Stoneworts belong to a group of green algae called charophytes. Although they are algae, they have a remarkably complex structure, and are often confused with so-called ‘higher plants’: they look like true waterweeds or even horsetails. One remarkable feature of some charophytes is the size of their individual cells, which can be up to 15 centimetres (6 inches) long.
Why are they important?
33 species of stonewort are known in the UK, but they are highly sensitive to pollution, and, as a result, many are now rare, including several species for which we have special responsibility in Scotland. Brackish lagoons, of which there are many on the Scottish coast, particularly in the Western Isles, are a special habitat for stoneworts. To properly protect stoneworts, we need to ensure the waters in which they live are unpolluted – and that is good news both for stoneworts and for people
Where do they grow?
Stoneworts grow in both clear, fresh water and in brackish water in lagoons and pools on the coast, where they are sometimes the commonest plants. Even man-made lagoons can make good habitats. The lagoon species live in water that is brackish, but not quite as salty as the sea (usually at a salinity between 1% and 3%, compared to around 3.5% or 35 parts per thousand in the sea). One of the most typical, but rarest species, is the beautiful foxtail stonewort Lamprothamnium papulosum found in lagoons in North and South Uist. Bird’s nest stonewort Tolypella nidifica and Baltic stonewort Chara baltica grow in a few similar lagoons in Shetland and the Western Isles.
Other stoneworts, such as the rough stonewort Chara aspera grow in machair lochs, formed where calcium-rich sand blows into lochs in the open grassland along the west coast of the Western Isles and parts of the north-west Highlands. Action to protect them is being taken forward by the Scottish Government through the Saline Lagoons Habitat Action Plan.