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The Effects Of Climate Change On Caribbean Coral Reef

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Climate change happens when changes in the Earth’s climate result in new weather patterns that remain. According to Alva-Basurto and Arias-Gonzalez (2014), air and ocean temperatures have increased over the past 200 years, and ocean pH and oxygen levels have decreased. These are all important factors surrounding the coral reef ecosystem. Coral reefs are among the most susceptible to climate change. The rise in global temperatures has resulted in coral stress which then results in bleaching or death. Coral bleaching results when the algae that lives within the coral, due to an endosymbiotic relationship, leaves. These algae are important for the coral because they provide a large percentage of energy to the coral. The bleaching events due to climate change have caused a loss of reef-building coral. The loss of coral results in a change of habitat and consequently a loss of species. The deoxygenation of ocean water, related to climate change, has also occurred due to rise of temperature. The deoxygenation has left fish, who rely on oxygen for their own health, in danger of predation and a limited growth capacity. Increased carbon dioxide (CO2) has caused a lowering of pH in marine ecosystems and led to a decrease in biodiversity. It is predicted by the year 2065, there could also be up to a 50% reduction in the availability of carbonate ions which are needed by organisms to create shells and exoskeletons.

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The main objective of the experiment done by Alva-Basurto and Arias-Gonzalez (2014) was to determine the possible effects of climate change stressors on the trophic webs of a coral reef model based on reefs within the Mesoamerican Reef System. The specific stressors being tested are increased ocean temperatures, decreased oxygen levels, and increased carbon dioxide levels. These stressors were to be tested individually and then combined. The researchers wanted to model climate change on these reefs for coastal conservation and management.

For their experiment, Alva-Basurto and Arias-Gonzalez (2014) simulated food web models based on information collected from 13 reefs along the eastern coast of the Yucatan Peninsula, which is a biodiversity hotspot in the Caribbean Sea. They used a program which allowed them to create these models called Ecopath with Ecosim. They created a simulation coral reef with information collected from over 170 species of reef fish, such as biomass and abundance and benthic communities from the 13 reef systems. The Ecopath program displays ecological relationships and considers fisheries and exports from the system. For the simulation of global warming, forcing functions that affect predator search rates were manipulated. Four scenarios were created to display the effects: increased water temperature on corals, decreased oxygen levels on fish functional groups, decreased pH on calcifying organisms, and combined effects of the three stressors. For increased temperatures, the researchers considered a loss of 80% coral biomass as a significant impact strength. The forcing function was set to -35 and directly applied to the coral functional group, and the time interval was set for two years. Two mediation functions were also set to observe the effects of this temperature on different species. These were high fish-coral relationships for small reef fish and medium fish-coral for intermediate fish. For the second stressor, the researchers used a median low oxygen level since the highest recorded level was 7.5 mg/l and levels less than 4.2 mg/l produce deadly effects. The decrease of sea deoxygenation produces a 90% loss of fish functional group biomass. The increase of CO2 is estimated to lower ocean’s pH by 0.77 pH units by the year 2300. The average pH is normally between 7.9 and 8.3, so Alva-Basurto and Arias-Gonzalez (2014) set their pH lowered by 0.4 and used pH 7 as the base. This forcing function was directly applied to the functional groups consisting of calcifying organisms. 

From their program results, the researchers found variance in biomass levels. In the first scenario of increased ocean temperature, the biomass of most fish increased. The largest increase of 36.5% recorded in grouper, but small parrotfish decreased by 37.8%. In non-fish groups the biomass decreased between less than 1% and up to 100%. The rising temperature caused a phase shift due to coral bleaching and coral biomass decreased by 81% (80% was considered a significant impact). In the second scenario of decreased oxygen levels, there was a decrease in biomass by all fish groups. Although there was a 100% increase in crustaceans. The phase shift index stated that deoxygenation in fish showed a pristine state which means there is high coral cover and low macroalgal. In the third scenario of increased sea pH, there were various biomass levels recorded. Most importantly, small reef fish biomass increased by 100% and non-fish organisms overall had high decreases in biomass, such as birds decreasing 100%. In the fourth scenario, all three stressors were combined. All fish groups experienced a decrease in biomass, but a greater decrease was seen in those having a stronger relationship with coral. In non-fish groups biomass increased. The phase change resulted in an 80.6% decrease in coral biomass, which, in turn, resulted in a severe decrease of biodiversity. The two largest declines were due to sea deoxygenation and the combined effects of all three.

The researchers concluded that potential effects of climate change on biomass depend on the stressor affecting the system. Increased temperature resulted in a biomass decrease of 81%, and an inverse relationship was observed between temperature and coral biomass. The phase shift labeled the coral in a severe phase which meant low coral cover and high macroalgal cover. This led to a major biomass decrease in those species that rely on coral cover. The overall biodiversity decreased, and large fish groups increased in biomass. In the decreased oxygen scenario, all fish groups had a decrease in biomass, this could cause economic problems in regions of fisheries. In scenario three, the direct relationship of increased carbon dioxide and increased calcifying organisms’ biomass was observed, along with the expected decrease in fish biomass. When the three stressors are combined in one scenario, there were extreme effects on the trophic groups. The researchers were surprised to find that not all species were negatively affected. This suggests that the species has the ability to adapt and evolve to the changing conditions.

In my opinion, it is very important to research the effects of climate change on Caribbean reefs. As stated earlier, reefs are very easily affected by climate change and there are countless species of plants and animals located in these ecosystems. The researchers from this article came up with a good experiment to test the possible effects of climate change. The downside is that the article was hard to read. The authors referenced numerous other experiments and researchers for facts. They did have to cite some of these for the data they used to help with the experiment but considering so many other facts also came from other researchers, they had to cite someone almost every sentence. Furthermore, when explaining some of the equations used in their methods, it could be kind of hard to follow. If I could, I would get better clarification on Kempton’s Q Index because it seemed to almost be thrown into the article without explaining it deeply.

09 March 2021

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