Effect of Artificial Upwelling on Preventing Coral Bleaching During Times of Thermal Stress: Strategies for Survival in a Warming Ocean

Moronke Harris1, Yvonne Sawall1, Mario Lebrato2, Marlene Wall3 and Ellias Feng3, (1)Bermuda Institute of Ocean Sciences, Coral Reef Ecology and Optics Laboratory, St. George's, Bermuda, (2)Bazaruto Center for Scientific Studies (BCSS), Benguerra Island, Mozambique, (3)GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany
Coral bleaching, caused by elevated ocean temperatures, is one of the most lethal and enduring threats to tropical marine ecosystems. As such, the impacts of rising sea-surface temperatures on the frequency and severity of bleaching events are well documented and urge scientists to develop novel strategies that mitigate negative effects on corals and associated habitats. Artificial upwelling (AU) of cooler deep water towards the ocean surface may be one of such interventions. As a climate engineering technology, AU may enhance the thermal tolerance of shallow water corals by allowing a more gradual adjustment to increasing sea-surface temperatures.

To evaluate the potential use of AU to mitigate heat stress and prevent bleaching events, this study investigated the effect of simulated pulses of Atlantic Ocean water, collected off the coast of Bermuda at 50 m (24°C) and 100 m (20°C) depths, on the symbiont biology of 3 scleractinian coral species (Pseudodiploria strigosa, Porites astreoides, and Montastraea cavernosa) under thermal stress. The following treatments were applied to coral colony fragments over a period of 3 weeks: (i) control at 28°C, (ii) heat at 31°C, (iii) heat at 31°C + pulsed water from 50 m depth (25 minute drops to 27.5°C once per day), and (iv) heat at 31°C + pulsed water from 100 m depth (25 minute drops to 26.3°C once per day). Simulated pulses resulted in a reduction of treatment temperature for 2 hours per day. Comparative analysis of photosynthetic rate, chlorophyll-a concentration, and zooxanthellae density revealed a significant reduction of coral bleaching in AU treatments in two of the three investigated species (Figure 1). Further, water from 100 m depth produced a stronger effect than that from 50 m depth.

Our results suggest that deep water pulses reduce heat stress responses in corals, supporting the notion that AU might be an important intervention strategy for coral survival in a warming ocean. Implementation of an effective in-situ AU strategy in coral reefs will require further exploration of the ecological benefits and associated potential ecosystem impacts. However, by investigating direct solutions for the reversal and prevention of bleaching threats, this study promotes new initiatives for sustainable development of the ocean.