Long‐term Dynamics and Drivers of Coral and Macroalgal Cover on Inshore Reefs of the Great Barrier Reef Marine Park

Marji Puotinen,Daniela M Ceccarelli,Richard D Evans,Garry R Russ,Philippa Mantel,Murray Logan,Caroline Petus,David H Williamson

doi:10.1002/bes2.1635

Abstract

Quantifying the role of biophysical and anthropogenic drivers of coral reef ecosystem processes can inform management strategies that aim to maintain or restore ecosystem structure and productivity. However, few studies have examined the combined effects of multiple drivers, partitioned their impacts, or established threshold values that may trigger shifts in benthic cover. Inshore fringing reefs of the Great Barrier Reef Marine Park (GBRMP) occur in high-sediment, high-nutrient environments and are under increasing pressure from multiple acute and chronic stressors. Despite world-leading management, including networks of no-take marine reserves, relative declines in hard coral cover of 40–50% have occurred in recent years, with localized but persistent shifts from coral to macroalgal dominance on some reefs. Here we use boosted regression tree analyses to test the relative importance of multiple biophysical drivers on coral and macroalgal cover using a long-term (12–18 yr) data set collected from reefs at four island groups. Coral and macroalgal cover were negatively correlated at all island groups, and particularly when macroalgal cover was above 20%. Although reefs at each island group had different disturbance-and-recovery histories, degree heating weeks (DHW) and routine wave exposure consistently emerged as common drivers of coral and macroalgal cover. In addition, different combinations of sea-surface temperature, nutrient and turbidity parameters, exposure to high turbidity (primary) floodwater, depth, grazing fish density, farming damselfish density, and management zoning variously contributed to changes in coral and macroalgal cover at each island group. Clear threshold values were apparent for multiple drivers including wave exposure, depth, and degree heating weeks for coral cover, and depth, degree heating weeks, chlorophyll a, and cyclone exposure for macroalgal cover, however, all threshold values were variable among island groups. Our findings demonstrate that inshore coral reef communities are typically structured by broadscale climatic perturbations, superimposed upon unique sets of local-scale drivers. Although rapidly escalating climate change impacts are the largest threat to coral reefs of the GBRMP and globally, our findings suggest that proactive management actions that effectively reduce chronic stressors at local scales should contribute to improved reef resistance and recovery potential following acute climatic disturbances.

Highlights

Coral reefs are governed by complex interactions between physical and biological drivers (Graham et al 2015, Zinke et al 2018), acute disturbances and chronic stressors (Ward and Myers 2005, Brodie and Pearson 2016, Hughes et al 2018b)
Clear threshold values were apparent for multiple drivers including wave exposure, depth and degree heating weeks for coral cover, and depth, degree heating weeks, chlorophyll-a and cyclone exposure for macroalgal cover, all threshold values were variable among island groups
Rapidly escalating climate change impacts are the largest threat to coral reefs of the Great Barrier Reef Marine Park (GBRMP) and globally, our findings suggest that proactive management actions that effectively reduce chronic stressors at local scales should contribute to improved reef resistance and recovery potential following acute climatic disturbances

Summary

Introduction

Coral reefs are governed by complex interactions between physical and biological drivers (Graham et al 2015, Zinke et al 2018), acute disturbances (e.g. storms, bleaching events, outbreaks of coral predators) and chronic stressors (e.g. fishing, terrestrial run-off, pollution) (Ward and Myers 2005, Brodie and Pearson 2016, Hughes et al 2018b). Repeated disturbances that kill corals, especially at broad spatial scales (e.g. cyclones and bleaching events), can trigger the persistent declines in coral cover that typically precede phase shifts to alternate (non-coral dominated) states (Norström et al 2009). Hard corals are the primary habitat-builders of coral reefs They provide food and the threedimensional structure for the multitude of reef-associated organisms that contribute to overall ecosystem function (Alvarez-Filip et al 2013, Graham and Nash 2013). Benthic community shifts from coral-dominated to algal-dominated states (phase shifts) have been widely recorded, even on isolated reefs far from direct human influence (Bruno and Valdivia 2016). Persistent phase shifts typically reduce biodiversity and degrade ecosystem functioning (Folke et al 2004, Norström et al 2009, Johns et al 2018)

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Conclusion