How Sedimentation, Nutrient Enrichment, and Overfishing Impact a Coral Reef Ecosystem Immediately Following a Disturbance

by Natalie Ireland

Coral reefs are regularly disturbed by natural phenomena such as bleaching, storms, and outbreaks of predators, such as the corallivorous sea star Ancanthaster planci. Corrallivores are animals that eat coral polyps. Coral reef ecosystems are resilient, and are often able to recover from large-scale disturbances quickly. However, anthropogenic stressors such as overfishing, nutrient enrichment, and sedimentation can prevent coral reefs from recovering. Nutrient enrichment, caused by terrestrial runoff, creates the perfect environment for benthic algae to grow on disturbed and broken coral reefs. Overfishing, working in tandem with nutrient enrichment, causes an overgrowth of algae if there are not enough fish to graze it, and the successive degradation of the reef. Sedimentation is also another side effect of terrestrial runoff. Sedimentation buries corals, which blocks light from reaching them and potentially stops coral recovery. However, sedimentation, when not paired with any other stressor, can also stop the growth of algae by burying surfaces for algae to grow on. Gil et. al. (2016) set out to test the interactive effects that overfishing, sedimentation, and nutrient enrichment have on coral reefs in French Polynesia. They hypothesized that these anthropogenic disturbances, when working interactively, will negatively impact corals, while promoting algal cover. Continue reading

Ocean Acidification can Mediate Biodiversity Shifts by changing Biogenic Habitat

by Elizabeth Rodarte

Ocean acidification is the process in which the pH of the world’s oceans decreases due to the production of atmospheric CO2. The increase of CO2 and decrease in pH leads to changes in calcification, growth, and abundance of species such as coral reefs, mussels, seagrass, and macroalgae. Habitats experience the indirect effects of such CO2 increases. They must remain resistant to sudden changes in pH and CO2 in order to benefit the organisms they support. By modeling the effects of lowering pH in habitats with corals, mussels, seagrass, and microalgae, we can determine the costs to these species. Coral reefs and mussels are calcifying organisms that are negatively affected by the pH which limits survival and stunts, or even stops, growth and development. Lower pH decreases the species complexity of corals and mussels and ultimately the species richness in habitats. Mytilus mussels, for example, require specific pH to function. The species of mussels, other than Mytilus, that survive decreases in pH lack “structural complexity” to support dense surrounding vegetation. Therefore, the loss of Mytilus mussels due to ocean acidification allows for a more stable yet less diverse habitat. Continue reading

The Important Role Small Herbivores Play on Degraded Coral Reefs

by Natalie Ireland

Biodiversity is constantly being altered by anthropogenic and natural variants. Due to ocean acidification, and rising ocean temperatures, coral reef systems have degraded, and algae has come to dominate some of these systems. Macroalgae are aggressive and quickly colonize areas where coral has been degraded, and heavy algae cover of dead coral substrates prevents recovery of dead coral communities. A study conducted by Kuempel and Altieri (2017) set out to discover how coral reefs adapt to changing environments and how individual species living along the reefs promote resilience. The presence of herbivores, such as parrotfish, sea urchins, and other small grazing fish around degraded coral reefs likely halts the shift from coral-dominated areas to algae-dominated areas. Understanding the rate of recovery for coral reef dynamics can help scientists predict future coral resiliency and aid conservation efforts.

Kuempel and Altieri studied coral reefs on the Caribbean coast of Panama after a recent hypoxic event killed over 90% of coral on some reefs in that area. They chose to study this area because it has high anthropogenic stress, increasing the chance of a higher rate of algal dominance after coral disturbances. Using field surveys, herbivore manipulation, caging, and algal transplant, Kuempel and Altieri were able to study the relationships between herbivore populations, pressures that herbivores face, and grazing importance in relation to other algal mitigating factors.

This study found that there was no correlation between mass coral reef deaths and high rates of macroalgae cover. A large number of herbivores, mostly small grazing fish and invertebrates, around dead coral areas was almost always able to prevent macroalgae from colonizing. Many species of smaller herbivores were able to escape the pressures of overfishing and effectively graze coral reefs in place of large keystone herbivores. This prevented macroalgae from aggressively colonizing places where live coral cover was very low. Initial diversity in coral reef fish species is important in degraded coral reefs to overcome anthropogenic pressures and stifle macroalgae growth. Further research must be done to determine whether grazing by small herbivores can shift a coral degraded area into a coral dominated area and how this will impact future coral resilience.

Kuempel, C.D., Altieri, A.H., 2017. The Emergent Role of Small-Bodied Herbivores in Pre-empting Phase Shifts on Degraded Coral Reefs. Scientific Reports 7, 10:1038.



