The hyporheic zone of Scottish rivers: its ecology, function and importance

Abstract

The hyporheic zone (HZ) has been defined as an active ecotone lying between the river bed and underlying groundwater; it is chemically and biologically distinct from these over- and underlying zones. Research into the chemistry, faunal composition and ecological relationships within the HZ have received relatively little attention until recent years and it has since been shown to have an important impact on riverine ecology. Initial work centred on the development of a simple, robust sampling methodology that could be used to obtain discrete, analysable samples of both invertebrates and water for faunal and chemical analysis. No single sampling methodology was found that fulfilled these criteria, consequently two separate methodologies were used in parallel: Karaman-Chappuis pits excavated in exposed river gravels were used to obtain a shallow (10 cm) sample; modified Bou-Rouch pumping was used to extract a deep (50 cm) sample from below the pit. Initial trials at three sites were used to determine that four replicate pit-pipe samples would extract a representative sample from a site. A total of 25 sites were surveyed across Scotland, these were selected to cover as wide a range of river types, water chemistries, geographical diversity and physical structure as possible. A degree of clustering within the samples was used to help assess between-site differences. The survey found that a well developed hyporheic fauna is present across Scotland. Over 92% of all invertebrates recovered were from pit samples indicating that the fauna is primarily shallow. The composition of the fauna differs from the benthos and is dominated by oligochaetes, cyclopoid copepods, nematodes and dipteran larvae; these four groups accounted for 77% of invertebrates from pits samples and 78% from pipe samples. The pipe samples were not faunally distinct from the pit samples. It appears that the HZ is an important nursery area for the smallest Plecoptera instars. The number of invertebrates recovered and taxonomic diversity were patchy at the local scale but regionally uniform; both tending to decrease northwards. Trends in chemical parameters from montane to lowland sites indicate that considerable changes in environmental chemistry occur along Scottish rivers. Trends at a local (site) scale are less clear, but seem to indicate a degree of within riffle processing, particularly with respect to DO; patterns were broadly similar in pit and pipe samples. While the total number of invertebrates and taxonomic richness in pits decreased downstream through bars this was not evident in pipe samples. It is suggested that the compactedness of sediments acts as a filter so that invertebrate assemblages are at their most developed in the downwelling zone at the head of the bar where the most intense chemical processing occurs. The key drivers of community composition were found to be distance to source, conductivity and source altitude in pits; site altitude, longitude and total alkalinity in pipes. Dissolved oxygen was found to be a key determinant of taxonomic richness. BMWP scores from taxa present in the samples were used to back-calculate scores for their role in bioindication within the HZ. While the revised invertebrate scores ranged from 2.1 to 12.4, the back-calculated results ranged from 5.50 to 7.12. The relationship between the two scoring systems was significant at the P < 0.05 level and indicates that within the HZ high- and low-scoring macroinvertebrates have a higher probability of co-occurring than they do in benthic communities.