Silikatne alge su dominantna grupa u zajednici fitobentosa slatkovodnih ekosistema, široko su rasprostranjene, prisutne tokom cele godine i naseljavaju raznovrsna staništa. S obzirom da su dobri bioindikatori, u mnogim zemljama su razvijeni ekološki indeksi zasnovani na silikatnim algama (dijatomni indeksi), sa ciljem ocene ekološkog statusa voda i praćenja promena u vodenim ekosistemima. U skladu sa odrednicama Okvirne direktive o vodama, u Srbiji je zakonskom i podzakonskom regulativom ozvaničena ocena ekološkog statusa/potencijala na osnovu bioloških elemenata kvaliteta vode, u koje se ubrajaju bentosne silikatne alge. Algološka istraživanja reka Velike Morave i Save do sada su se prvenstveno odnosila na zajednicu fitoplanktona, dok su ispitivanja zajednica bentosnih silikatnih algi ovih reka započeta poslednjih godina. Istraživanja koja uključuju procenu ekološkog statusa velikih reka na osnovu bentosnih silikatnih algi do sada nisu rađena na teritoriji Srbije. Ciljevi našeg istraživanja su bili: analiza kvalitativnog i kvantitativnog sastava bentosnih silikatnih algi, utvrđivanje njihove prostorne i sezonske dinamike, utvrđivanje vrednosti fizičkih i hemijskih parametara na lokalitetima i određivanje najznačajnijih sredinskih faktora koji deluju na ispitivane zajednice, utvrđivanje kvaliteta vode na osnovu vrednosti dijatomnih indeksa korišćenjem OMNIDIA softvera, ocena ekološkog potencijala Velike Morave i Save (deo toka kroz Srbiju), u skladu sa zakonskom regulativom Republike Srbije, razmatranje efikasnosti korišćenja bentosnih silikatnih algi kao indikatora u monitoringu voda i delotvornosti predviđenih ekoloških indeksa, ocena ekološkog potencijala i ukazivanje na prisustvo specifičnih indikatorskih taksona i formi. Uzorci fitobentosa iz Velike Morave su prikupljeni tokom jednogodišnjeg perioda (od 2010. do 2011.) sa pet lokaliteta, dok su uzorci iz Save prikupljeni tokom septembra četiri godine (2011., 2012., 2014. i 2015.) sa ukupno 33 lokaliteta duž celog toka. Nakon obrade materijala, izvršena je analiza zajednica bentosnih silikatnih algi ispitivanih reka prema ciljevima istraživanja. U Velikoj Moravi je utvrđeno prisustvo 162, dok je u Savi utvrđeno prisustvo 184 taksona. Dominantni i učestali taksoni Save i Velike Morave ubrajaju se u eurtofne i hipereutrofne taksone. Na sezonsku dinamiku bentosnih silikatnih algi Velike Morave, najveći uticaj imaju pH, temperatura i arsen. Najznačajniji sredinski parametri koji utiču na zajednicu silikatnih algi bentosa Save su arsen i silicijum, pri čemu je najveći uticaj identifikovan na lokalitetima u donjem toku ove reke. Duž toka Save sastav zajednice bentosnih silikatnih algi se menja od dominacije formi blisko pričvršćenih za supstrat (gornji tok), do pokretnih formi (srednji i donji tok), u skladu sa opštim promenama reke Save, gde ona prelazi iz sub- alpijske reke u nizijsku ravničarsku reku, sa dominacijom sitnijih frakcija supstrata. Visoke vode zabeležene u septembru 2014. godine na Savi, nisu uticale na smanjenje vrednosti Shannon-ovog indeksa diverziteta, što potvrđuje otpornost bentosnih silikatnih algi na takvu vrstu pritiska. Naša istraživanja ukazuju da su i velike reke važno stanište za bentosne silikatne alge. Vrsta Mayamaea cahabaensis po prvi put identifikovana u Velikoj Moravi i Savi (a samim tim i u flori silikatnih algi Srbije), zabeležena je sa velikom brojnošću. U Savi su zabeležene dve potencijalno invazivne vrste silikatnih algi ‒ Didymosphenia geminata i Diadesmis confervacea, prisutne sa malom brojnošću. Teratološke forme silikatnih algi primećene su na svim lokalitetima na Velikoj Moravi i na nekoliko lokaliteta na Savi. Potvrđeno je da udeo teratoloških formi u zajednici silikatnih algi ima bioindikatorski potencijal. Indeks IPS pruža pouzdaniju ocenu u odnosu na drugi indeks propisan nacionalnom regulativom – CEE, u slučaju Velike Morave. Potrebno je razmotriti promenu granica klasa za CEE indeks za tip 2 vodotokova. Dijatomni indeksi su osetljivi na povećane koncentracije arsena i gvožđa, iako su osmišljeni prvenstveno kao indikatori organskog zagađenja i opterećenja nutrijentima. Na osnovu ovoga razmatranja, silikatne alge se mogu smatrati pouzdanim indikatorom prisustva višestrukih pritisaka u slučaju velikih ravničarskih reka, odnosno mogu se koristiti kao parametar opšte degradacije. Upotreba standardne metodologije uzorkovanja bentosnih silikatnih algi za rutinski monitoring je potvređena kao pouzdana, za tipove voda kojima pripadaju Velika Morava i Sava. Diatoms are dominant group in a phytobenthos community of freshwater ecosystems, widely distributed, present throughout the year and they inhabit diverse habitats. Considering