Using Projected Climate Change Impact on Coral Reefs to Explore a New Framework for Equity

by Wendy Noreña

The effect of greenhouse gas (GHG) emissions on ecosystem services is a subject of major concern in climate policy and conservation. Coral reefs are considered an especially vulnerable ecosystem as they are projected to be highly affected by ocean warming and acidification, both of which are generally thought to be likely consequences of climate change. While much research has already been conducted to determine the damage coral reefs will suffer as a result of climate change, surveys of how individual countries will be affected by coral reef devastation have not yet been implemented. Wolff et al. model both in this study, showcasing projected climate stress on reefs from 1875 to 2050 alongside measures of vulnerability and equity for individual countries and regions based on GHG emissions per capita and expected reef devastation. The study finds an alarming decoupling between total GHG emissions and reef impact, indicating that, in general, countries that emit the most GHG will often experience less reef impact while the opposite is true for countries that emit very little GHG. Continue reading

Potential Coral Reef Structure Changes from Climate Change

by Kimberly Coombs

Coral reefs vary in structural architecture, meaning that the structure can be very complex or relatively simple. The more structurally complex a coral reef is, the more species diversity may be supported. The reef building corals that create the complex coral reef structures need to have a sustainable carbonate budget in order to continue the processes of accretion and erosion to build the coral reefs. These corals have been experiencing reductions in their carbonate budget; as a result, they have declined around the world. Continue reading

The Economic Value of Coral Reefs

by Kimberly Coombs

Coral reefs are known for supporting a habitat rich in species diversity and abundance. Besides the benefit coral reefs provide to other species, they also offer a benefit to humans. Coral reefs provide a source of economic gain in terms of tourism and fisheries, usually bringing in about $30 billion each year. However, climate change is threatening to diminish this revenue as corals become bleached and experience higher rates of mortality.

Chen et al. (2015) conducted a study to estimate the global economic impact from loss of corals as a result of climate change. They identified three main factors from climate change that impact coral reefs the most: sea surface temperatures, CO2 concentrations in the water, and sea level rise. In order to assess the impact of these factors on coral reefs, Chen et al. used a threshold model in which they found that there are two temperature thresholds that may negatively impact coral reefs. When sea surface temperatures are between 22.37 and 26.85, coral cover may increase; conversely, when sea surface temperatures drop below 22.37 or rise above 26.85, coral cover decreases. Chen et al. found that increasing CO2 concentrations also cause a decrease in coral cover, while sea level fluctuations were found to have no significant effect.

In order to evaluate the value of coral reefs, Chen et al. used a meta-analysis that incorporated the percent coral cover, number of visitors to the reefs, GDP per capita, and the tourism expenditure for each visitor. They found that when coral cover decreased, reef value was reduced. The number of visitors correlates negatively with coral reef value because visitors prefer to visit uncrowded coral reefs. The GDP per capita and the tourism expenditure for each visitor were found to have positive effects on coral reef value.

Lastly, Chen et al. developed four different mitigation scenarios in response to climate change to evaluate coral reef value. The impact of these different mitigation scenarios on tourism and recreation revenue varies as coral cover varies under these scenarios. The economic loss ranges from $1.88 billion to $12.02 billion by the year 2100. Chen et al. noted that this result only represents the coral reef value from tourism and recreation and that there are many other factors that will be impacted by a decline in coral cover; therefore, they create a crude economic loss estimate under these four mitigation scenarios that ranges from $3.72 billion to $23.78 billion.

Overall, CO2 and sea surface temperatures will affect coral cover, which will reduce the coral reef value. A reduction in coral reef value reduces the recreation and tourism expenditures amongst other factors; therefore, ensuring coral cover remains high will give a higher guarantee that recreation, tourism.

Chen, P., Chen, C., Chu, L., McCarl, B., 2015. Evaluating the economic damage of climate change on global coral reefs. Global Environmental Change, 30, 12-20.

Potential for Coral Reefs to Recover after Coral Bleaching Events

by Kimberly Coombs

In 1998, a mass coral bleaching event resulted from increased water temperatures due to climate change and impacted corals world wide. This event caused much of the coral cover to be greatly reduced as many corals have a narrow set of temperature ranges that they can survive, and most live near their upper thermal maximum; therefore, slight increases in temperatures can have negative affects on coral survivorship. Not much is known about the ability of corals to recover after coral bleaching events or the likelihood of the environment switching to an algae dominated environment.

Graham et al. (2015) conducted a study in order to identify reef recovery, the amount of coral cover being greater than macroalgal cover post-disturbance, or a regime shift, the amount of macroalgal cover being greater than coral cover post-disturbance, at the Seychelles reefs. This study observed 21 reef sites from 1994 to 2011 in which about 90% of the coral cover was lost in 1998. They found that 12 of the 21 reef sites were able to recover post-disturbance, yet it took about 10 years to see any major improvements in the amount of coral cover. On the other hand, the other 9 reef sites switched to a macroalgae dominated environment. Before the mass bleaching event, the macroalgae and coral cover percent were the same between the 12 reefs and 9 reefs, suggesting that the regime shift resulted from coral bleaching. Continue reading