Expansion of emergent plants has the potential to alter aquatic ecosystems. Especially, lotus（Nelumbo nucifera）forms a dense canopy and may dramatically influence environmental conditions and food webs. We tried to reveal food webs of a lotus-covered（60% of surface area）lake Izunuma and the effects of N. nucifera on it. The carbon and nitrogen stable isotope ratios of organic sources and of dominant invertebrates and common two fish species were measured. Isotope signatures indicated that epiphytes and particulate organic matter（POM）are dominant sources of invertebrate diets. Moreover, the ¹³C-depleted isotopic signatures of some invertebrates suggest that methane-oxidizing bacteria（MOB）is a potentially important food source. Fish was considered to ingest mainly epiphytes and invertebrates. Expanding N. nucifera increase the dietary contribution of epiphytes in food webs and may increase of MOB because dense beds of it can cause localized oxygen depletion, which provides MOB with suitable environment.