Neal E. Cantin

Global warming and recurrent mass bleaching of corals

During 2015–2016, record temperatures triggered a pan-tropical episode of coral bleaching, the third global-scale event since mass bleaching was first documented in the 1980s. Here we examine how and why the severity of recurrent major bleaching events has varied at multiple scales, using aerial and underwater surveys of Australian reefs combined with satellite-derived sea surface temperatures. The distinctive geographic footprints of recurrent bleaching on the Great Barrier Reef in 1998, 2002 and 2016 were determined by the spatial pattern of sea temperatures in each year. Water quality and fishing pressure had minimal effect on the unprecedented bleaching in 2016, suggesting that local protection of reefs affords little or no resistance to extreme heat. Similarly, past exposure to bleaching in 1998 and 2002 did not lessen the severity of bleaching in 2016. Consequently, immediate global action to curb future warming is essential to secure a future for coral reefs.

Real-time PCR reveals a high incidence of Symbiodinium clade D at low levels in four scleractinian corals across the Great Barrier Reef: implications for symbiont shuffling

Reef corals form associations with an array of genetically and physiologically distinct endosymbionts from the genus Symbiodinium. Some corals harbor different clades of symbionts simultaneously, and over time the relative abundances of these clades may change through a process called symbiont shuffling. It is hypothesized that this process provides a mechanism for corals to respond to environmental threats such as global warming. However, only a minority of coral species have been found to harbor more than one symbiont clade simultaneously and the current view is that the potential for symbiont shuffling is limited. Using a newly developed real-time PCR assay, this paper demonstrates that previous studies have underestimated the presence of background symbionts because of the low sensitivity of the techniques used. The assay used here targets the multi-copy rDNA ITS1 region and is able to detect Symbiodinium clades C and D with >100-fold higher sensitivity compared to conventional techniques. Technical considerations relating to intragenomic variation, estimating copy number and non-symbiotic contamination are discussed. Eighty-two colonies from four common scleractinian species (Acropora millepora, Acropora tenuis, Stylophora pistillata and Turbinaria reniformis) and 11 locations on the Great Barrier Reef were tested for background Symbiodinium clades. Although these colonies had been previously identified as harboring only a single clade based on SSCP analyses, background clades were detected in 78% of the samples, indicating that the potential for symbiont shuffling may be much larger than currently thought.

Surviving Coral Bleaching Events: Porites Growth Anomalies on the Great Barrier Reef

Mass coral bleaching affected large parts of the Great Barrier Reef (GBR) in 1998 and 2002. In this study, we assessed if signatures of these major thermal stress events were recorded in the growth characteristics of massive Porites colonies. In 2005 a suite of short (<50 cm) cores were collected from apparently healthy, surviving Porites colonies, from reefs in the central GBR (18–19°S) that have documented observations of widespread bleaching. Sites included inshore (Nelly Bay, Pandora Reef), annually affected by freshwater flood events, midshelf (Rib Reef), only occasionally affected by freshwater floods and offshore (Myrmidon Reef) locations primarily exposed to open ocean conditions. Annual growth characteristics (extension, density and calcification) were measured in 144 cores from 79 coral colonies and analysed over the common 24-year period, 1980–2003. Visual examination of the annual density bands revealed growth hiatuses associated with the bleaching years in the form of abrupt decreases in annual linear extension rates, high density stress bands and partial mortality. The 1998 mass-bleaching event reduced Porites calcification by 13 and 18% on the two inshore locations for 4 years, followed by recovery to baseline calcification rates in 2002. Evidence of partial mortality was apparent in 10% of the offshore colonies in 2002; however no significant effects of the bleaching events were evident in the calcification rates at the mid shelf and offshore sites. These results highlight the spatial variation of mass bleaching events and that all reef locations within the GBR were not equally stressed by the 1998 and 2002 mass bleaching events, as some models tend to suggest, which enabled recovery of calcification on the GBR within 4 years. The dynamics in annual calcification rates and recovery displayed here should be used to improve model outputs that project how coral calcification will respond to ongoing warming of the tropical oceans.

Impacts of Ocean Acidification on Early Life-History Stages and Settlement of the Coral-Eating Sea Star Acanthaster planci

Coral reefs are marine biodiversity hotspots, but their existence is threatened by global change and local pressures such as land-runoff and overfishing. Population explosions of coral-eating crown of thorns sea stars (COTS) are a major contributor to recent decline in coral cover on the Great Barrier Reef. Here, we investigate how projected near-future ocean acidification (OA) conditions can affect early life history stages of COTS, by investigating important milestones including sperm motility, fertilisation rates, and larval development and settlement. OA (increased pCO2 to 900–1200 µatm pCO2) significantly reduced sperm motility and, to a lesser extent, velocity, which strongly reduced fertilization rates at environmentally relevant sperm concentrations. Normal development of 10 d old larvae was significantly lower under elevated pCO2 but larval size was not significantly different between treatments. Settlement of COTS larvae was significantly reduced on crustose coralline algae (known settlement inducers of COTS) that had been exposed to OA conditions for 85 d prior to settlement assays. Effect size analyses illustrated that reduced settlement may be the largest bottleneck for overall juvenile production. Results indicate that reductions in fertilisation and settlement success alone would reduce COTS population replenishment by over 50%. However, it is unlikely that this effect is sufficient to provide respite for corals from other negative anthropogenic impacts and direct stress from OA and warming on corals.

