Callitriche brutia

About

Callitriche brutia, the pedunculate water-starwort, is a dwarf amphibious annual or perennial dicotyledon herb in the genus Callitriche. The common name of this species is water starwort. It colonizes aquatic environments, such as wetlands, lakes, rivers, streams, and pools, and it grows in oligotrophic conditions, meaning it grows in an environment that has little to sustain life. Pedunculate water-starwort was found in Ireland and England, but is now an invasive species due to its resistance to environmental stressors, and its ability to grow in still water.

Summary from Wikipedia