The hyporheic zone
The hyphorheic zone can be regarded as the heart of a river. Without a healthy, intact hyphoreic zone, river ecosystems could not function satisfactorily.
The hyphorheic zone is the pore space at the interface between surface- and ground- water, and encompasses the areas both beneath, and laterally of, the bed of a river or lake. Water in this zone necessarily comprises a mixture of both surface and ground water components.
Interactions between surface waters and groundwater are strongly affected by the composition of the pore system. The hyporheic zone harbours the most part of the water bodies’ biomass (animals, micro organisms). Thus, the decomposition of organic material, the so-called natural purification, occurs primarily within the hyporheic zone.
Currently, the main problem with many flowing water systems is the transportation of fine sedimentary material from the catchment area, which accumulates and causes blockages of the porous system. This is known as Clogging or Colmation, and leads to a rigorous reduction of species richness and abundances, and to a decrease of the natural purification.
Colmation is the main cause of the reduction of freshwater pearl mussel populations. Furthermore as some fish, such as salmon and trout, require freshwater stream gravels for spawning, their reproduction is directly endangered by clogging.
Surface waters (lakes, rivers, estuaries and coastal waters) must reach at least ‘good’ ecological status by 2015. This is the intention of the EU water framework directive (WFD, 2000/60/EC). For this purpose analysis and assessment of the condition and deficits of surface waters are required and water management plans have to be developed and implemented.
Surprisingly, the hyporheic zone is not incorporated into the WFD. Thus, although some rivers attain a “good” evaluation, this is based upon the hydrochemical and structural quality only; aquatic communities are not considered. Clearly however, the importance of the hyporheic zone must be considered, in order to achieve a realistic ecological “good” status.
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