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 necessari...

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