| Summary: | 243 leaves 30 cm. Includes bibliography. University of Otago department: Marine Science. Ocean acidification is the alteration of seawater carbonate chemistry due to the sequestration of atmospheric carbon dioxide (CO₂) into the ocean. An increase in dissolved CO₂ (pCO₂ (aq)) in surface waters has caused a decrease in present day ocean surface pH by approximately 0.1 pH units since the industrial revolution. Atmospheric CO₂ levels are rising annually by 0.5% and if emissions are not mediated, ocean surface pH is predicted to decrease by 0.3 - 0.5 pH units by the year 2100 and 0.7 - 0.77 pH units by the year 2300. This MSc research examined the effects of ocean acidification on fertilisation and early development in two polar organisms; the nemertean worm Parborlasia corrugatus and sea urchin Sterechinus neumayeri, and the temperate mussel Perna canaliculus. Fertilisation success and embryological development were observed in P.corrugatus and S.neumayeri gametes and embryos exposed to ambient pH seawater (8.0), seawater pH predicted for the years 2100 (pH 7.7) and 2300 (pH 7.3) and an extreme pH (pH 7.0), adjusted through direct addition of CO₂ into seawater. Fertilisation, larval growth, survival, calcification and skeletal morphology are described for P.canaliculus larvae reared in ambient seawater (pH 8.0), and acidified seawater (pH 7.7 and 7.3). An additional treatment of pH 8.3 (pre-industrial ocean pH) examined whether P.canaliculus is already being affected by a drop in pH from pre-industrial levels. Fertilisation success was not affected by pH in P.corrugatus but declined significantly in S.neumayeri in pH 7.0 and 7.3 seawater at low sperm concentrations. Fertilisation in P.canaliculus decreased significantly in pH 7.3, 7.7 and 8.3 seawater in a number of trials, but not in others. Early cell division in both S.neumayeri and P.corrugatus were robust to pH changes with a negative effect detected in P.corrugatus only when seawater pH was lowered to 7.0. Sterechinus neumayeri gastrula development was abnormal in pH 7.0 seawater, while there were more abnormal P.corrugatus embryos in pH 7.0 at the blastula stage, and also in pH 7.3 during coeloblastulation. Perna canaliculus survival, calcification and skeletal morphology were not affected by ocean acidification. Shell length and thickness in P.canaliculus larvae significantly decreased in pH 7.3 and pH 7.7 seawater indicating that growth is delayed during hypercapnia. Alkalisation of seawater (pH 8.3) had no effect on P.canaliculus growth, survival, calcification or skeletogenesis. This study found that negative effects of acidification increased with development and responses were species-specific, suggesting that a range of physiological and environmental factors play key roles in determining an organism's response to reduced pH. Effects of lowered seawater pH on organism function are complex and it is uncertain what effect ocean acidification will have on temperate and polar organisms over the coming centuries. However, no positive responses to acidification were seen during any studied life stages in P.canaliculus, S.neumayeri or P.corrugatus and acclimation to decreasing pH over short time scales is unlikely. Further studies are crucial to understand what effects anthropogenic emissions will have on marine invertebrates at individual, community and ecosystem levels.
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