The Other Ocean Acidification Problem: CO2 as a Resource

by Dawn Barlow

This study addresses the effects of enhanced CO2 levels in the ocean by looking at how increased acidity might indirectly cause phase shifts in community structure of coral reef and kelp forest ecosystems in temperate and tropical waters. Under elevated acidity and temperature conditions, productivity of certain photosynthetic organisms such as mat-forming algae (low-profile ground-covering macroalgal and turf communities) can increase, making CO2 not only a direct stressor but also an indirect stressor by being a resource for certain competitive organisms, creating enormous potential for shifts in species dominance. Additionally, ocean acidification acts together with other environmental stressors and primary consumers, and these factors also influence community response to acidic conditions. Connell et al. (2013) investigate the prevalence of mat-forming algae in three different scenarios where CO2 levels were either ambient or elevated: in the laboratory, in mesocosms in the field, and at naturally occurring CO2 vents that locally alter the seawater chemistry. They find that in all the scenarios, the algae mats respond positively to the elevated conditions, increasing growth rate and cover to so that the algae became a majority space holder regardless of any herbivory. This is likely because the new environmental conditions favor species with fast growth and colonization rates and short generation times, and these are the species that are capable of completely… displacing space-holding species with slower growth and longer lifespans.

In this study, Connell et al. attempt to examine the indirect effects that ocean acidification has on coral reef and kelp forest ecosystems due to competition for space under elevated CO2 and temperature conditions. This is a very broad investigation, where several different techniques are carried out in several different locations. All of the acidification questions are addressed for both temperate and tropical regions—the temperate experiments are carried out in the Mediterranean and the tropical experiments are conducted on the Great Barrier Reef. One thing to keep in mind when considering this study is the fact that that these individual investigations in different regions were carried out over different timespans and using different techniques in addition to being conducted in different places. The idea behind using these three different techniques for each region is to get the most holistic picture of how entire communities will respond to elevated conditions. In the laboratory it is possible to easily eliminate certain variables that could be offsetting results, but on the other hand it is difficult to account for all the factors in an ecological community in a laboratory setting. The mesocosms allow for an experiment to be carried out in the field but with certain manipulated factors, in this case pH. The future pH conditions were chosen to represent projected near-future conditions anticipated for the end of the century; 7.7 –7.8 in the temperate region and 6.8 –7.8 in the tropics. The vents allowed for an observational study on the existing community under naturally elevated CO2 conditions where local ecological communities have adapted. The drawback with the field studies at these vents is that the spatial and temporal variation of pH in these acclimatized communities does not necessarily behave as would future ocean conditions, although the fact that it is an observational study eliminates the possibility that manipulations are skewing the results.

Connell et al. found that in all cases, mat-forming algae cover increased under elevated CO2 conditions, regardless of herbivory. Net productivity in the lab increased significantly, but there were several sources of potential loss that were not accounted for in the laboratory. In field mesocosms, the growth rate of the algae mats increased under elevated conditions to two to three times what it is under ambient conditions. At temperate vents, the algae mats covered 40% more space and at tropical vents cover was 50% more, in both cases expanding from a minority space holder to a majority space holder. In the community phase-shift that took place, calcareous taxa (primarily crustose coralline algae and barnacles) maintained a percentage cover in the elevated conditions that was similar to the ambient in the early stages of succession, but were then quickly overgrown by mat-forming algae after 3.5 months. Mat-forming algae appear to inhibit the percentage cover of other taxa, given that the development was 26% in ambient conditions and 17% in elevated conditions. Whether there was a change herbivory was not made clear by this study, as densities of calcareous grazers such as urchins did not differ between ambient and elevated conditions, and the densities of herbivorous fish at the vents was is unknown and fish had the ability to easily move in and out of the low pH area. This suggests that the increase in cover of algae mats is not due to an absence of grazers, and that they still do hold an important role in mediating competitive interactions between space holders.

This study demonstrated the fact that under projected future acidification conditions, species with fast rates of colonization and growth and short generation times are favored and competitively displace slower-growing and longer-living space holders. This shows that the negative effect on coral growth rates that happens in the presence of increased CO2 is strengthened by increased growth rates of mat-forming algae. The authors of this study suggest that because CO2 acts as a non-additive stressor in combination with local stressors, the potential for acidification-related phase shifts in response to gradual acidification is likely to increase with additional stressors. The implications of this finding are that by reducing local stressors such as nutrient pollution and overfishing, it is possible to reduce the effects of global stressors such as ocean acidification and temperature, and that reduction of these local stressors is something that needs to happen for effective management if there is hope for the reduction of loss of coral reef and kelp forest ecosystems.

Connell, Sean D., Kroeker, Kristy J., Fabricius, Katharina E., Kline, David I., Russell, Bayden D., 2013. The other ocean acidification problem: CO2 as a resource among competitors for ecosystem dominance. Philosophical Transactions of the Royal Society B: Biological Sciences 368.1627  Paper at http://bit.ly/1lKBwmI

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