| Summary: | The intertidal, purple tipped sea urchin {Psammechinus miliaris) was exposed to artificially acidified seawater treatments (pHw 6.16, 6.63. 7.44) over a period of 8 days. 100% mortality was reached at pHw6.16 after 7 days due to a build up of pC02 in the coelomic fluid resulting in irrecoverable acidosis. Coelomic fluid acid-base status measures showed an accumulation of PCO2 and a significant reduction in pH in all treatments compared with controls. Bicarbonate buffering was employed in all cases to reduce acidosis but compensation was unsuccessful even under moderate hypercapnia. Significant test dissolution was evident due to increases in magnesium ions under all pH treatments. The test is however, not thought to be a major source of bicarbonate ions for compensation. Results suggest that a reduction of surface water pH of below 7.5 would be severely detrimental to the acid-base balance of this intertidal species of echinoid; despite its ability to tolerate fluctuations in carbon dioxide partial pressure (pCOi) and pH in the rock pool environment. A lack of respiratory pigment, poor capacity for ionic regulation and dependency on a magnesian calcite test make both intertidal and non-intertidal echinoids vulnerable to anthropogenic acidification. Geological sequestration leaks may result in dramatic localised pH reductions e.g. pH 5.8. P. miliaris is intolerant of pH 6.16 seawater and significant mortality is seen at pH 6.63. These results indicate that potential future CO2 storage leaks would result in reduced survival of adult sea urchin populations. KEY WORDS: Hypercapnia • Oceanic sequestration • Acid-base balance • Intertidal echinoid • Bicarbonate buffer system. Faculty of Science, Plymouth Marine Laboratory
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