Stoneworts, or Charophytes, are structurally complex green algae which thrive in relatively unpolluted freshwater habitats. Nick Stewart, BSBI’s Charophyte referee, has put together a register of the 15 species occurring in Somerset, see the Rare Plant Register menu. The ditches of the Levels are the main area of interest in Somerset, which hold the largest UK populations of Great Tassel Stonewort (Tolypella prolifera).
An excellent written introduction to the group and their habitat needs is available in Nick’s report on Important Stonewort Areas for Plantlife. There are also links to presentations and an identification key on the BSBI Charophyte page and a Stonewort Identification video on the BSBI Aquatic Plants Project page.