Water pollution a major threat to Stoneworts

By Jim Hurley

Stoneworts grow underwater, usually in freshwater, and are not flowering plants.
Stoneworts grow underwater, usually in freshwater, and are not flowering plants.

Stoneworts are interesting plants. A 'wort' is an old English name for a plant, herb or weed and the 'stone' part of their name refers to the fact that, in hard waters, the plants can become encrusted with lime making them feel gritty in the hand.

They grow underwater, usually in freshwater, and are not flowering plants; they are algae, freshwater seaweeds, if you like. While they can tolerate some salt, and are often found in brackish lagoons by the coast, they do not survive in pure seawater.

Brackish water is water that is neither fresh nor salt; it is a mixture of the two and can range along a spectrum from almost pure freshwater to almost pure seawater. Seawater around Ireland generally contains about 35 gramme of salt per kilogramme of water. Its salinity is therefore 35g/kg.

Another way of putting it is that the water contains 35 parts of salt per thousand parts of water. The modern preferred equivalent unit of measurement is the 'psu', the 'practical salinity unit', so the salinity of seawater measures 35psu.

Water can and does evaporate from trapped bodies of seawater so the salinity can rise above 35 as water is lost and the amount of salt remains constant; the Dead Sea, for example, contains water with a salinity of about 40psu.

Returning to brackish lagoons by the coast, coastal lagoons are defined as expanses of shallow water of varying salinity and water volume, wholly or partially separated from the sea by a barrier. Stoneworts thrive in coastal lagoons. Other important places for them are lakes, turloughs, sand dune and machair pools, gravel pits and quarries, peat cuttings, canals, ditches and rivers.

Their stems are branched at regular intervals. Like the ribs in an umbrella, the side branches come off in groups are regular intervals along the stem giving them a superficial resemblance to land plants like horsetails.

Some 400 species of stonewort are known worldwide. Out National Botanic Gardens lists 34 of these as having been recorded in Ireland. While two species, Foxtail Stonewort and Slender Stonewort, are afforded legal protection under the Flora (Protection) Order, 2015, the future of the group is by no means secure.

Many species are now very rare in Ireland to the extent that some are confined to just one or two sites and are rated in danger of becoming extinct.

Water pollution is one of the biggest threats facing the future wellbeing of our waters and the stoneworts they support.

Wexford People

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