Oxygen supply and the adaptations of animals in groundwater

1. The first part of this review focuses on the oxygen status of natural gr oundwater systems (mainly porous aquifers) and hyporheic zones of streams. The second part examines the sensitivity of groundwater organisms, especial ly crustaceans, to low oxygen concentrations (< 3.0 mg L-1 O-2) 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 compos ition and structure, groundwater flow velocity, organic matter content, and the abundance and activity of micro-organisms. Dissolved oxygen also exhib its strong temporal changes in the hyporheic zone of streams as well as in the recharge area of aquifers, but these fluctuations should be strongly at tenuated with increasing distance from the stream and the recharge zone. 3. Dissolved oxygen gradients along flow paths in groundwater systems and h yporheic zones vary over several orders of magnitude (e.g. declines of 9 x 10(-5) to 1.5 x10(-2) mg L-1 O-2 m(-1) in confined aquifers and 2 x 10(-2) to 1 mg L-1 O-2 m(-1) in parafluvial water). Several factors explain this s trong variation. Where the water table is close to the surface, oxygen is l ikely to be consumed rapidly in the first few metres below the water table because of incomplete degradation of soil-generated labile dissolved organi c carbon (DOC) in the vadose zone. Where the water table is far from the su rface, strong oxygen depletion in the vicinity of the water table does not occur, DO being then gradually consumed as groundwater flows down the hydra ulic gradient. In unconfined groundwater systems, oxygen consumption along flow paths may be compensated by down-gradient replenishment of DO, resulti ng either from the ingress of atmospheric oxygen or water recharge through the vadose zone. In confined groundwater systems, where replenishment of ox ygen 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 se diment. Comparison of the hyporheic zones between systems also revealed str ong differences in the removal time and length of underground pathways for DO. This strong variability among systems seems related to differences in c ontact time of water with sediment. 4. Although groundwater macro-crustaceans are much more resistant to hypoxi a than epigean species, they cannot survive severe hypoxia (DO < 0.01 mg L- 1 O-2) 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 mai nly a result of the combination of three mechanisms: a high storage of ferm entable fuels (glycogen and phosphagen), a low metabolic rate in normoxia, and a further reduction in metabolic rate by reducing locomotion and ventil ation. It is suggested here that the low metabolic rate of many hypogean sp ecies 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 ab ility to recover from anaerobic stress and, more specifically, rapidly to r esynthesize glycogen stores during post-hypoxic recovery. A high storage an d rapid restoration of fermentable fuels (without feeding) allows groundwat er crustaceans to exploit a moving mosaic of suboxic (< 0.3 mg L-1 O-2), dy Soxic (0.3-3.0 mg L-1 O-2) and oxic (> 3 mg L-1 O-2) 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 hypox ia are clearly suited for life in groundwater characterized by spatially he terogeneous or highly dynamic DO concentrations. Their capacity to survive severe hypoxia for a few days and to recover rapidly would explain partly w hy ecological field studies often reveal the occurrence of interstitial tax a in groundwater with a wide range of DO.