Symbiotic Relationship Between Coral And Macroalgae

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In addition to their necessary symbiotic relationship with coral, zooxanthellae also serve as the component responsible for the lush and vibrant colors of the coral reefs. A good indication of whether or not a reef is healthy is the amount of color present. Since the algae contains photosynthetic pigments, the degree of light it receives can alter the colors, hence the colorful reefs. The algae are sensitive to light and temperature, and expel zooxanthellae cells when they undergo conditions that place stress on them, such as an increase in temperature brought on by climate change. Consequently, a process known as coral bleaching occurs, in which the reefs turn white due to the absence of the algae, which could lead to the death
In the U.S. Virgin Islands specifically, coral reef cover dropped from over 40% to less than 25% from the 1970’s to the 1990’s. As of 2008, it dropped to below 12% on most reefs, although some deeper reefs have a higher percentage of about 30% (noaa). Long term monitoring shows the connection between the decline of coral and the increase in macroalgae cover, which is believed to be a consequence of the coral bleaching and disease in 2005. The growth of macroalgae due to the actions of natural threats can be considerably damaging to coral reefs if the correct circumstances are present. When reef-building corals are destroyed by bleaching or disease, the competing algae invades the dead coral and prevent light or oxygen from reaching them. With this disruption in the ecosystem, the organisms that formerly depended on the reefs for their source of food or protection are now unable to survive, upsetting the communities and species of reef-colonizing
Virgin Islands. Construction of unpaved roads and driveways due to the growing island populations stimulate an increase in sedimentation and erosion. Sediment runoff presents several problems for these vulnerable coral reefs, including a reduced rate of photosynthesis. The overflow of sediment in the water causes less sunlight to penetrate to the corals, making it difficult for the algae to provide the necessary nutrients obtained from photosynthesis for the coral. Seasons of heavy rainfall pose a threat too, as sedimentation can increase to tremendous rates during these times. Mangroves are advantageous in the case of sedimentation and runoff, although the bulk of mangroves in the U.S. Virgin Islands have been removed due to the destructive actions of dredging, filling, and hurricanes. The loss of mangroves combined with everyday exposure to the damaging stresses of the inhabitants around them have caused significant reef