). Some species were epibenthic (living in the upper few centimetres of sediment and only inhabiting the hyporheic zone occasionally) whereas others were hyporheobionts