Evidence of extensive reef development and high coral cover in nearshore environments: implications for understanding coral adaptation in turbid settings

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.

Calcium Carbonate Production, Coral Cover and Diversity along a Distance Gradient from Stone Town: A Case Study from Zanzibar, Tanzania

“…High coral cover favored by benthic ecological competition due to limited substrate availability has been reported from some locations with turbid waters. Similar results were found in nearshore reefs at the GBR, where hard coral cover was two times higher than mid-and outer-shelf reefs (Morgan et al, 2016). However, biodiversity indices gave us a finer overview; consistent with other studies reporting reduced biodiversity values close to highly populated cities in the coral triangle .…”

The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments

“…Corals employ physiological plasticity to persist in turbid waters, including flexible photosynthetic properties, which may include a change of algal endosymbiont genetic type, and/or enhanced host heterotrophy (Anthony, 2000;Anthony and Fabricius, 2000;Hennige et al, 2008Hennige et al, , 2010Suggett et al, 2012b). Morgan et al (2016) recently described over 21 genera of coral surviving in muddy waters on the inshore GBR suggesting that turbid systems may afford important range (niche) extension beyond that currently considered for many key "blue water" tropical oligotrophic reef-forming coral taxa.…”

Palaeoecological records of coral community development on a turbid, nearshore reef complex: baselines for assessing ecological change

“…Fabricius et al 2005;Roff et al 2013). One explanation for the apparent disparity between existing ecological records is that the impact of increasing sediment influx is spatially heterogeneous, with an emerging hypothesis that nearshore coral communities are preadapted to, and thus better able to cope with, conditions of low light availability and sedimentation (Perry et al 2008;Morgan et al 2016a;Ryan et al 2016). This pre-adaption may therefore have instilled a greater resilience within these nearshore coral communities to changes in water quality than those of inshore reefs, which are located further offshore towards the inner/mid-shelf boundary, and where the influence of suspended sediments and the frequency of exposure to river flood plume-associated particulates (i.e.…”

“…gov.au/docs/data-centre/reef-monitoring-surveys.html), but can be as high as *40% (e.g. Browne et al 2010;Morgan et al 2016a). However, declining water quality is considered a major threat to the health of inner-shelf reefs across the GBR .…”

“…In the shallow (\5 m depth) coastal areas of the inner shelf, these finegrained sediments are frequently resuspended by waves, producing wide fluctuations in turbidity (Larcombe et al 2001;Browne et al 2012b). Based on the prevailing environmental conditions, a distinction has recently been made between inner-shelf reefs that occur within coastal 'nearshore' (\10 m isobath) areas and those located further offshore in more 'inshore' settings ([10 m isobath), where the local natural sedimentary background conditions are less extreme (Morgan et al 2016a).…”