ALBERT

Notes: 1. Longitudinal changes in physicochemical factors and the composition of the invertebrate community were examined in the hyporheic zone of a glacial river (Val Roseg, Switzerland) over a distance of 11 km from the glacier terminus. Multivariate analysis was used to determine the habitat preferences of taxa along an upstream-downstream gradient of increasing temperature and groundwater contribution to river flow.2. The hyporheos conformed to the longitudinal distribution model described for zoobenthic communities of glacial rivers in that taxonomic richness increased with distance from the glacier terminus. Spatial variation in taxonomic richness was best explained by temperature, the influence of groundwater, and the amount of organic matter. The overriding importance of these variables on the distribution of taxa was confirmed by the multivariate analysis.3. The hyporheic zone contributed significantly to the overall biodiversity of the Roseg River. Whereas insect larvae were predominant in the benthos, hyporheic invertebrates were dominated by taxa belonging to the true groundwater fauna and the permanent hyporheos. Several permanently aquatic taxa (e.g. Nematoda, Ostracoda, Cyclopoida, Harpacticoida, Oligochaeta) appeared exclusively in the hyporheic zone or they extended farther upstream in the hyporheic layer than in the benthic layer. Leuctridae, Nemouridae, and Heptageniidae colonised hyporheic sediments where maximum water temperature was only 4 °C.4. Despite strong seasonal changes in river discharge and physicochemistry in hyporheic water, the density and distribution of the hyporheos varied little over time.5. Taxonomic richness increased markedly in the downstream part of a floodplain reach with an extensive upwelling zone. Upwelling groundwater not only maintained a permanent flow of water but also created several species-rich habitats that added many species to the community of the main channel.

Notes: 1. The first part of this review focuses on the oxygen status of natural groundwater systems (mainly porous aquifers) and hyporheic zones of streams. The second part examines the sensitivity of groundwater organisms, especially crustaceans, to low oxygen concentrations (〈 3.0 mg L−1 O2).2. Dissolved oxygen (DO) in groundwater is spatially heterogeneous at macro- (km), meso- (m) and micro- (cm) scales. This heterogeneity, an essential feature of the groundwater environment, reflects changes in sediment composition and structure, groundwater flow velocity, organic matter content, and the abundance and activity of micro-organisms. Dissolved oxygen also exhibits strong temporal changes in the hyporheic zone of streams as well as in the recharge area of aquifers, but these fluctuations should be strongly attenuated with increasing distance from the stream and the recharge zone.3. Dissolved oxygen gradients along flow paths in groundwater systems and hyporheic zones vary over several orders of magnitude (e.g. declines of 9 × 10−5 to 1.5 ×10−2 mg L−1 O2 m−1 in confined aquifers and 2 × 10−2 to 1 mg L−1 O2 m−1 in parafluvial water). Several factors explain this strong variation. Where the water table is close to the surface, oxygen is likely to be consumed rapidly in the first few metres below the water table because of incomplete degradation of soil-generated labile dissolved organic carbon (DOC) in the vadose zone. Where the water table is far from the surface, strong oxygen depletion in the vicinity of the water table does not occur, DO being then gradually consumed as groundwater flows down the hydraulic gradient. In unconfined groundwater systems, oxygen consumption along flow paths may be compensated by down-gradient replenishment of DO, resulting either from the ingress of atmospheric oxygen or water recharge through the vadose zone. In confined groundwater systems, where replenishment of oxygen is impossible, the removal time of DO varies from a few years to more than 10 000 years, depending mainly on the organic carbon content of the sediment. Comparison of the hyporheic zones between systems also revealed strong differences in the removal time and length of underground pathways for DO. This strong variability among systems seems related to differences in contact time of water with sediment.4. Although groundwater macro-crustaceans are much more resistant to hypoxia than epigean species, they cannot survive severe hypoxia (DO 〈 0.01 mg L−1 O2) for very long (lethal time for 50% of the population ranged from 46.7 to 61.7 h). In severe hypoxia, none of the hypogean crustaceans examined utilized a high-ATP yielding metabolic pathway. High survival times are mainly a result of the combination of three mechanisms: a high storage of fermentable fuels (glycogen and phosphagen), a low metabolic rate in normoxia, and a further reduction in metabolic rate by reducing locomotion and ventilation. It is suggested here that the low metabolic rate of many hypogean species may be an adaptation to low oxygen and not necessarily result from an impoverished food supply.5. An interesting physiological feature of hypogean crustaceans is their ability to recover from anaerobic stress and, more specifically, rapidly to resynthesize glycogen stores during post-hypoxic recovery. A high storage and rapid restoration of fermentable fuels (without feeding) allows groundwater crustaceans to exploit a moving mosaic of suboxic (〈 0.3 mg L−1 O2), dysoxic (0.3–3.0 mg L−1 O2) and oxic (〉 3 mg L−1 O2) patches.6. It is concluded that although hypogean animals are probably unsuited for life in extensively or permanently suboxic groundwater, they can be found in small or temporarily suboxic patches. Indeed, their adaptations to hypoxia are clearly suited for life in groundwater characterized by spatially heterogeneous or highly dynamic DO concentrations. Their capacity to survive severe hypoxia for a few days and to recover rapidly would explain partly why ecological field studies often reveal the occurrence of interstitial taxa in groundwater with a wide range of DO.

Notes: SUMMARY 1. We examined the thermal patterns of the surface waters in the catchment of the Roseg River, which is fed by the meltwaters of two valley glaciers. One of the glaciers has a lake at its terminus. The river corridor comprised a proglacial stream reach below one glacier, the glacier lake outlet stream, a 2.5-km long complex floodplain and a constrained reach extending to the end of the catchment.2. Temperatures were continuously measured with temperature loggers at 27 sites between 1997 and 1998. Moreover, from 1997 to 1999, spot measurements were taken at 33–165 floodplain sites (depending on water level) at monthly intervals.3. The temperature regime of glacial streams, including the glacier lake outlet, was characterised by rapidly increasing temperatures in April and May, a moderate decline from June to September (period of glacial melt) and a subsequent fast decline in autumn. During summer, the lake increased temperatures in the outlet stream by 2–4 °C, compared with the adjacent proglacial stream reach.4. In the main channel (thalweg) of the Roseg River, annual degree-days (DD) ranged from 176 DD in the upper proglacial reach to 1227 DD at the end of the catchment.5. Thermal variation among different channels within the floodplain was higher than the variation along the entire main channel. Floodplain channels lacking surface connection to the main channel accumulated up to 1661 annual DDs.6. Thermal heterogeneity within the floodplain was linked to the glacial flow pulse. With the onset of ice melt, temperatures in the main channel and in channels surface-connected to the main channel began to decline, whereas in surface-disconnected channels temperatures continued to increase; as a consequence, thermal heterogeneity at the floodplain scale rose slightly until September.7. High thermal heterogeneity was not anticipated in the harsh environment of a largely glacierised alpine catchment. The relatively wide range of thermal environments may contribute to the highly diverse zoobenthic community.