PETRIFYING SPRING WATERS THAT SUPPORT RARE, ECOLOGICALLY SIGNIFICANT BRYOPHYTE SPECIES HAVE VERY LOW NITRATE AND PHOSPHATE LEVELS

Petrifying (tufa-precipitating) springs contain rare stenotopic bryophyte species in specialised plant communities of high nature conservation value. We surveyed petrifying springs from throughout Ireland, recording bryophyte species and collecting and analysing water samples. We demonstrate that holding down levels of eutrophication is a critical issue for the survival of some of our rarest species and habitats (including those that are legally protected).With increased nitrate and/or phosphate levels, spring waters contain fewer species and lack rare species. At the highest nutrient levels, spring specialists are replaced by generalists.The rarest and most ecologically significant species, e.g. Catoscopium nigritum, Orthothecium rufescens, Seligeria oelandica and Tomentypnum nitens, were confined to waters with <2.0mg l^-1 NO₃^- and <15.0µg l^-1 PO₄^3-. Characteristic petrifying spring species, including Palustriella commutata, P. falcata, Philonotis calcarea and Scorpidium cossonii, were typically found in waters with <8.0mg l^-1 NO₃^- and <20.0µg l^-1 PO₄^3-. Brachythecium rivulare, Cratoneuron filicinum and Rhynchostegium riparioides, in contrast, often indicated nutrient-enriched waters. Water quality thresholds for nutrient levels in ground and surface waters in Ireland greatly exceed those required to maintain vulnerable, ecologically sensitive plant communities.We identify critical thresholds to be 2.0mg l^-1 NO₃^- and 15.0µg