Local human activities and climate-driven marine heatwaves are significantly altering coral reef ecosystems. Managers aiming to increase reef resilience often face challenges in effectively integrating ridge-to-reef initiatives within their plans for coral reef conservation, despite the significant land-based impacts on reefs. This study, which analyzed over 370 reef surveys over the last 17 years and 20 years of land-sea impact data, identified key factors affecting coral reef health before, during, and after a major heatwave in Hawaii. The research revealed that simultaneously increasing herbivorous fish populations and reducing land-based impacts, such as water pollution, are crucial for positive coral growth and reduced mortality under severe heat stress.  

Before the heatwave, thriving reefs that showed increasing coral cover over time had larger herbivorous fish populations. Meanwhile, those with declining coral cover had smaller herbivorous fish populations and experienced 40-80% more wastewater pollution, nutrient loading, and urban runoff. 

The coral response to the 2015 marine heatwave varied according to environmental factors and fish biomass. Reefs with less urban runoff and sediment input experienced less coral mortality due to reduced compounding stress from pollutants and sediments that decrease the resilience of coral. Total fish and scraper (algae-eating fish) biomass were also significant factors in predicting coral mortality, but much less so.  

Four years post-heatwave, the main indicators of a reef’s higher reef-building capacity (measured as coral and crustose coralline algae cover) were reduced wastewater pollution and increased scraper biomass. 

Lastly, the study evaluated the impacts of models of different management strategies. It found that an integrated approach that included both land and ocean management was three to six times more effective in achieving high reef-building cover compared to separate land or coastal management — underscoring the importance of integrated land and coastal management in fostering resilient coral reef ecosystems in the face of environmental stressors. 

Implications for managers 

  • Effective reef management should integrate land-based threats into its management plan to maximize coral reef survival in a changing climate. 
  • Relying on indirect proxy measures for human impacts like population density and general water quality measures may not provide enough precise information for effective conservation. Prioritizing accurate, localized data on land-sea impacts is crucial. 
  • Underutilized policies like the U.S. Clean Water Act can be effective in managing land-based stressors affecting marine environments. Applying these policies, especially in urban areas, can improve coral resilience to severe marine heat waves. 

Authors: Gove, J.M., G.J. Williams, J. Lecky, E. Brown, E. Conklin, C. Counsell, G. Davis, M.K. Donovan, K. Falinski, L. Kramer, K. Kozar, N. Ling, J.A. Maynard, A. McCutcheon, S.A. McKenna, B.J. Neilson, A. Safaie, C. Teague, R. Whittier, and G.P. Asner 

Year: 2023 

Nature 621: 536–542. doi:10.1038/s41586-023-06394-w 

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