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Vanduo upėse

Augalus, gyvus organizmus bei vandenį, sausumą, ir atmosferą į nedalomą visumą jungia ne vien vandens apytakos ratas. Visa susieja gyvybės saitai, kartais akivaizdūs, o dažnai tik numanomi, pažįstami tiems, kurie sugeba įžvelgti gamtos vyksmų esmę. Dažnai sunku nusakyti vandens ekosistemos ribas, nes jos dalių tarpusavio priklausomybė ir medžiagų apytaka neapsiriboja vandens telkiniu ir aprėpia didesnes aplinkos sritis. Svarbi upės, upelio ekosistemos ypatybė – vandens tėkmė. “Negalima dukart įbristi į tą pačią upę”, – sakydavo senovės išminčiai, norėdami pabrėžti nenutrūkstamą pasaulio kaitą. Gamtinėje ekosistemoje nusistovi organizmų rūšinė sudėtis ir jų tarpusavio ryšiai, gyvūnai gerai prisiderina prie savo gyvenamosios aplinkos. Vienok, vandens ekosistemos vis dažniau patiria antropogeninį poveikį. Teršalai į vandenį gali pakliūti iš taškinių taršos šaltinių – buitinės kanalizacijos vamzdžių, pramonės įmonių vandens išleidėjų. Dažnai vandenį užteršia išsklaidytos taršos šaltiniai. Tai iš dirvožemio išplautas trąšų ir pesticidų perteklius, lietaus vandens nuotekos iš miestų gatvių, kuriose yra naftos produktų ir sunkiųjų metalų, įvairių avarijų metu į vandenį pakliūvančios nuodingos medžiagos. Taršos padariniai gali pasireikšti net labai toli nuo taršos šaltinio.

upe rudeniBuitinės nuotekos ir iš dirvožemio išplautos trąšos (fosfatai, nitratai) padidina fosforo ir azoto koncentraciją vandenyje. Tai sukelia eutrofikaciją. Eutrofikaciją patiria beveik visos Lietuvos upės. Svarbiausias eutrofikacijos požymis – vandens “žydėjimas”. Fosforas ir azotas skatina dumblių ir aukštesniųjų augalų augimą. Organinių medžiagų perteklius nusėda į dugną, dumblėja pakrantės. Skaidydamos vandenyje organines atliekas (augalų dalis, buitines bei maisto pramonės nuotekas it kt.), daugelis bakterijų taip pat vartoja deguonį, todėl jo ima trūkti.

Svarbus išsklaidytos vandens taršos šaltinis – oro tarša. Visos dalelės ir dujos iš oro pagaliau patenka į vandenį ar dirvožemį. Iš dirvožemio jos taip pat gali būti išplaunamos į upelius, upes, ežerus. Azoto oksidai (NOx) iš automobilių išmetimo vamzdžių ir sieros dioksidas (SO2) iš iškastinį kurą deginančių šiluminių elektrinių kaminų ištirpsta vandens garų lašeliuose ir sugrįžta į žemę rūgštaus lietaus pavidalu. Ypatingai pavojingi gamtai dioksinai. Vėjas juos gali pernešti dideliais atstumais, dirvoje ir vandenyje jie išlieka ne vienerius metus. Gamtoje dioksinai savaime nesusidaro, todėl jie gyvieji organizmai neatsparūs jo poveikiui. Dioksinai kaupiasi vandens gyvūnų, ypač žuvų, riebaliniuose audiniuose, sukelia jiems vėžį, kepenų veiklos ir dauginimosi sutrikimus. Rūpesčių kelia ne tik vandens tarša, bet ir krantų erozija, kurią pagreitina netinkamas ūkininkavimas pakrantėse. Dėl erozijos kyla dvi problemos – siaurėja upelio vaga ir padaugėja sąnašų. Drumstas vanduo užkemša žuvų žiaunas ir apsunkina kvėpavimą, dėl prasto matomumo sunkiau susirasti maisto ir vengti pavojų. Uždengusios dugno žvyrą sąnašos apsunkina žuvų neršimą (ikrams trūksta deguonies), jos užpildo duburius, kur gyvena mailius. Dėl vandens drumstumo į vandenį patenka mažiau saulės spindulių, todėl sumažėja fotosintezės intensyvumas. Beje, ir vaikai dažnai skatina eroziją, šokinėdami, šliuoždami nuo krantų į vandenį, važinėdamiesi pakrantėje dviračiais ir taip sunaikindami augmeniją. Medžių ir kitų augalų šaknys sulaiko krantų eroziją, saugo dirvą nuo išplovimo.

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