Weakening macroalgal feedbacks through shading on degraded coral reefs

Abstract

Abstract Extensive and dense macroalgal fields can compromise the ecosystem function of habitat mosaics on reefs owing to their limiting effect on patch connectivity. Macroalgae can maintain and increase their dominance with effective self‐reinforcing feedback mechanisms. For example, macroalgae can form dense beds, supressing coral settlement and grazing by herbivores. This compromised ecosystem function can lead to major socioeconomic and ecological changes. Dense macroalgal beds were shaded with submerged shade sails of two sizes and changes to the underlying benthos and feeding rates of herbivorous fishes were recorded. The shade sails reduced the algae's ability to photosynthesize by 29%. After 6 weeks, macroalgal cover was reduced by 24% under small sails and by 51% under large sails. Small shade sails reduced turf algal growth by 23%, while large sails reduced growth by 82%. Three months after removal of the shade sails, algal beds had almost completely regrown. During this regrowth period, herbivore bites taken from the experiment's substrates were recorded, with grazing impact reducing significantly with time. This study is the first to achieve macroalgal reduction via the alteration of the light regime. While macroalgae regrew in this relatively short‐term experiment, shading may be a viable reef management approach that aims to maximize habitat mosaics on coral reefs, particularly if used in combination with other intervention methods.