Ur-desbideratzeen eragina ibaietako komunitateetan

Abstract

[EUS] Ekosistema osasuntsuak mantentzeko ibaietan garraiatu behar den ur kopurua zenbatekoa den jakitea galdegai errepikakorra bihurtu da ibai-ekologiaren ikerketetan. Ur-desbideratzeak eragin larriak izan ditzake ibai-ekosistemen erregimen hidrologikoan, uraren ezaugarri fisiko-kimikoetan eta ibaietako bizidunen komunitateetan, ekosistemako prozesuetan aldaketak eragiten baititu. Etorkizunean ur-desbideratzeek gora egingo dutenez gero, desbideratze horiek ibaiko komunitateetan dituzten eraginetan sakonkiago aztertzeko ikerketa bat burutu da Deba ibaiko tarte batean, Bergarako Osintxu auzoan (Gipuzkoa), non Sologoen zentral hidroelektrikoaren presa kokatuta dagoen. Ur-murrizpenak ibai-ekosisteman dituen eraginak ebaluatzeko, control eta impact tarteetako (hurrenez hurren, presatik gora eta behera kokatutako 100 metroko tarteak) bizidunen komunitateak konparatu dira. Makroornogabe bentikoen, biofilmaren eta arrainen komunitateak aztertu dira, eta horiekin batera, ibaiko materia organikoaren kopurua ere. Uraren ezaugarri fisiko-kimikoak ere neurtu dira urtebeteko epean zehar. Ur-desbideratzeak nabarmenki murriztu ditu uraren abiadura, emaria eta ubide hezearen zabalera presatik beherako tartean. Hala ere, konduktibitatean, pH balioetan eta mantenugaien kopuruetan ez da ezberdintasun esanguratsurik aurkitu bi tarteen artean. Ikerketaren emaitzek makroornogabe bentikoen murrizpen gogoangarria erakusten dute impact tartean. Izan ere, ornogabe kopurua % 9ra murriztu da presatik beherako tartean. Perifiton komunitatearen barruan, alga berdeen murrizpena bereziki nabarmena da, eta biofilmaz elikatzen diren makroornogabeen taxoi batzuen murrizpena beherakada horrekin lotuta egon daitekeela uste da. Presatik beherako tartean behatutako materia organikoaren murrizpena tarte horretako uraren abiaduraren eta emariaren murrizpenari egotzi zaio, bai eta kanalak desbideratutako uretan galdutako materia organiko kopuruari ere. Amaitzeko, bi tarteen artean, ez da ezberdintasun esanguratsurik aurkitu arrainen ugaritasunean ez eta arrainen tamainan ere. Ezberdintasun nabarmenik aurkitu ez izana lotuta egon daiteke arrainak mugikortasun handiko animaliak izatearekin, ez baitute ikuspegi fidagarria ematen uraren desbideratzeak dituen eraginetan. Ikerketa honen emaitzek bistaratzen dute ur-desbideratzeak eragin esangarria daukala ibaiko bizidunen komunitateetan, eta, gainera, epe luzean, ibai-ekosistemen funtzionamenduan eragina izan dezakeela.

[EN] Understanding how much water must remain in a stream to maintain a healthy functioning ecosystem has become an important focus in stream ecology research. Water abstraction can impact streams by altering the hydrological regime, water physicochemical characteristics, fluvial communities and ecosystem processes. Because water diversion is likely to increase in the near future, a research experiment was carried out in a section of the Deba River in Osintxu, Bergara (Gipuzkoa), where the dam of the Sologoen power plant is located, to further investigate on the effects of water abstraction on fluvial biota. To assess the impact of reduced discharge on the stream ecosystem, fluvial communities from control and impact reaches –upstream and downstream from the dam– were compared. Benthic macroinvertebrate, biofilm and fish communities were studied, as well as the storage of wet organic matter. Water physicochemical characteristics were also measured over the course of one year. Water diversion significantly affected water velocity, river-flow and the width of the wetted channel on the downstream reach. However, conductivity and pH levels, as well as nutrient amount did not show significant differences between the two reaches. The results also point to a considerable loss of benthic macroinvertebrates on the impact reach: out of all the invertebrates that were collected, only 9% belonged to the reach below the dam. Within the periphyton community, the decrease of green algae coverage was particularly evident, and might have had an influence on the decrease of certain macroinvertebrate taxa that feed on biofilm. The decrease of organic matter abundance on the reach below the dam was attributed to the decrease of water velocity and river flow on that reach, as well as to the loss of organic matter through the diversion-canal. Finally, no significant differences were found in fish abundance nor fish average size between control and impact reaches. This might possibly be related to the fact that they are highly mobile species, and therefore do not offer a reliable view on the impact of water diversion. In conclusion, it is clear that water abstraction has a significant effect on fluvial biota and might also affect the overall functioning of stream ecosystems over an increased period of time.