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The ocean's surface acidification with increasing atmospheric carbon dioxide and the threats for coral reef calcification
Author(s)
Date Issued
April 1, 2017
Type
Book Chapter
Abstract
In some tropical, shallow water, coastal areas located around the world, coral cover has been decreasing. This decline in coral reefs is possibly considered to be due to global environmental changes, such as increasing atmospheric carbon dioxide (CO2) produced by anthropogenic sources. The extra atmospheric CO2 is leading to increased seawater surface temperature (SST) as well as surface ocean acidification. This study investigates the relationship between the coral calcification rate and four different controlling factors, seawater pH, aragonite saturation state (Oarag), temperature, and light intensity. Seawater temperature, pH, and Oarag are all being directly affected by the increase in atmospheric carbon dioxide occurring. In this study, coral reef observations were performed in Jicchaku, Okinawa, Japan. A medium sized tide-pool (250 X 225 cm2, average water depth = 22.6 cm) was utilized for experimental observation periods. Two different types of trials were also run. Normal pH (~8.2) trials were run with no alteration to the seawater. Low pH trials were run after adding ~20 mL of 12M HCl to lower the pH in the tide-pool to a pH of ~7.8. During each trial run, a seawater sample was taken once at the beginning and end of the trial run and once during the trial run. In the laboratory, the samples collected from the field were utilized to determine pH and salinity. Total alkalinity (TA) was determined using the Gran plot method that uses a titration pH curve to determine TA. Seawater Oarag was calculated using a computer program called CO2SYS. The calcification rate was calculated using the change in TA over the change in time. Data from a daytime normal pH trial at the tide-pool showed TA decreased in relation to time for this trial indicating that calcification was occurring at that time. Data from a nighttime showed TA increased in relation to time for the trial indicating that calcium carbonate dissolution was occurring during nighttime. We also observed that there is a positive relationship between light intensity and the calcification rate suggesting 'light-enhanced' calcification. It could be observed that there is little correlation between the temperature and calcification rate. This suggests that temperature had no effect on changes in the calcification rate for the tide-pool experiments. On the other hand, seawater pH and Oarag were observed to have a strong influence on the calcification rate. From the field experiments we suggest that coral reef calcification is closely coupled to seawater CO2 pH and Oarag, and reduced coral calcification is predicted in the near future when atmospheric CO2 further increases.
Citation
Advances in Environmental Research, 56, 113-161, 2017