Fig 1.
Examples of the major reef algal assemblages on the Great Barrier Reef.
(a, b) In turbid waters especially on shallow wave-exposed inshore sites, Sargassum, Padina, Lobophora and other members of the class Phaeophyceae can dominate reef communities (a: Magnetic Island 2020; b: Havannah Reef 2007). (c) The red alga Asparagopsis can be common on disturbed high-current reef sites (Havannah Reef). (d) Relatively short-lived blooms of ephemeral Phaeophyceae can create anoxic conditions that may kill corals (Magnetic Island 2019). (e) Spring blooms of filamentous algae predominantly occur in high-nutrient environments and can be lethal for other reef benthos especially during senescence (Dingo Beach, Whitsundays). (f) On offshore reefs, the calcareous macroalga Halimeda can occupy extensive areas (Green Island). (g) Cyanobacteria may cover large reef areas after disturbance, but these blooms are functionally different from macroalgae and are excluded here from ‘total macroalgal cover’ (John Brewer Reef). (h) Turf algae, rather than macroalgae, occupy GBR reefs after disturbances in many environmental conditions (see Results) (Russell Island). Photos: a,c,d,e,g,h: K. Fabricius, AIMS; b,f: LTMP, AIMS.
Fig 2.
Macroalgal survey locations on the Great Barrier Reef, north-eastern Australia.
Colour shading indicates mean macroalgal cover. a. One-off surveys, here shown with values for 2 m depth, b. AIMS Long-Term Monitoring Program at 6–8 m depth. Coastline: Reprinted under a Creative Commons Attribution 4.0 International license, with permission from Australian Bureau of Statistics, original copyright 2011. Reef outlines: Reprinted under a CC BY license with permission from the Great Barrier Reef Marine Park Authority, original copyright 1998. Great Barrier Reef World Heritage Area: Reprinted from Australian Government Department of Climate Change, Energy, the Environment and Water under a CC BY license, with permission from Australian Government Department of Climate Change, Energy, the Environment and Water, original copyright 2022.
Fig 3.
Total macroalgal cover (MA) as a function of the three strongest spatial variables.
Partial dependence plots, showing changes in MA as deviation from their mean value of 9.3% (vertical axis and colour contours, in percent cover) attributable to the three interacting spatial predictors: relative distance across the continental shelf (0 = coast, 1 = offshore), latitude (°S), and depth (m) (a–c). Relative percentual influence of the predictors to main effects without interaction: across = 40.8%, latitude = 26.4%, depth = 20.0%, within-reef location = 6.71%, reef type = 6.10%.
Fig 4.
Total macroalgal cover (MA) in response to long-term mean environmental conditions.
Partial dependence plots, showing the relative influence (percentage contribution) of each predictor, and the changes in MA to the predictors (solid lines), and their 95% confidence intervals (grey envelopes). The y-axis represents the change in MA relative to the global mean (9.3% cover) attributable to each predictor. The ticks on the x-axis show the deciles of the predictors’ data distributions. Definitions and units of the predictors in S1 Table in S1 File.
Fig 5.
Relationship between macroalgal cover (where MA was present), and hard coral cover.
Plots based on the one-off surveys across five depth zones and three shelf positions (Red = Inshore, Orange = Mid-shelf, Light orange = Outer shelf). Solid lines are modelled higher posterior distribution means, shadings are 95% Bayesian Credible Intervals. Statistical differences can be inferred where 95% CIs do not overlap.
Fig 6.
Redundancy analysis, showing the relationships between benthos and predictors.
Plot based on the one-off surveys, and the main benthic groups (black arrows: cover of total macroalgae, hard corals, soft corals, coralline algae and turf algae, and their spatial and environmental predictors (blue arrows; definitions in S1 Table in S1 File). Species and environmental data scaled by eigenvalues (significance of relationships: Table 1). Each symbol represents a site, colors indicate shelf position, size increases with MA (range: 0% - 100% cover), and the symbols and hulls denote survey depths.
Table 1.
Significance of the predictors for the community data.
Fig 7.
Temporal trend in total macroalgal cover (%) in the Great Barrier Reef between 1995 and 2021.
Data are modelled hierarchical posterior distribution means and associated 95% Credible Intervals (CIs) from Bayesian hierarchical linear mixed models. (a) GBR-wide trend (black), as well as the trends for the inshore (red), mid-shelf (orange) and outer-shelf reefs (light orange) reefs. (b) Trends for the 12 reefs with the highest modelled MA values in anyone survey year. Pink: Havannah Reef (Townsville inshore); Light blue: Green Island Reef (Cairns inshore); Dark blue: Low Isles Reef (Cairns inshore); Red: Davies Reef (Townsville mid-shelf); Purple: Central mid-shelf reefs; Grey: Southern mid-shelf reefs; Light orange: Hyde Reef (Whitsunday outer-shelf).