Nutrients fuelling problem of coral bleaching
(CNS): New research from the University of Southampton and the UK's National Oceanography Centre points to an imbalance of nutrients in reef waters as one of the causes of corals susceptibility to bleaching. In common with the rest of the Caribbean region, the problem of bleaching has been identified throughout the Cayman Islands ocean environment. In 2009 local researchers from the Department of Environment reported that nearly all corals in the shallow reefs to about 30 feet now show signs of moderate to severe bleaching, with around 80% of corals in the deeper reefs exhibiting the early signs.
Although the DoE said bleaching is closely associated with global warming, they are also aware that other issues are at play, as revealed by this latest research. The University of Southampton study, published in the latest issue of the journal Nature Climate Change, has found that nutrient enrichment of the water can increase the probability of corals suffering from heat-induced bleaching.
Dr Jörg Wiedenmann, Senior Lecturer of Biological Oceanography at the University of Southampton and Head of the Coral Reef Laboratory, who led the study, said the findings suggest that the most severe impact on coral health might be from the depletion of phosphate that is caused by the increased demand of algae populations.
"Our results have strong implications for coastal management. The findings suggest that a balanced reduction of the nutrient input in coastal waters could help to mitigate the effects of increasing seawater temperatures on coral reefs,” he said.
Corals are made up of many polyps that jointly form a layer of living tissue covering the calcareous skeletons. They depend on single-celled algae called zooxanthellae, which live within the coral polyps.
The coral animal and the associated zooxanthellae depend on each other for survival in a symbiotic relationship, where the coral supplies the algae with nutrients and a place to live. In turn, the algae offer the coral some products of their photosynthesis, providing them with an important energy source.
High water temperatures can block photosynthetic reactions in the algal cells, causing a build-up of toxic oxygen compounds, which threaten the coral and can result in a loss of the zooxanthellae.
Without the algae, corals appear white, a state which is often referred to as 'bleached'. Bleaching often leads to coral death and mass coral bleaching has had already devastating effects on coral reef ecosystems.
Within the coral, the growth of zooxanthellae is restricted by the limited supply of nutrients. This allows the algae to transfer a substantial amount of their photosynthetically fixed carbon to the coral, which is crucial for the symbiotic relationship.
Algal growth becomes unbalanced when the availability of a specific nutrient decreases compared to the cellular demand, a condition called nutrient starvation.
Researchers found that an increased supply of dissolved nitrogen compounds in combination with a restricted availability of phosphate results in phosphate starvation of the algae. This condition is associated with a reduction in photosynthetic efficiency and increases the susceptibility of corals to temperature and light-induced bleaching.
Wiedenmann said balancing the nutrients in the oceans may help but it was important to stop the warming of the oceans, which will otherwise destroy most of the reefs in their present form in the near future. However, he said the results should help the design of functioning marine reserves in the short term.
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