In the past three decades, bleaching events, a stress response that results from extended warm periods, have caused tremendous declines in coral across the Great Barrier Reef. Not only do increasing temperatures impact ocean acidification but it also controls seawater pH, both of which are important drivers of coral calcification. Coral calcification plays an important role in determining the health of reef ecosystems, as numerous species associated with reefs depend on the foundations provided by coral skeletons. Evidence of species tested to date indicate that calcification rates of tropical reef building corals will be reduced by 20-60%. A numerical modeling approach was taken to solve the interactions between the drivers of calcification. This approach analyzed the effects of environmental parameters on coral calcification. A majority of the marine calcifiers tested were sensitive to changes in the carbonate saturation state and have shown a decline in calcification rates. Results show that the impacts of climate change/heat stress on the global oceans have reached unprecedented extremes and changes in the chemistry of the seawater have caused coral to grow improperly. When seawater absorbs carbon dioxide, chemical reactions occur that reduce seawater pH, thus leading to ocean acidification. Increased levels of carbon dioxide reduces the ability of corals, that are still reef-building, to produce their hard skeletons. The results from this study suggest that climate change is the most severe threat to the Great Barrier Reef and is projected to lead to coral bleaching and changes to the abundance of marine species. Managing climate change is crucial to ensure the long-term survival of the Great Barrier Reef, and if the Great Barrier Reef is to be properly managed into the future, further research on seawater chemistry should be conducted in order to better understand the correlation between climate change and effects on coral growth.
This map shows coral reef distribution is in Queensland, Australia. The black lines show the depth contours. The orange dashed lines show transient currents. The red lines show surface currents. The scattered colorful dots show individual reefs separated into different geographical zones1 Map from Mongin, M., et al. (2016).
Above is a picture retrieved the National Center for Ecological Analysis and Synthesis (National Center for Ecological Analysis and Synthesis) depicting a lively and healthy coral reef. Healthy coral reefs contain thousands of fish and other species that can be found nowhere else on earth. Below is a picture retrieved from the Ocean Agency (The Ocean Agency). depicting a lifeless, acidified reef in Queensland, Australia. Weather-related damage to reefs lead to the decline in marine life biodiversity. The continuation of stress on coral could result in their disappearance in the ocean.
Above is a picture retrieved from the Ocean Agency (The Ocean Agency) showing the same coral reef over the span of three major bleaching events due to warming ocean temperatures. Below is a graph showing the relationship between heat exposure and the amount of bleaching measured underwater in March and April of 2017. Graph from Hughes,T., et al. (2017).
The table above is showing estimated trends in years and sea-surface temperature (Cooper, T., et al., 2008). These estimates for skeletal density, annual extension and calcification were all based on different coral colony profiles as part of a mixed model analysis. Results show that over the years skeletal density, annual extension, and calcification have declined. As a result to climate change, thermal stress and warming oceans are few of the many environmental factors that have influenced the decline in calcification. Table modified from Cooper, T., et al (2008).
About 30% of the carbon dioxide emitted into the Atmosphere every year is absorbed by the world’s surface oceans. Thus, ultimately leading to a shift in the seawater carbonate chemistry (Albright, R., et al., 2013). Concentrations of carbon dioxide increases and the pH of seawater decreases. This relationship is known as ocean acidification (Albright, R., et al., 2013). Since preindustrial times, carbon dioxide uptake by the surface ocean waters have lowered seawater pH, increasing acidity of about 30% (Albright, R., et al., 2013). Much of the carbon dioxide that the ocean absorbs is anthropogenic; meaning that the carbon dioxide absorbed comes from human activity. Coral reefs can experience fluctuations in seawater carbonate chemistry on both daily and seasonal timescales (Albright, R., et al., 2013). These processes can alter based on a wide range of environmental factors, including biological activity, water depth, physical forcing, tide water activity, community composition, and residence time (Albright, R., et al., 2013)..Chemical conditions in seawater vary from reef to reef and will differ from open ocean conditions (Albright, R., et al., 2013).
Coral calcification is an important determinant of the health of reef ecosystems, as tens of thousands of species associated with reefs depend on the structural complexity provided by coral skeletons (De’ath, G., et al., 2009). Calcification increases linearly with increasing sea surface temperature and responds nonlinearly to annual temperature anomalies6. Increasing temperature stress and a declined state of seawater saturation could potentially diminish the ability for the Great Barrier Reefs’ corals to deposit calcium carbonate (Cooper, T., et al., 2008).
Coral reefs are one of the most diverse marine ecosystems on earth. Reef-building coral are able to support a wide range of ecosystem services due to their ability to build calcium carbonate reef systems (Courtney, T., et al. (2017). Various environmental variables such as, climate change, warming ocean temperatures, increased levels of carbon dioxide, and ocean acidification all influence the progression of coral calcification. Decreased levels of anthropogenic carbon dioxide emissions will allow calcification rates to become steady and maintainable. Considering that colonies respond differently to varied levels of carbon dioxide emissions, it is important that these environmental parameters are evaluated, and tests are done on multiple coral species to get a better understanding of future coral reef stability.