Refugia under threat: Mass bleaching of coral assemblages in high‐latitude eastern Australia

Abstract

Environmental anomalies that trigger adverse physiological responses and mortality are occurring with increasing frequency due to climate change. At species' range peripheries, environmental anomalies are particularly concerning because species often exist at their environmental tolerance limits and may not be able to migrate to escape unfavourable conditions. Here, we investigated the bleaching response and mortality of 14 coral genera across high‐latitude eastern Australia during a global heat stress event in 2016. We evaluated whether the severity of assemblage‐scale and genus‐level bleaching responses was associated with cumulative heat stress and/or local environmental history, including long‐term mean temperatures during the hottest month of each year (SSTLTMAX), and annual fluctuations in water temperature (SSTVAR) and solar irradiance (PARZVAR). The most severely‐bleached genera included species that were either endemic to the region (Pocillopora aliciae) or rare in the tropics (e.g. Porites heronensis). Pocillopora spp., in particular, showed high rates of immediate mortality. Bleaching severity of Pocillopora was high where SSTLTMAX was low or PARZVAR was high, whereas bleaching severity of Porites was directly associated with cumulative heat stress. While many tropical Acropora species are extremely vulnerable to bleaching, the Acropora species common at high latitudes, such as A. glauca and A. solitaryensis, showed little incidence of bleaching and immediate mortality. Two other regionally‐abundant genera, Goniastrea and Turbinaria, were also largely unaffected by the thermal anomaly. The severity of assemblage‐scale bleaching responses was poorly explained by the environmental parameters we examined. Instead, the severity of assemblage‐scale bleaching was associated with local differences in species abundance and taxon‐specific bleaching responses. The marked taxonomic disparity in bleaching severity, coupled with high mortality of high‐latitude endemics, point to climate‐driven simplification of assemblage structures and progressive homogenisation of reef functions at these high‐latitude locations.

Paradise lost: End‐of‐century warming and acidification under business‐as‐usual emissions have severe consequences for symbiotic corals

“…Although warming is the dominant driver of holobiont response in both species (Figure 2), temperature impacts fundamental physiological and metabolic properties more strongly in A. intermedia. Aside from some exceptions (Kim et al, 2019), this is in accordance with findings from previous research that classify Porites sp. as temperature tolerant and Acropora sp.…”

“…Meanwhile, some species are close to their upper limit in short‐term thermal acclimation (Schoepf et al, ), and may not be able to keep pace under the rapidly increasing temperature conditions of the RCP8.5 scenario (Bay et al, ; Hoegh‐Guldberg, ). Thermally sensitive groups (e.g., Acroporids) have been severely impacted by warming in recent years (Kim et al, ; Le Nohaïc et al, ) and are already facing local extinction (Riegl et al, ). Recurring thermal anomalies predicted under RCP8.5 emission pathways will likely cause the disappearance of thermally sensitive coral species from reefs globally before 2100 (Lough et al, ), while annually recurring bleaching could prove devastating to even some of the most thermally tolerant species (Grottoli et al, ).…”

Conservation value of a subtropical reef in south-eastern Queensland, Australia, highlighted by citizen-science efforts

“…Subtropical reefs are commonly promoted as potential refuges for the conservation of tropical reef species moving poleward as a result of climate change (Beger et al 2011(Beger et al , 2014Baird et al 2012;Makino et al 2014). Like their tropical counterparts, these subtropical reefs are subject to the effects of climate change, such as changes in water temperature and chemistry (Beger et al 2014;Sommer et al 2014;Kim et al 2019), as well as more localised anthropogenic stressors including pollution, eutrophication, overfishing and physical habitat damage (Gibbes et al 2014;McPhee 2017). In some instances, these issues may have even more profound and immediate effects on subtropical reefs because of the innate transitional nature of their environments (Beger et al 2011).…”

“…In some instances, these issues may have even more profound and immediate effects on subtropical reefs because of the innate transitional nature of their environments (Beger et al 2011). Research studies along the tropical-to-temperate transition in eastern Australia focus mainly on the ecological understanding of subtropical reefs at a regional or subregional spatial scale (Sommer et al 2018;Kim et al 2019). Detailed information of changes in community composition at finer spatial scales is often limited, which may hinder the development of management strategies for these unique ecosystems.…”

Coral bleaching susceptibility is predictive of subsequent mortality within but not between coral species

“…Indeed, the global loss of many thermally sensitive coral species from reef communities (e.g. (Loya et al , 2001; McClanahan, 2004; Hughes, Kerry, et al , 2018; Kim et al , 2019) may be a contributing factor in rising bleaching thresholds. Similarly, intraspecific diversity may also contribute to changes in the bleaching threshold if tolerant genotypes persist in populations and sensitive genotypes are lost.…”