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Подземные воды и пещеры

Подземные воды: фауна и адаптации к дыханию

Volume 41 Issue 1 Page 1 - February 1999

Oxygen supply and the adaptations of animals in groundwater

F. Malard* and F. Hervant

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 (<3.0mgL1O2).

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 9105 to 1.5

102mgL1O2m1 in confined aquifers and 2102 to 1mgL1O2m1 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 10000years, 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<0.01mgL1O2) for

very long (lethal time for 50% of the population ranged from 46.7 to 61.7h). 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 (<0.3mgL1O2), dysoxic (0.3-3.0mgL1O2) and oxic (>3

mgL1O2) 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.

Биология пресных вод: о науке.

The state of freshwater ecology

Colin S. Reynolds

1. The case is advanced that freshwater ecologists need to champion the relevance

of their work to the development of ecological theory and to the understanding of

ecosystem function and behaviour, not least for its importance in addressing

pressing applications to the stewardship of the biosphere. An essential step is to

review, and update where necessary, the paradigms of aquatic ecology.

2. It is proposed that the major constraint on the organisms, their attributes and

adaptations are related first to the physical properties of the medium in which they

live. The drives to grow and reproduce relate to the trophic transfer of reduced

carbon with important microbial interventions. General principles of emergy apply.

The supportive capacities of given environments may be set by chemical

constraints, but it is suggested that, with the exception of chronically resource

deficient waters, population dynamics relate to opportunities incumbent upon

system variability and the consequent pulsation of resources.

3. Variability affects diversity, through frequent revision of the thermodynamic

base. Frequent structural change promotes species diversity and, because function

is maintained, it appears that efficient function is dependent upon high diversity.

Caution is necessary because high productivity and high diversity are both products

of the disturbances consequent upon external forcing and manifestly nonequilibrium

conditions.

4. Reactions to these statements are canvassed.