Freshwater macroalga, Ulva pilifera (Ulvaceae, Chlorophyta) as an indicator of the trophic state of waters for small water bodies

•Use of U. pilifera as an indicator is compatible with the WFD guidelines.•Freshwater U. pilifera occurs in eutrophic ponds.•U. pilifera can be easily identified even by non-expert.•U. pilifera can be used as trophic indicator for pond systems.•U. pilifera should be included into pond monitoring protocols.