Autumn coral bleaching in Hawai‘i

Abstract

Coral bleaching in Hawai‘i was first reported in 1996 and subsequently occurred in 2014 and 2015. These largely summer events did not persist past November. In autumn 2019, Hawai‘i experienced persistent warm water until December, resulting in bleaching of many pocilloporid coral colonies, particularly Pocillopora meandrina. We followed the fates of 357 adult P. meandrina and 377 juvenile Pocillopora spp. off Waikīkī on the south shore of O‘ahu. Water temperatures at 5-18 m depth at this site consistently remained above a bleaching threshold of 28°C from September until mid-December. For adult colonies, 96% bleached at least partially, 4% bleached fully, and 11% died by March. Among juvenile colonies, 21% bleached, of which 47% died by March 2020. While mortality was low compared to previous bleaching events in Hawai‘i, such protracted bleaching late into the autumn increases the annual period of temperature stress, which may not bode well for the future of Hawaiian and other high-latitude coral reefs.

Anomalous sea temperatures can impair coral reef fish recruitment

“…It appears that recruiting in early summer, before July, is not beneficial for recruits as they are exposed to SST below the thermal optimum. While the long-term mean SST of November is similar to July, and could provide recruits the optimal temperature to develop in, recruiting after July is more likely to expose the recruits to SST that are warmer than optimal, where heatwaves tend to occur during September onwards in Hawai'i [26,48].…”

“…recruitment delayed until after July). This, in turn, increases the likelihood that recruits are exposed to high temperatures which typically occur in late summer in Hawai'i; for example, heatwaves in the region usually occur during September [48]. Management strategies that focus on protecting recruits, such as minimum size limits or bans on aquarium fishing, will be essential to preventing significant declines in coral reef fish in Hawai'i.…”

Seasonal variability in resilience of a coral reef fish to marine heatwaves and hypoxia

“…We found no significant differences in the frequency, duration, heating rate, mean, maximum, and cumulative intensities of MHWs between winter and summer (Figure 1). By defining MHWs relative to the baseline temperature of a particular time of year, our analysis successfully identified the major past summer MHWs that resulted in widespread coral bleaching events in Kāneʻohe Bay during the summers of 1996, 2015, and 2019, and the persistent bleaching that followed the summer 2019 event (Bahr et al, 2015; Jones et al, 2021; Winston et al, 2020; Figure S1). The consequences of winter MHWs have been documented to a lesser extent but noteworthy examples indeed exist (e.g., Bond et al, 2015; Mills et al, 2013; von Biela et al, 2019).…”

Importance of depth and temperature variability as drivers of coral symbiont composition despite a mass bleaching event

“…The environmental heterogeneity of the bay and the complexity of symbiont communities in M. capitata , create an ideal system to investigate factors influencing coral bleaching response and resilience 84 , 89 , 93 , 94 . We previously quantified the algal symbiont across Kāne’ohe Bay in 2018 81 before using this baseline to re-sample colonies after the 2019 bleaching event 95 to compare algal symbiont community change through time. Here, we take advantage of this natural bleaching event to examine whether Symbiodiniaceae community structure changes in response to thermal stress, and if so, whether environmental factors modify the community response of algal symbionts within individual M. capitata colonies across the environmental mosaic of Kāne’ohe Bay.…”