Shifting paradigms in restoration of the world's coral reefs

Abstract Many ecosystems around the world are rapidly deteriorating due to both local and global pressures, and perhaps none so precipitously as coral reefs. Management of coral reefs through maintenance (e.g., marine‐protected areas, catchment management to improve water quality), restoration, as well as global and national governmental agreements to reduce greenhouse gas emissions (e.g., the 2015 Paris Agreement) is critical for the persistence of coral reefs. Despite these initiatives, the health and abundance of corals reefs are rapidly declining and other solutions will soon be required. We have recently discussed options for using assisted evolution (i.e., selective breeding, assisted gene flow, conditioning or epigenetic programming, and the manipulation of the coral microbiome) as a means to enhance environmental stress tolerance of corals and the success of coral reef restoration efforts. The 2014–2016 global coral bleaching event has sharpened the focus on such interventionist approaches. We highlight the necessity for consideration of alternative (e.g., hybrid) ecosystem states, discuss traits of resilient corals and coral reef ecosystems, and propose a decision tree for incorporating assisted evolution into restoration initiatives to enhance climate resilience of coral reefs.

Perspectives on Massive Coral Growth Rates in a Changing Ocean

The tropical ocean environment is changing at an unprecedented rate, with warming and severe tropical cyclones creating obvious impacts to coral reefs within the last few decades and projections of acidification raising concerns for the future of these iconic and economically important ecosystems. Documenting variability and detecting change in global and regional climate relies upon high-quality observational records of climate variables supplemented, prior to the mid-19th century, with reconstructions from various sources of proxy climate information. Here we review how annual density banding patterns that are recorded in the skeletons of massive reef-building corals have been used to document environmental change and impacts within coral reefs. Massive corals provide a historical perspective of continuous calcification processes that pre-date most ecological observations of coral reefs. High-density stress bands, abrupt declines in annual linear extension, and evidence of partial mortality within the skeletal growth record reveal signatures of catastrophic stress events that have recently been attributed to mass bleaching events caused by unprecedented thermal stress. Comparison of recent trends in annual calcification with century-scale baseline calcification rates reveals that the frequency of growth anomalies has increased since the late 1990s throughout most of the world’s coral reef ecosystems. Continuous coral growth histories provide valuable retrospective information on the coral response to environmental change and the consequences of anthropogenic climate change. Co-ordinated efforts to synthesize and combine global calcification histories will greatly enhance our understanding of current calcification responses to a changing ocean.

Coral reef metabolism and carbon chemistry dynamics of a coral reef flat

Global carbon emissions continue to acidify the oceans, motivating growing concern for the ability of coral reefs to maintain net positive calcification rates. Efforts to develop robust relationships between coral reef calcification and carbonate parameters such as aragonite saturation state (Ωarag) aim to facilitate meaningful predictions of how reef calcification will change in the face of ocean acidification. Here we investigate natural trends in carbonate chemistry of a coral reef flat over diel cycles and relate these trends to benthic carbon fluxes by quantifying net community calcification and net community production. We find that, despite an apparent dependence of calcification on Ωarag seen in a simple pairwise relationship, if the dependence of net calcification on net photosynthesis is accounted for, knowing Ωarag does not add substantial explanatory value. This suggests that, over short time scales, the control of Ωarag on net calcification is weak relative to factors governing net photosynthesis.

Rapid adaptive responses to climate change in corals

Pivotal to projecting the fate of coral reefs is the capacity of reef-building corals to acclimatize and adapt to climate change. Transgenerational plasticity may enable some marine organisms to acclimatize over several generations and it has been hypothesized that epigenetic processes and microbial associations might facilitate adaptive responses. However, current evidence is equivocal and understanding of the underlying processes is limited. Here, we discuss prospects for observing transgenerational plasticity in corals and the mechanisms that could enable adaptive plasticity in the coral holobiont, including the potential role of epigenetics and coral-associated microbes. Well-designed and strictly controlled experiments are needed to distinguish transgenerational plasticity from other forms of plasticity, and to elucidate the underlying mechanisms and their relative importance compared with genetic adaptation.

Variation in growth rates of branching corals along Australia’s Great Barrier Reef

Coral growth is an important component of reef health and resilience. However, few studies have investigated temporal and/or spatial variation in growth of branching corals, which are important contributors to the structure and function of reef habitats. This study assessed growth (linear extension, density, and calcification) of three branching coral species (Acropora muricata, Pocillopora damicornis and Isopora palifera) at three distinct locations (Lizard Island, Davies/Trunk Reef, and Heron Island) along Australia’s Great Barrier Reef (GBR). Annual growth rates of all species were highest at Lizard Island and declined with increasing latitude, corresponding with differences in temperature. Within locations, however, seasonal variation in growth did not directly correlate with temperature. Between October 2012 and October 2014, the highest growth of A. muricata was in the 2013–14 summer at Lizard Island, which was unusually cool and ~0.5 °C less than the long-term summer average temperature. At locations where temperatures reached or exceeded the long-term summer maxima, coral growth during summer periods was equal to, if not lower than, winter periods. This study shows that temperature has a significant influence on spatiotemporal patterns of branching coral growth, and high summer temperatures in the northern GBR may already be constraining coral growth and reef resilience.