Hyporheic zone is a part of a fluvial system. The term “hyporheic’originates form Greek: hypo is a prefix meaning “below”, the noun rheos means “stream”. Hyporheic zone is a transitional ecotone between surface and groundwater in porous-bed rivers, characterized by the water exchange, which affects water quality. The lateral extension of a hyporheic zone below the riparian zone is known as a parafluvial zone (Fig. 1) Hyporheic ecology is a branch of a young discipline known as a groundwater ecology, which studies the interactions between groundwater organisms and their environment, both aquifer and connected terrestrial system. This discipline has been developing very intensively from the 1990s. Then the first papers on the surface water/groundwater ecotone concept were published. The ecotone approach to groundwater ecology was a turning point because surface and groundwaters were considered separately before, e.g. in 1975 in Schlitz, Germany, took place “International Colloquium on Niphargus, Gammarus and groundwater ecology”, continued in next years in Blacksburg (USA) and Łódź (Poland).The simplified mechanisms of a hyporheic filtration are as follows. The riverine surface water transporting different chemical substances (organic matter, oxygen), infiltrates into the hyporheic interstices (hyporheal), moves through and out of it in a downstream direction (Fig. 2). Three mechanisms which alter water quality occur in the hyporheic zone: physical filtration by the sediment matrix, biological filtration by the microbial biofilm and chemical filtration by reactions such as mineral and redox processes. Hyporheic interstitial organisms (hyporheos): bacteria, fungi, meio- and macrofauna, use an organic matter input as a source of energy, utilizing it to the carbon dioxide and water. There is lack of information concerning efficiency of hyporheic filtration. The problem is interesting not only from the scientific point of view but also has practical implications, especially when taking into consideration that quality of the exploitable sources of fresh water is still getting worse. It seems that bioenergetics might be a very usefull tool which help us to find the answer for a key question: how many organic matter hyporheic organisms are able to utilize? The Vistula’s River sand banks will be a proper study area. It is expected that sand banks can play an important role in the river self-purification processes in the sense of an organic matter utilization. Studies carried out in a Baltic Sea sandy beach showed that coastal sands are very active heterotrophic systems and can exhibit high mineralization rates (Figs. 3,4). If it is proved that sand bank are efficient natural filters, one more argument against the Vistula River regulation will be gained. One has to remember that the natural uniqueness of the Vistula River is endangered by the planned river regulation and land use.
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