Coral bleaching, mortality and benthic community assemblages on the reefs within the Pacific Island Network national parks

McCutcheon, Amanda; McKenna, Sheila

doi:10.36967/nrr-2287992

The 2014–2017 global-scale coral bleaching event caused extensive coral injury and mortality. Since 2006, the National Park Service Pacific Island Inventory and Monitoring Network has monitored benthic marine communities annually at reef sites within four parks: Kaloko-Honokōhau National Historical Park (KAHO) on Hawaiʻi Island, Hawaiʻi; Kalaupapa National Historical Park (KALA) on Molokaʻi Island, Hawaiʻi; the National Park of American Samoa (NPSA) on Tutuila Island in the U.S. territory of American Samoa; and War in the Pacific National Historical Park (WAPA) in the U.S. territory of Guam. Photo quadrats were taken annually along 15 fixed transects (25 m length) in each park on hard-bottom substrate between 10- and 20-m depth. Benthic cover and coral health were analyzed using standard point-count image analyses to determine percent substrate type and incidence of coral bleaching. Trends in benthic cover, bleaching, and mortality were assessed across the time-series and in relation to temperature data. Results indicated that KAHO was the most impacted by coral bleaching with 77% bleached coral cover in 2015, a 63% reduction in coral cover from 2014 to 2016, and significant post-bleaching mortality of the most abundant coral taxa (Porites lobata, Porites compressa, Pocillopora meandrina, and Montipora capitata). At KALA, coral cover significantly declined (8% in 2006 to 4% in 2019), but 2014–2016 image analysis suggested this was not bleaching related. At NPSA, coral cover significantly increased (24% in 2007 to 33% in 2019) and bleached cover was greatest in 2017 (8%) with no significant mortality detected. At WAPA, time of survey did not coincide with peak bleaching conditions, but coral cover remained stable from 2008 to 2019 (Asan 22%, Agat 3%), indicating no significant mortality within our observed depths of 10- to 20-m. Bleaching assessment was limited by the sampling frame and time of survey of the monitoring program. To strengthen our ability to detect and assess future coral bleaching and other disturbance events, we recommend adjusting annual benthic monitoring for heat stress events when feasible, adding shallow reef transects, and implementing regular manta tow surveys to detect and document large-scale disturbances. In light of forecasted increases in the frequency of coral bleaching events, we recommend proactive visitor management strategies, identification and mitigation of point source pollution events, and continued evaluation of heat-tolerant coral species to improve reef resiliency to future bleaching events.

Coral bleaching, mortality and benthic community assemblages on the reefs within the Pacific Island Network national parks

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