Seawater carbonate chemistry, productivity and calcification of Marginopora vertebralis in a laboratory experiment

Changes in the seawater carbonate chemistry (ocean acidification) from increasing atmospheric carbon dioxide (CO2 ) concentrations negatively affect many marine calcifying organisms, but may benefit primary producers under dissolved inorganic carbon (DIC) limitation. To improve predictions of the ecological effects of ocean acidification, the net gains and losses between the processes of photosynthesis and calcification need to be studied jointly on physiological and population levels. We studied productivity, respiration, and abundances of the symbiont-bearing foraminifer species Marginopora vertebralis on natural CO2 seeps in Papua New Guinea and conducted additional studies on production and calcification on the Great Barrier Reef (GBR) using artificially enhanced pCO2 . Net oxygen production increased up to 90% with increasing pCO2 ; temperature, light, and pH together explaining 61% of the variance in production. Production increased with increasing light and increasing pCO2 and declined at higher temperatures. Respiration was also significantly elevated (~25%), whereas calcification was reduced (16-39%) at low pH/high pCO2 compared to present-day conditions. In the field, M. vertebralis was absent at three CO2 seep sites at pHTotal levels below ~7.9 (pCO2 ~700 µatm), but it was found in densities of over 1000 m(-2) at all three control sites. The study showed that endosymbiotic algae in foraminifera benefit from increased DIC availability and may be naturally carbon limited. The observed reduction in calcification may have been caused either by increased energy demands for proton pumping (measured as elevated rates of respiration) or by stronger competition for DIC from the more productive symbionts. The net outcome of these two competing processes is that M. vertebralis cannot maintain populations under pCO2 exceeding 700 µatm, thus are likely to be extinct in the next century.

Seawater carbonate chemistry and community calcification during a Molokai reef (Hawaii) study 2006

The severity of the impact of elevated atmospheric pCO2 to coral reef ecosystems depends, in part, on how seawater pCO2 affects the balance between calcification and dissolution of carbonate sediments. Presently, there are insufficient published data that relate concentrations of pCO2 and CO3 to in situ rates of reef calcification in natural settings to accurately predict the impact of elevated atmospheric pCO2 on calcification and dissolution processes. Rates of net calcification and dissolution, CO3 concentrations, and pCO2 were measured, in situ, on patch reefs, bare sand, and coral rubble on the Molokai reef flat in Hawaii. Rates of calcification ranged from 0.03 to 2.30 mmol CaCO3 m**-2 h**-1 and dissolution ranged from -0.05 to -3.3 mmol CaCO3 m**-2 h**-1. Calcification and dissolution varied diurnally with net calcification primarily occurring during the day and net dissolution occurring at night. These data were used to calculate threshold values for pCO2 and CO3 at which rates of calcification and dissolution are equivalent. Results indicate that calcification and dissolution are linearly correlated with both CO3 and pCO2. Threshold pCO2 and CO3 values for individual substrate types showed considerable variation. The average pCO2 threshold value for all substrate types was 654±195 µatm and ranged from 467 to 1003 µatm. The average CO3 threshold value was 152±24 µmol/kg, ranging from 113 to 184 µmol/kg. Ambient seawater measurements of pCO2 and CO3 indicate that CO3 and pCO2 threshold values for all substrate types were both exceeded, simultaneously, 13% of the time at present day atmospheric pCO2 concentrations. It is predicted that atmospheric pCO2 will exceed the average pCO2 threshold value for calcification and dissolution on the Molokai reef flat by the year 2100.