In order to examine the effects of ocean acidification on coral growth rates, D. Manzello (2010) measured the extension rates of six different coral species over a two-year period. The values for growth rate obtained by Manzello were compared to values reported in primary literature to determine the change in growth rate over a period of about thirty years. Of the species examined, the linear extension of one of the most important reef building coral species, Pocillopora damicornis, declined by almost one-third over a thirty-two year period. The extension rates of the coral P. elegans were found to be fairly consistent with the P. damicornis results, showing a similar decline in growth rate of the coral. The extension rates of the massive coral species examined showed little change between the study and the twenty-seven year period before. Even with the decline in growth rate, the two Pocillopora species still had a much higher growth rate than the massive species. The density and calcification of the corals was also highest in the Pocillopora species. Two Pavona species showed similar or higher growth rates where Ω (a measure of the concentration of CO32- ions) was lower, in contrast to the Pocillopora species, which had higher extension rates at higher Ω values. Although seawater temperature and coral growth strategy also play a role in determining the response of corals to changing ocean chemistry, the combination of results shows that the branching Pocillopora species are potentially more vulnerable to ocean acidification. As the dominant species in the area, this could affect coral reef composition if ocean acidification continues. — Rachel King
Manzello, D., 2010. Coral growth with thermal stress and ocean acidification: lessons from the eastern tropical Pacific. Coral Reefs 29, 749–758.
The species studied in this investigation included Pocillopora damicornis, Pocillopora elegans, Pavona clavus, Pavona gigantea, Pavona varians, and Gardineroseris planulata. Corals were placed at various reef sites by tying coral colonies to a rebar hammered into a reef 2–3 m below the low tide line. The water temperature was recorded on temperature loggers every 30 minutes for the duration of the coral deployments. The corals were collected after one year, and the extension, density, and calcification for each coral species was determined from the samples. In massive corals, each coral was sliced perpendicular to the growth axis and growth was measured with a ruler to the nearest millimeter. On the branching corals, growth was measured for each branch with calipers after slicing the branch tips adjacent to the top of the stain line.
In addition to the decline in extension rate of the main reef building coral P. damicornis near Panama, a decline in extension rate was also noticed in Costa Rica, indicating a potential large-scale factor contributing to the decreasing extension rate in the eastern Pacific Ocean. The extension rates of the massive coral species were not shown to change over a twenty-seven year period, although the extension rates were still half that of the Pocillopora species. In general, the average density of each species was shown to be inversely related to the mean extension rate in every species except P. varians, though the relationship was only significant in P. damicornis. P. damicornis also displayed lower extension rates in water with low Ω values, which is the saturation state of carbonate minerals in seawater. Locations with lower Ω values are expected to be more affected by ocean acidification.
The author also discusses how coral growth strategy and temperature effects could influence the growth of corals in the future. Four models were discussed for declining coral growth rate: cumulative ocean acidification, acute thermal stress, and the combined thermal stress and acidification either as additive or synergistic effects. Depending on which model is most accurate, the effects of acidification on coral growth rate could vary significantly. In spite of this, the study shows that continuing acidification of the ocean from the anthropogenic input of CO2 into the atmosphere has the potential to significantly effect the growth of the branching Pocillopora coral species in the Pacific ocean.