Macroalgal cover on coral reefs: Spatial and environmental predictors, and decadal trends in the Great Barrier Reef

Abstract

Macroalgae are an important component of coral reef ecosystems. We identified spatial patterns, environmental drivers and long-term trends of total cover of upright fleshy and calcareous coral reef inhabiting macroalgae in the Great Barrier Reef. The spatial study comprised of one-off surveys of 1257 sites (latitude 11–24°S, coastal to offshore, 0–18 m depth), while the temporal trends analysis was based on 26 years of long-term monitoring data from 93 reefs. Environmental predictors were obtained from in situ data and from the coupled hydrodynamic-biochemical model eReefs. Macroalgae dominated the benthos (≥50% cover) on at least one site of 40.4% of surveyed inshore reefs. Spatially, macroalgal cover increased steeply towards the coast, with latitude away from the equator, and towards shallow (≤3 m) depth. Environmental conditions associated with macroalgal dominance were: high tidal range, wave exposure and irradiance, and low aragonite saturation state, Secchi depth, total alkalinity and temperature. Evidence of space competition between macroalgal cover and hard coral cover was restricted to shallow inshore sites. Temporally, macroalgal cover on inshore and mid-shelf reefs showed some fluctuations, but unlike hard corals they showed no systematic trends. Our extensive empirical data may serve to parameterize ecosystem models, and to refine reef condition indices based on macroalgal data for Pacific coral reefs.

Past disturbances and local conditions influence the recovery rates of coral reefs

“…In addition, high nutrient loads (Graham et al., 2015) and the direct influence of rivers (Golbuu et al., 2011) can deter coral recovery. Biological circumstances can also play a role in coral recovery, such as the density of herbivorous fishes (Graham et al., 2015), the presence of macroalgae (Fabricius et al., 2023; Smith et al., 2023), and community composition (Darling et al., 2019; McWilliam et al., 2020). To date, most coral reef recovery studies have been localized (Gilmour et al., 2013; Golbuu et al., 2007; Gouezo et al., 2019; Schoepf et al., 2020a; but see González‐Barrios et al., 2023; Tebbett et al., 2023).…”

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

“…Inshore regions of Australia's Great Barrier Reef (GBR) are generally exposed to the factors that favour macroalgal growth (Fabricius et al, 2023), especially in comparison with mid‐ and outer‐shelf reef regions. For example, inshore reefs experience elevated nutrient and sediment inputs resulting from erosion following land clearing, riverine run‐off linked to agricultural development, port‐associated dredging and other coastal development activities (De'ath & Fabricius, 2010; Williamson et al, 2019).…”

“…For example, inshore reefs experience elevated nutrient and sediment inputs resulting from erosion following land clearing, riverine run‐off linked to agricultural development, port‐associated dredging and other coastal development activities (De'ath & Fabricius, 2010; Williamson et al, 2019). Despite inshore reefs on the GBR exhibiting naturally higher levels of macroalgae relative to their offshore counterparts (Fabricius et al, 2023), over recent decades some inshore reefs have experienced substantial increases of macroalgal abundance, with concurrent declines in hard coral cover (Ceccarelli et al, 2020; De'ath et al, 2012; De'ath & Fabricius, 2010; Thompson et al, 2023). As the climate warms and anthropogenic pressures increase, macroalgae are expected to become more pervasive, thus threatening the resilience of coral reefs, particularly inshore reefs (Graham et al, 2015).…”

Macroalgae exhibit diverse responses to human disturbances on coral reefs

“…Furthermore, environmental factors can influence macroalgal cover on reefs, such as exposure to wind and waves (Fabricius et al, 2023; Gove et al, 2015; Page‐Albins et al, 2012), seasonality (Brown et al, 2018; Fulton et al, 2014), and sea surface temperature (SST; Graba‐Landry et al, 2020; Tanaka et al, 2012). Studies endeavoring to assess links between local human disturbance and macroalgae cover should therefore consider these environmental factors in analyses.…”

“…Studies endeavoring to assess links between local human disturbance and macroalgae cover should therefore consider these environmental factors in analyses. Macroalgae taxa also exhibit variability in their responses to local human and environmental stressors, including temperature (Anton et al, 2020; Fabricius et al, 2023), fishing pressure (Gilby et al, 2015), water pollution (Fabricius, 2005; McClanahan et al, 2004; McCook, 1999), and sedimentation (Fabricius, 2005; Harris et al, 2021). Research investigating these taxon‐specific responses to local stressors are lacking for all but the most common macroalgae.…”