Abstract
Mean coral cover has reportedly declined by over 15% during the last 30 years across the central Great Barrier Reef (GBR). Here, we present new data that documents widespread reef development within the more poorly studied turbid nearshore areas (<10 m depth), and show that coral cover on these reefs averages 38% (twice that reported on mid- and outer-shelf reefs). Of the surveyed seafloor area, 11% had distinct reef or coral community cover. Although the survey area represents a small subset of the nearshore zone (15.5 km2), this reef density is comparable to that measured across the wider GBR shelf (9%). We also show that cross-shelf coral cover declines with distance from the coast (R2 = 0.596). Identified coral taxa (21 genera) exhibited clear depth-stratification, corresponding closely to light attenuation and seafloor topography, with reefal development restricted to submarine antecedent bedforms. Data from this first assessment of nearshore reef occurrence and ecology measured across meaningful spatial scales suggests that these coral communities may exhibit an unexpected capacity to tolerate documented declines in water quality. Indeed, these shallow-water nearshore reefs may share many characteristics with their deep-water (>30 m) mesophotic equivalents and may have similar potential as refugia from large-scale disturbances.
Highlights
Despite the potential significance of such reef-building settings, empirical data on the structure, community composition and diversity of contemporary reefs within nearshore light-limited environments remains sparse
Mean (±s.d.) coral cover within the Paluma Shoals Reef Complex (PSRC) was 38 ± 24%, but locally attained coverage of up to 75% across a large area (0.012 km2)
Highest mean cover (40 ± 36%) was recorded in shallow waters (
Summary
Despite the potential significance of such reef-building settings, empirical data on the structure, community composition and diversity of contemporary reefs within nearshore light-limited environments (defined here as “shallow-water mesophotic” reefs, as corals experience low light conditions as deep-water mesophotic coral ecosystems due to very high turbidity) remains sparse. This is largely due to difficult field working conditions especially from poor visibility. Our results highlight the capacity for naturally marginal marine settings to support productive and diverse reefal habitats, and provide a framework for testing hypotheses about the apparent resilience of these reefs to some of the drivers of coral community degradation associated within seemingly more optimum (high light, lower nutrient) conditions on mid- and outer shelf areas of the central GBR
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