Sun on Ocean

Artificial upwelling: an opportunity to reverse coral bleaching events

With 2020 being on course to be the warmest year on record, and the concentration of carbon dioxide (CO2) in the atmosphere being the highest it has been in human history (as of May 2020), the oceans are being affected significantly. Thermal-stress events associated with climate change cause coral bleaching and mortality that threatens coral reefs survival globally. This is considered to be the most severe threat to coral reefs, which makes it important for scientists to develop novel strategies that mitigate the impact of warming on corals and associated habitats. Artificial upwelling provides the opportunity to reduce coral bleaching, potentially reversing it during peak years. The Bazaruto Center of Scientific Studies (BCSS) has contributed to two recent papers researching the potential benefits of artificial upwelling (geo-engineering) in the context of coral bleaching.

Coral bleaching
Bleaching events leave corals vulnerable to disease, stunts their growth, affects their reproduction, and impacts other species that depend on the coral communities. When corals die, they rarely come back, resulting in a decline in the number of coral reefs and coral species globally. Coral reefs cover just 0.1% of the world’s oceans, and yet they support nearly 25% of marine life. They function as the base of complex ecosystems and offer shelter to numerous juvenile fish species. Both wildlife and humans depend on healthy coral reefs – especially coastal communities. Coral bleaching impacts peoples’ livelihoods, safety and food security, as they function as natural barriers that absorb the force of waves and storm surges, protecting coastal communities directly.

The problem
The productivity of the ocean depends on how well the nutrient-rich deep waters mix with the sun-lit surface layer. In large areas of the global ocean, the movement of the waters is blocked by a temperature-induced density gradient as the lighter, warm waters reside on top of heavier cold waters. These large parts of the ocean are often referred to as “ocean deserts” by scientists and are expanding due to surface-ocean warming as a result of climate change. Enhancing the upward movement of large amounts of nutrient-rich waters through artificial upwelling can break the blockade and increase the waters’ productivity.

Artificial upwelling
Geo-engineering, also known as climate engineering, is the active intervention in the oceans and Earth system to reduce, increase or mitigate the adverse effects of climate change, global warming or anthropogenic impacts. Artificial upwelling (AU) is a novel geo-engineering technology that brings seawater from deep in the ocean to the surface. The technique aims to artificially pump up cooler, nutrient-rich waters from the deep to stimulate phytoplankton activity and draw down carbon dioxide. Another potential benefit of artificial upwelling is that the pressure of fish stocks could be alleviated.

The findings
The results of the papers that BCSS has contributed to indicate that artificial upwelling could be an effective strategy to mitigate coral bleaching during heat stress events allowing corals to adjust to increasing temperatures more gradually. The studies focused on the effect of artificial upwelling on coral reefs from an experimental and a modelling perspective. In both studies it was found that a proper implementation and management of artificial upwelling techniques can certainly decrease coral bleaching, buying coral more time to physiologically adapt to the rising seawater temperatures.

Coral reefs in the Indian Ocean and the Bazaruto Archipelago are in a mostly healthy state, with relatively few bleaching events per year during the warm season. However, bleaching is increasingly seen on the reefs in the West Indian Ocean, including the Bazaruto Archipelago. This year, the BCSS team has perceived more bleaching than normal and received a mass bleaching warning. Luckily, the reefs here do seem to be rather resilient as they usually regain their health in a short period of time. This last statement is based solely on observation by our research team. We are collecting data to validate whether the shorter heating periods allow for corals to bounce back more quickly. The BCSS Ocean Observatory has various moorings and sensors in coral reefs habitats to monitor the well-being of coral reefs and their future. This data will be increasingly valuable as time goes by, giving us an accurate perspective on the development of coral reefs under the influence of a changing environment as part of climate change.

Read the two papers BCSS contributed to here:

Sawall Y, Harris M, Lebrato M, Wall M and Feng EY (2020) Discrete Pulses of Cooler Deep Water Can Decelerate Coral Bleaching During Thermal Stress: Implications for Artificial Upwelling During Heat Stress Events. Front. Mar. Sci. 7:720. doi: 10.3389/fmars.2020.00720

Feng EY, Sawall Y, Wall M, Lebrato M, Fu Y (2020) Modelling coral bleaching mitigation potential of water vertical translocation – an analogue to geoengineered artificial upwelling Front. Mar. Sci. | doi: 10.3389/fmars.2020.556192

If you have any questions regarding the studies, please feel free to get in touch with Dr. Lebrato directly via

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