Rehabilitation of coral reefs through removal of macroalgae: state of knowledge and considerations for management and implementation

Abstract

Coral reef ecosystems are under increasing pressure by multiple stressors that degrade reef condition and function. Although improved management systems have yielded benefits in many regions, broad‐scale declines continue and additional practical and effective solutions for reef conservation and management are urgently needed. Ecological interventions to assist or enhance ecosystem recovery are standard practice in many terrestrial management regimes, and they are now increasingly being implemented in the marine environment. Intervention activities in coral reef systems include the control of coral predators (e.g. crown‐of‐thorns starfish), substrate modification, the creation of artificial habitats and the cultivation, transplantation, and assisted recruitment of corals. On many coastal reefs, corals face competition and overgrowth by fleshy macroalgae whose abundance may be elevated due to acute disturbance events, chronic nutrient enrichment, and reduced herbivory. Active macroalgae removal has been proposed and trialed as a management tool to reduce competition between algae and corals and provide space for coral recruitment, in the hope of restoring the spatial dominance of habitat‐forming corals. However, macroalgae removal has received little formal attention as a method of reef restoration. This review synthesizes available knowledge of the ecological role of macroalgae on coral reefs and the potential benefits and risks associated with their active removal.

Uncovering drivers of juvenile coral density following mass bleaching

“…High levels of herbivory also tend to push algal communities towards those dominated by calcareous forms (Littler and Littler 1984;Belliveau and Paul 2002), adding to habitats favouring coral reinforcing feedbacks, underlined by the positive effect of CCA detected in our study. A recent review by Ceccarelli et al (2018) highlights the potential for physical removal of macroalgae to benefit coral, yet its effectiveness over large spatial and temporal scales will depend on whether the underlying drivers (e.g. eutrophication, overfishing of herbivores, ocean warming) that keep reefs locked in negative or reinforcing feedback loops favouring macroalgal dominance (Johns et al 2018) are addressed (Norström et al 2016).…”

Interactions between coral restoration and fish assemblages: implications for reef management

“…Indeed, reef restoration is not recommended in areas where grazing populations of fishes and/or invertebrates are scarce as this would prevent restored corals from recruiting in the future, therefore rendering the exercise futile (Edwards, 2010). Surveys of existing fish populations at proposed sites are, therefore, essential (Edwards & Gomez, 2007), and the active removal of macroalgae has been suggested on reefs with reduced herbivory in association with coral reef restoration efforts to improve chances of coral survival while coral fragments establish (Ceccarelli et al, 2018). for coral recruits and reducing coral-algal interactions (Abelson et al, 2016b;Bellwood et al, 2004;Ogden & Lobel, 1978), either through targeting algae directly (Adam et al, 2018) or indirectly while feeding as microphages (Clements et al, 2017).…”

Marine protected areas enhance coral reef functioning by promoting fish biodiversity

“…Although herbivory has a strong impact on benthic structure (Burkepile & Hay 2010) and has been linked to coral reef resilience for decades (Bellwood et al, 2004), macroalgae are also an integral feature of coral reef ecosystems (Ceccarelli et al, 2018). Further, the global prevalence of reef phase shifts to macroalgae continues to be debated (Bruno et al, 2009;Jouffray et al, 2015;Smith et al, 2016), in part because some regions are more prone to macroalgal blooms than others (Roff & Mumby 2012), and multiple states (e.g., turf-algal mats) are possible following reef disturbance (Donovan et al, 2018).…”