If current trends continue and countries fail to reduce greenhouse gas emissions, nearly all of the world's coral reefs will suffer severe bleaching on an annual basis, according to a new United Nations environment agency study. Bleaching has a hugely negative effect on the health of a coral reef which, in turn, is vital to ocean health. The Conversation Africa's Samantha Spooner asked Leonard Chauka to shares his insights into coral bleaching on Tanzania's coastline.
What is coral bleaching?
Corals, that make structures we call reefs, are comprised of two partner organisms: an animal (coral) and a unicellular photosynthetic algae (dinoflagellate). The interaction whereby two different organisms live together for mutual benefit is called symbiosis. In this symbiosis, coral (the animal) provides protection and raw materials for photosynthesis while photosynthetic algae produces food. This enables both partners to grow and reproduce. The beautiful colour we see in coral comes from the photosynthetic algae because they have pigment that makes them colourful.
Coral bleaching is the disappearance of the coral's colour, revealing the white coral skeleton. It occurs due to either the breakdown of symbiosis between coral and photosynthetic algal cells or the degradation of photosynthetic pigments of photosynthetic alga cells. This occurs as a result of stressful conditions caused by, in most cases, high temperatures.
Therefore, major coral bleaching events occur during high sea surface temperatures associated with El Nino - a warm extreme weather pattern. But other factors such as cold conditions, elevated solar radiations and pollution have been found to cause minor coral bleaching at local scale.
Is coral along Tanzania's coastline being affected?
Yes. For example, the 1997-1998 global coral bleaching event caused mortalities of up to 80% in some of Tanzania's reefs. This worldwide bleaching event was caused by elevated sea surface temperature due to El Nino. Sea surface temperatures were 2°C higher than average (over 30°C).
The Misali and Tutia reefs in Pemba and Mafia Islands were the most affected with about 90% of these reefs suffering coral mortality.
Normally prolonged bleaching events lead to coral mortality (deaths) because the provider of food - photosynthetic algae - is no longer there to play its role. Most of these reefs have not recovered and there is no hope for them to recover completely.
Another bleaching event occurred in 2016 between February and June along the Tanzanian coast. Like previous events, this was caused by high sea surface temperatures due to El Niño. But this one didn't cause much mortality. Few coral mortalities were observed in reefs in Dar es Salaam and Zanzibar. Most coral recovered and are now in good health.
While these were two significant coral bleaching events, there are also minor seasonal bleaching events. These are not recorded and occur between February and June as these are the hottest months in Tanzania.
What has the impact been?
Coral reefs are an important habitat for fish species as breeding grounds. As a result, fisheries have been largely affected. Fisheries statistics show a decline in fish numbers.
The statistics also show changes in fish community structure, for example herbivores increasing in abundance. This is probably due to a shift from coral dominated reef to macroalgae dominated reef. Normally coral deaths resulting from bleaching encourage macroalgae to inhabit the area quickly and take over even the few remaining patches of live coral reefs.
Coral reefs are also very important in providing coastal protection and stabilisation by reducing wave energy and mitigating both routine erosion and damage from waves associated with small and moderate storms. Current trends of climate change are accompanied by global warming, sea level rise and increased storm intensity. It will therefore be detrimental to future coastal protection to lose these reefs.
What are the solutions to preventing or, if possible, reversing it?
Coral bleaching is caused by rises in sea surface temperature which is why there is concern over the impact of global warming. There are, however, other minor bleaching events that are caused by factors such as low temperature, solar radiation, lowered salinity and pollution. But there is no scientific data established to show the contribution of these other factors on bleaching events that we have experienced in Tanzania.
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There is no way we can stop current trends of global warming at the local level. What is being done in Tanzania is the restoration of highly degraded coral reefs. This involves growing nursery reared coral transplants in areas with highly degraded reefs.
Currently I am involved in active restoration of highly degraded coral reefs off Dar es Salaam. In this project we have been able to successfully restore about 5,000 meters squared of degraded coral reef since July 2016 when the project started. The data are yet to be published.
Disclosure statement
Leonard Jones Chauka has received funding from the Western Indian Ocean Marine Sciences Association. His PhD was sponsored by the World Bank and his coral restoration project in Dar es Salaam is funded by the Vice President's office of United Republic of Tanzania.
