Comeau S., R.C. Carpenter, C.A. Lantz, and P.J. Edmunds: Ocean acidification accelerates dissolution of experimental coral reef communities, Biogeosciences, 12, 365-372, 2015. Doi: 10.5194/bg-12-365/2015
Coral reefs are beautiful, important, natural structures in the tropical oceans that provide a habitat for various fish species and protect the shores of paradises from wave erosion. You probably have already been informed that the rise in atmospheric carbon dioxide (CO2) poses a threat to the survival of reef communities globally; maybe you’ve heard it referred to as “ocean acidification” (OA). The relationship between CO2 and the acidity of the ocean is such: as CO2 in the atmosphere increases some of it dissolves into the ocean, forcing the ocean pH to go down. The fear in the future is that the pH will be forced so low that it could cause the dissolution (chemical breakdown) of coral reefs to exceed their calcification (formation). The long-term effects of OA could be detrimental to the survival of reef communities.
Specific impacts of OA on community structure and the roles of different community members are still vaguely understood. Previous research has observed that there are specie-specific responses to OA. These responses can be used to infer the impact on communities. There have also been observations of in situ communities impacted by volcanic activity and mesocosm research that provide some information of how the community structure may change, however directed research on the response of specific community members to OA and their role in the response of the whole community has had little attention. To investigate the topic deeper, biologists from California State University set up experiments that enabled them to monitor the response of specific community members and evaluate their roles in the community as a whole to varying concentrations of dissolved CO2.
Biologists designed the experiments with an ‘ex situ’ approach so that they could have complete control of the conditions but still apply their research to whole communities found in the natural environment. Four flumes were used to house the ex situ reef communities (figure 1), which were designed with the intention of representing reefs in 3-6 feet of water off the shores of Moorea, French Polynesia. Species used in the study were chosen to best represent the natural assemblage, they include the four most abundant species of Corals (Porites spp., Porites rus., Montipora spp., and Pocillopora spp.), coralline algae, and dead coral fragments (rubble).
Sediment was taken 200 miles from the reef crest in a lagoon north of Moorea using sediment boxes. The sediment used took a total of four days to collect so that it could re-establish chemical stratification in the boxes before being transferred to the flumes. The importance of including sediment in the flumes is the dissolution that can occur in pore fluid, often related to the CO2 increase from biological activity. There has also been work suggesting that the sediment response to OA can be observed when atmospheric CO2 exceeds 800 uatm.
With this set up scientists were able to manipulate the chemical and physical conditions of the flumes. They set up a pair of flumes to mimic an ambient ocean environment similar to present day (atmospheric CO2 ~400 uatm) and a second pair to mimic conditions based on predictions for the end of this century (atmospheric CO2 ~1300 uatm). In addition, the flumes were filled with seawater with a controlled flow, pH was monitored and varied to mimic day and night conditions, the flumes were exposed to natural sunlight, and temperatures were maintained at 27 degrees Celsius.
Calcification was measured on three levels: the whole community, the sediment, and the marco-calcifiers (algae and corals) with the intent to distinguish the sensitivity of each to OA. Net calcification rates were calculated using the total alkalinity anomaly method based on a known relationship between alkalinity and calcium carbonate precipitation that allows scientists to use changes in alkalinity to infer changes in calcium carbonate. The whole community was monitored every 7 days and the sediment was monitored at day 7, 30, and 56. Coral and algae rates were determined by subtracting the net sediment calcification from the community calcification.
Researchers were successful in their experiment (figure 2)! The experiment replicated between the duplicate flumes, the researchers were successful at maintaining each pair of flumes at the conditions desired, and they were able to obtain results that compared reasonably to other studies. They did encounter one complication when an outbreak impacted 10% of one coral species in both tanks. Two of the corals, the least abundant of the four, died. Around 70% of the coralline algae died, but it was attributed to abrasion via sediment
For the whole community net calcification was higher than dissolution in the ambient flumes during both day and night. In the high CO2 flumes dissolution was greater than precipitation, but to a greater degree during the night. Overall calcification was 59% lower in the high CO2 flumes.
The sediment calcification varied between treatments and between day and night. There was net dissolution during both day and night in the high CO2 flumes, and during the night in the ambient CO2 flumes. Overall the sediment in the high CO2 environment experienced net dissolution and the sediment in the ambient flume had a balance between calcification and dissolution.
Corals and Algae (macro-calcifiers):
For corals and algae net calcification was positive in the high CO2 flumes at night and in ambient flumes during the day and night. Overall there was 29% more net calcification in the ambient CO2 flume than the high CO2 flume. Corals were the greatest contributors to calcification. Porites spp. had a greater role in the high CO2flumes. P. rus, Montipora spp. And Pocillopora spp. were less important at high CO2. The low contribution of calcification from algae was attributed to its death by abrasion.
Based on this research is it plausible that the ex situ community reef observation can be used to make more precious observations of the roles of community members during OA effects than inferring based on species specific laboratory studies. Net calcification rates in the ambient flume were comparable to previous in situ studies on the Moorea reef and mesocosm studies in Hawaii. The difference in results between this studies and previous studies is that this study estimates a greater influence of high CO2 on the community level. The difference can be explained by the sediment, which attributed to less calcification and more dissolution in the higher CO2 flume. It is important to remember that even in high CO2 conditions communities may still see net calcification, it just may be must closer to the rate of dissolution.
This project was important because it demonstrated the success of ex situ flumes, particularly accurately handling flow conditions. It also highlights that there is an important role of ecological balance in the calcification budget: the balance of calcification and dissolution will likely be related to the balance between macro-calcifiers and sediment. It is important to monitor such changes because reef communities provide a safe habitat of many fish species, and project tropical shores of wave erosion.