Successive marine heatwaves cause disproportionate coral bleaching during a fast phase transition from El Niño to La Niña.

The frequency and intensity of marine heatwaves that result in coral bleaching events have increased over recent decades and led to catastrophic losses of reef-building corals in many regions. The high-latitude coral assemblages at Lord Howe Island, which is a UNESCO listed site is the world southernmost coral community, were exposed to successive thermal anomalies following a fast phase-transition of the record-breaking 2009 to 2010 warm pool El Niño in the Central Pacific to a strong La Niña event in late 2010. The coral community experienced severe and unprecedented consecutive bleaching in both 2010 and 2011. Coral health surveys completed between March 2010 and September 2012 quantified the response and recovery of approximately 43,700 coral colonies to these successive marine heatwaves. In March 2010, coral bleaching ranged from severe, with 99% of colonies bleached at some shallow lagoon sites, to mild at deeper reef slope sites, with only 17% of individuals affected. Significant immediate mortality from thermal stress was evident during the peak of the bleaching event. Overall, species in the genera Pocillopora, Stylophora, Seriatopora and Porites were the most affected, while minimal bleaching and mortality was recorded among members of other coral families (e.g. Acroporidae, Dendrophyllidae & Merulinidae). Surviving corals underwent a subsequent, but much less intense, thermal anomaly in 2011 that led to a disproportionate bleaching response among susceptible taxa. While this observation indicates that the capacity of thermally susceptible high-latitude corals to acclimatize to future ocean warming may be limited, particularly if bleaching events occur annually, our long-term survey data shows that coral cover at most sites recovered to pre-bleaching levels within three years in the absence of further thermal anomalies.

Multiple field-based methods to assess the potential impacts of seismic surveys on scallops.

Marine seismic surveys are an important tool to map geology beneath the seafloor and manage petroleum resources, but they are also a source of underwater noise pollution. A mass mortality of scallops in the Bass Strait, Australia occurred a few months after a marine seismic survey in 2010, and fishing groups were concerned about the potential relationship between the two events. The current study used three field-based methods to investigate the potential impact of marine seismic surveys on scallops in the region: 1) dredging and 2) deployment of Autonomous Underwater Vehicles (AUVs) were undertaken to examine the potential response of two species of scallops (Pecten fumatus, Mimachlamys asperrima) before, two months after, and ten months after a 2015 marine seismic survey; and 3) MODIS satellite data revealed patterns of sea surface temperatures from 2006-2016. Results from the dredging and AUV components show no evidence of scallop mortality attributable to the seismic survey, although sub-lethal effects cannot be excluded. The remote sensing revealed a pronounced thermal spike in the eastern Bass Strait between February and May 2010, overlapping the scallop beds that suffered extensive mortality and coinciding almost exactly with dates of operation for the 2010 seismic survey. The acquisition of in situ data coupled with consideration of commercial seismic arrays meant that results were ecologically realistic, while the paired field-based components (dredging, AUV imagery) provided a failsafe against challenges associated with working wholly in the field. This study expands our knowledge of the potential environmental impacts of marine seismic survey and will inform future applications for marine seismic surveys, as well as the assessment of such applications by regulatory authorities.

Exploring the Symbiodinium rare biosphere provides evidence for symbiont switching in reef-building corals.

Reef-building corals possess a range of acclimatisation and adaptation mechanisms to respond to seawater temperature increases. In some corals, thermal tolerance increases through community composition changes of their dinoflagellate endosymbionts (Symbiodinium spp.), but this mechanism is believed to be limited to the Symbiodinium types already present in the coral tissue acquired during early life stages. Compelling evidence for symbiont switching, that is, the acquisition of novel Symbiodinium types from the environment, by adult coral colonies, is currently lacking. Using deep sequencing analysis of Symbiodinium rDNA internal transcribed spacer 2 (ITS2) PCR amplicons from two pocilloporid coral species, we show evidence consistent with de novo acquisition of Symbiodinium types from the environment by adult corals following two consecutive bleaching events. Most of these newly detected symbionts remained in the rare biosphere (background types occurring below 1% relative abundance), but one novel type reached a relative abundance of ~33%. Two de novo acquired Symbiodinium types belong to the thermally resistant clade D, suggesting that this switching may have been driven by consecutive thermal bleaching events. Our results are particularly important given the maternal mode of Symbiodinium transmission in the study species, which generally results in high symbiont specificity. These findings will cause a paradigm shift in our understanding of coral-Symbiodinium symbiosis flexibility and mechanisms of environmental acclimatisation in corals.