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Severe thermal stress events occurring on the backdrop of globally warming oceans can result in mass coral mortality. Tracking the ability of a reef community to return to pre-disturbance composition is important to inform the likelihood of recovery or the need for active management to conserve these ecosystems. Here, we quantified annual, temporal changes in the benthic communities for the three years following mass coral mortality at Jarvis Island—an uninhabited island in the Pacific Remote Islands Marine National Monument. While Jarvis experienced catastrophic coral mortality in 2015 due to heat stress resulting from the 2015/16 El Niño, significant annual shifts were documented in the benthic community in the three years post-disturbance. Macroalgal and turf dominance of the benthos was temporary—likely reflecting the high biomass of herbivorous reef fishes post-bleaching—giving way to calcifiers such as crustose coralline algae and Halimeda, which may facilitate rather than impede coral recovery. By 2018, indications of recovery were detectable in the coral community itself as juvenile densities increased and stress-tolerant genera, such as Pavona, exceeded their pre-disturbance densities. However, densities of Montipora and Pocillopora remain low, suggesting recovery will be slow for these formerly dominant taxa. Collectively, the assemblage and taxon-specific shifts observed in the benthic and coral community support cautious optimism for the potential recovery of Jarvis Island's coral reefs to their pre-disturbance state. Continued monitoring will be essential to assess whether reassembly is achieved before further climate-related disturbance events affect this reef system. [ABSTRACT FROM AUTHOR]

The 2014–2017 Global Coral Bleaching Event is the longest, most widespread, and impactful on record. Rapid ecological assessment surveys by NOAA's Pacific Reef Assessment and Monitoring Program reported widespread coral mortality at Jarvis Island in the aftermath of the 2015–2016 super-El Niño warming event; hard coral cover declined from 18.7% in April 2015 (pre-bleaching) to 0.4% in May 2016 (post-bleaching), representing a catastrophic > 98% decline. Between 2015 and 2016, corals at Jarvis experienced maximum heat stress of 22.25 °C-weeks exceeding the bleaching threshold (28.72 °C) for 66 consecutive weeks. Mass coral bleaching was observed in November 2015, which resulted in mass mortality across all coral taxa, depths, and island sectors. The bleaching event altered the benthic community composition including the coral assemblage. In the 2 yrs post-bleaching, the benthic community has transitioned from a short-lived increase of encrusting macroalgae to a more recent near-recovery of crustose coralline algae. Coral cover had not recovered by 2017 and could be potentially delayed by fast-growing turf algae. Within the coral community, the pre-bleaching dominant genus Montipora exhibited extreme mortality and only a handful of colonies of this taxon were enumerated in the 2016 surveys and none in 2017. Some coral taxa have persisted in low densities, including the ESA-threatened Acropora retusa and colonies of encrusting Pavona, Psammocora, and the free living Fungia. As the frequency and intensity of these high-temperature events is projected to increase in coming years, it is essential to track how remote ecosystems normally undisturbed by human influence, such as Jarvis, respond to a climate change. [ABSTRACT FROM AUTHOR]

Recovery of the threatened staghorn coral ( Acropora cervicornis) is posited to play a key role in Caribbean reef resilience. At four Caribbean locations (including one restored and three extant populations), we quantified characteristics of contemporary staghorn coral across increasing conspecific densities, and investigated a hypothesis of facilitation between staghorn coral and reef fishes. High staghorn densities in the Dry Tortugas exhibited significantly less partial mortality, higher branch growth, and supported greater fish abundances compared to lower densities within the same population. In contrast, partial mortality, branch growth, and fish community composition did not vary with staghorn density at the three other study locations where staghorn densities were lower overall. This suggests that density-dependent effects between the coral and fish community may only manifest at high staghorn densities. We then evaluated one facilitative mechanism for such density-dependence, whereby abundant fishes sheltering in dense staghorn aggregations deliver nutrients back to the coral, fueling faster coral growth, thereby creating more fish habitat. Indeed, dense staghorn aggregations within the Dry Tortugas exhibited significantly higher growth rates, tissue nitrogen, and zooxanthellae densities than sparse aggregations. Similarly, higher tissue nitrogen was induced in a macroalgae bioassay outplanted into the same dense and sparse aggregations, confirming greater bioavailability of nutrients at high staghorn densities. Our findings inform staghorn restoration efforts, suggesting that the most effective targets may be higher coral densities than previously thought. These coral-dense aggregations may reap the benefits of positive facilitation between the staghorn and fish community, favoring the growth and survivorship of this threatened species. [ABSTRACT FROM AUTHOR]