Ocean Acidification Reduces Recruitment of the Reef-Building Coral Acropora palmata

Coral reefs are one of the most diverse ecosystems on the planet. As CO2 emissions grow, the oceans absorb a greater amount of CO2 in a process known as ocean acidification. Recent studies have focused on the impact of ocean acidification on corals’ ability to grow a calcified skeleton. However, very little experimentation has been done to examine the impact of acidification on the early life stages of coral. A recent study has shown that an increase in dissolved CO2 has reduced the recruitment abilities of Acropora palmata juveniles (Albright et al. 2010). Albright et al. found that increased acidity resulted in a decrease in fertilization, settlement, and early growth. Reductions in recruitment can severely impact the size of coral reefs and limit the ability of reefs to recover from large-scale disasters. Thus, ocean acidification places further stress on an already threatened ecosystem.––Emily Putnam
Albright, R., Mason, B., Miller, M., Langdon, C., 2010. Ocean acidification compromises recruitment success of the threatened Caribbean coral Acropora palmata. Proceedings of the National Academy of Sciences 107, 20400–20404.

Albright and colleagues at University of Miami and the National Oceanic and Atmospheric Administration performed assays to determine the effect of increased ocean acidity on the three components of recruitment––the successful development of larvae, the ability of larvae to settle and the growth of larvae after settling. A fertilization assay, a settlement assay, and a growth assay were executed. Each test was performed at dissolved CO2 levels either consistent with today’s values, at the lowest value estimated for the year 2100, or at the highest estimate for the year 2100. The fertilization assay determined the number of successful fertilizations for a range of optimal sperm concentrations as a function of dissolved CO2. The settlement assay determined the number of larvae settled onto tiles. The growth assay measured the upward growth of the larvae once they were settled. Taken together, these assays measure the recruitment success of Acropora palmata juveniles.
Albright et al. found a significant decrease in the recruitment success of Acropora palmata juveniles. Ocean acidification reduced fertilization success by an average of 12–13%. The number of settled larvae was reduced by 45% at the low estimate and by 69% at the high estimate for dissolved CO2 levels in the year 2100. Growth was limited by 39% at the low end of the range and by 50% at the high end. The authors conclude that there was a net reduction of 52­­–73% in larval recruitment. Further studies are necessary to determine whether the mechanisms involved in reducing recruitment success are direct or indirect.
Reduced growth in larval settlement stages further supports the observation of reduced coral skeleton growth in adults under acidified conditions. The authors have found that ocean acidification will negatively impact larval coral stages in addition to the widely recognized impacts on adult stages. The magnitude of this problem is further revealed when you consider that approximately 75% of coral species reproduce in the same way as Acropora palmata. Thus, coral communities throughout the globe may be at risk for reductions in recruitment.
Catastrophic events further degrade and damage coral reefs. Global climate change has contributed to an increase in frequency of catastrophic storms and to an increase in global seawater temperatures. These events have severely weakened the resiliency and health of coral reef systems throughout the world. The success of recruitment is vital to recovery following destructive events. An increase in acidity over the next century will further limit recovery and reduce coral population sizes. 

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