Sea-weeding: Manual removal of macroalgae facilitates rapid coral recovery

The proliferation of macroalgae poses significant threats to coral reefs, impacting their health and resilience. Globally, many reefs have shifted from coral to macroalgal dominance due to factors like increased nutrient loads, reduced herbivory, and decreased coral competition following coral mortality events. This situation is expected to worsen with climate change and increasing anthropogenic pressures. To address this issue, managers and scientists are exploring methods to restore damaged reefs and prevent further coral loss.

One proposed method is the manual removal of macroalgae, considered a cost-effective intervention to boost coral recovery. While some studies have shown benefits, the effectiveness of manual removal likely depends on factors such as timing and method of removal. Most existing research has been short-term or based on single removal events, leaving the long-term effects unknown.

A three-year field experiment was conducted on two macroalgal-dominated coral reefs in the central Great Barrier Reef to investigate the long-term effects of manual macroalgae removal. SCUBA divers, assisted by snorkelers, manually removed macroalgae (primarily Sargassum spp.) from experimental plots eight times over three years, targeting the holdfasts of the algae. Changes in the benthic community composition were documented through photographic surveys before and after each removal event. Additionally, in situ surveys using a stratified transect method were conducted to account for canopy-forming macroalgae obstructing the view of organisms underneath.

Each removal event reduced the macroalgae cover by about half (52%). The difficulty of removing some species and limited field time prohibited complete removal. Results showed that macroalgae cover in experimental plots reduced significantly from 81% to 37% over three years, while control plots remained relatively unchanged (87% to 83%). Coral cover increased significantly in removal plots, from 6% to 35%, compared to a modest increase in control plots from 7% to 10%. Transect-derived estimates corroborated these findings, showing similar patterns of macroalgae reduction and coral cover increase.

Initially dominated by Sargassum spp., the macroalgae community in the removal plots diversified by the end of the study. Control plots remained low in species diversity. Additionally, coral community composition became more diverse in the experimental plots, while control plots remained low.

In summary, the study demonstrates that regular manual removal of macroalgae can effectively reduce macroalgae cover and significantly increase coral cover and diversity. The cost of the macroalgae removal project, including materials, vessel and vehicle hire, dive equipment, SCUBA tank air fills, ferry costs, and marine berths, amounted to approximately $23,000 (USD, base year 2010). Volunteer labor was primarily used, and this estimate excludes staff salaries. With this investment, they were able to double the coral cover in a 300 m² area, equivalent to $77 per m². The study estimates that it would cost $67,250 per hectare per removal event, although costs would vary depending on local labor costs and other variables.

Implications for managers

• Hand removal of macroalgae is a cost-effective way to decrease macroalgae cover, increase coral cover and diversity, and improve the health of coral reef ecosystems.
• Make macroalgae removal projects multiyear, as it takes several years of repeated removal for significant effects on coral communities to emerge. Regular long-term monitoring is also important.
• This low-tech, high-impact method can be integrated into local reef management strategies and can be done using volunteer divers with minimal training.
• Macroalgae removal should be done in tandem with other actions to manage macroalgae, such as limiting nutrient pollution and overfishing.

Authors: Smith, H.A, S.E. Fulton, I.M. McLeod, C.A. Page and D.G. Bourne
Year: 2023

Journal of Applied Ecology 60(11): 2459-2471. doi: 10.1111/1365-2664.14502
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