Higher ocean temperatures and other threats are leading to worldwide decline of Coral Reefs
A research conducted to understand what drives reef decline, recently published results of a three-year-long field experiment that simulated overfishing and nutrient pollution on a coral reef in the Florida Keys. The findings were made by researchers from six institutions, including Florida International University, the University of California/Santa Barbara, Penn State University, and the Laboratoire d'Excellence.
Findings of research
One of the longest and largest studies of coral reef health ever undertaken finds corals are declining worldwide because of a variety of threats—overfishing, nutrient pollution and pathogenic disease that ultimately become deadly when coupled with higher ocean temperatures. In Australia, threats to the Great Barrier Reef are confronting the tourism sector.
The research, which was funded by the Virginia-based National Science Foundation's Division of Ocean Sciences, found impacts of both anthropogenic stressors and local stressors such as overfishing and nutrient pollution on coral reefs. A reduction in resilience of coral reefs was observed due to increase in coral-algal competition and reducing coral recruitment, growth and survivorship.
It found an increase in turf and macro algal cover which destabilises microbiomes, thus elevating putative pathogen loads. This led to a two-fold increase in the disease rate and eight-fold increase in mortality rate of the corals.
The situation was worse at above average temperature, further disrupting microbes of unhealthy corals and concentrating 80 per cent of mortality in the warmest seasons. Increase in nutrient concentration leads to an exponential increase in bacterial opportunism and mortality in corals bitten by parrot fish, turning even normal tropic interactions deadly for corals.
Coral degradation and erosion
Previous studies had observed that bleaching is one of the major causes of coral degradation and erosion. According to a report published by UNEP in May 2016, there has been unprecedented coral bleaching on the northern and central Great Barrier Reef, one of the world's most iconic reefs and a world heritage site. Bleaching in the central Indian Ocean is also severe, especially in the Maldives, Sri Lanka and in the Lakshadweep islands of India, where up to 100 per cent of corals are bleached in some locations. Many will not survive.
However, this new study points towards a more serious concern of anthropogenic stressors (over fishing and nutrient pollution) and pathogenic disease being leading causes for coral erosions. Anthropogenic and Pathogenic diseases together make the coral less immune to disturbances in the sea. It is a more concerning factor because it doesn’t destroy the coral directly but does so by compromising its resilience towards disturbances such as hurricanes and thermal anomalies.
How was the study conducted?
Studies on corals and their response to global warming were conducted before, but on a smaller scale using lab and field experiments. So, to conduct experiments at a larger scale, the scientists stimulated overfishing and nutrient pollution on a reef in the Florida Keys, on four different plots of 9 sq m each. It is on these plots that the researchers tracked the impact on the benthic community, coral–microbe dynamics and coral survival across seasons. They were able to simulate nutrient pollution by enriching the plots of reef with nitrogen and phosphorus, while other four “control” plots remained at ambient nutrient levels.
What were the results?
Due to the removal of herbivorous fishes and excessive nitrogen and phosphorus, there was an exponential increase in algal cover (up to six times) and species richness (up to three-fold) at the expense of coralline algae and closely-cropped turf algae which are beneficial or neutral for corals.This facilitated the growth of algae that is known to increase coral tissue loss or mortality via shading, abrasion and allelopathy.
To identify how algal communities and nutrient pollution affected the coral microbiome, the researchers collected DNA samples from the surface mucus layer of 80 corals at monthly intervals.
The study found that increasing algal cover or elevated temperature suppressed the typical, dominated microbiome of healthy corals and facilitated blooms of other microbes, including many putative opportunists or pathogens. Another significant finding of the research was the change in stable state of the coral microbiomes under stressful condition. It also found out radical changes in coral microbiome to be strongly correlated with coral tissue loss and mortality in the field.
When asked about the implications of the report and their future plans, Jesse Zaneveld, one of the authors of the study, said, “These findings about the interaction between local pollution and overfishing, temperature, the coral microbiome and coral health is important for coral conservation. Our co-author Jeff Maynard has shown in a separate paper that many reefs will be exposed to moderate warming that can promote bacterial infections of the coral even when you can't see coral bleaching. In those cases, local conservation measures can make a big difference, even as we push towards global frameworks for reducing carbon emissions.” Zaneveld also added that a worldwide study of coral microbiology is being conducted “to understand diverse symbiotic interactions between corals and their microbes that we have only begun to explore”.
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