to be good bioindicators, many countries developed assessment of the ecological status of waters and monitoring of changes in aquatic ecosystems based on diatom indices. In accordance with the guidelines of the Water Framework Directive, the legislation of the Republic of Serbia formalized the assessment of ecological status/potential on the bases of biological water quality elements, which include benthic diatoms. Until now algological investigations of the Velika Morava River and the Sava River primarily concerned the phytoplankton community, while the studies of the benthic diatoms communities recently began. Studies that include the assessment of the ecological status of large rivers based on benthic diatoms have not been done on the territory of Serbia so far. The goals of our research were: qualitative and quantitative analysis of the benthic diatoms community composition, to determine their spatial and seasonal dynamics, to determine the values of physical and chemical parameters at sampling sites and the most important environmental parameters that affect the investigated communities, assessment of the water quality based on the diatom indices values using OMNIDIA software, assessment of the ecological potential of the Velika Morava River and the Sava River (part of the flow through Serbia), in accordance with the legislation of the Republic of Serbia, consideration of the efficiency of benthic diatoms as indicators in the water monitoring and the effectiveness of selected ecological indices, evaluation of environmental potential and identification of the specific indicator taxa and forms. Phytobenthos samples from the Velika Morava River were collected annually (from 2010 to 2011) at five sampling sites, while samples from the Sava River were collected each September during a period of four years (2011, 2012, 2014 and 2015) at a total of 33 sites along the entire watercourse. After processing the material, the analysis of the benthic diatoms community was carried out according to the research objectives. In the Velika Morava River presence of 162 taxa was detected, while in the Sava River presence of 184 taxa was registered. The dominant and frequent taxa of the Sava and the Velika Morava rivers are considered as eutrophic and hypereutrophic taxa. The greatest influence on seasonal dynamic of benthic diatoms of the Velika Morava have environmental parameters pH, temperature and arsenic. The most important environmental parameters affecting the community of benthic diatoms of the Sava are arsenic and silicon, with the greatest influence on sites in the lower course of the river. Along the Sava River, the composition of the benthic diatoms communities changes from the dominance of the forms closely attached to the substrate (upstream) to the dominance of the motile forms (middle and lower flow), which is in accordance with the general changes in the Sava River, from the sub-alpine river to the lowland river, with the dominance of the smaller fractions of the substrate. The high water levels recorded in September 2014 on the Sava River, didn't result in decrease of Shannon's diversity index values, which confirms the resistance of benthic diatoms to this type of pressure. Our research suggests that large rivers are an important habitat for benthic diatoms. The species Mayamaea cahabaensis, first time identified in the Sava and the Velika Morava rivers (and therefore in diatom flora of the Serbia), was recorded with a high abundance. Two potentially invasive taxa ‒ Didymosphenia geminata and Diadesmis confervacea are present in the Sava River with a low abundance. Teratological forms of diatoms have been recorded at all sites in the Velika Morava River and at several sites on the Sava River. It has been confirmed that the share of teratological forms in diatom community has a bioindicator potential. In the case of the Velika Morava River, our research indicate that assessment according to national legislation is more reliable using IPS index in comparison to CEE. It is necessary to consider changing class boundaries for the CEE index for type 2 watercourses. Diatom indices are sensitive to increased concentrations of arsenic and iron, although indices were primarily designed as indicators of organic pollution and nutrient load. Having this in mind, benthic diatoms can be considered as a reliable indicator of the presence of multiple pressures in the case of large lowland rivers, and they can be used as a parameter of general degradation. The reliability of the standard methodology for benthic diatoms sampling in routine monitoring, in the case of the Velika Morava and Sava rivers, is confirmed.