Approximately one-quarter of the anthropogenic carbon dioxide released into the atmosphere each year is absorbed by the global oceans, causing measurable declines in surface ocean pH, carbonate ion concentration ([CO3(2-)]), and saturation state of carbonate minerals (Ω). This process, referred to as ocean acidification, represents a major threat to marine ecosystems, in particular marine calcifiers such as oysters, crabs, and corals. Laboratory and field studies have shown that calcification rates of many organisms decrease with declining pH, [CO3(2-)], and Ω. Coral reefs are widely regarded as one of the most vulnerable marine ecosystems to ocean acidification, in part because the very architecture of the ecosystem is reliant on carbonate-secreting organisms. Acidification-induced reductions in calcification are projected to shift coral reefs from a state of net accretion to one of net dissolution this century. While retrospective studies show large-scale declines in coral, and community, calcification over recent decades, determining the contribution of ocean acidification to these changes is difficult, if not impossible, owing to the confounding effects of other environmental factors such as temperature. Here we quantify the net calcification response of a coral reef flat to alkalinity enrichment, and show that, when ocean chemistry is restored closer to pre-industrial conditions, net community calcification increases. In providing results from the first seawater chemistry manipulation experiment of a natural coral reef community, we provide evidence that net community calcification is depressed compared with values expected for pre-industrial conditions, indicating that ocean acidification may already be impairing coral reef growth.

Coral reefs feed millions of people worldwide, provide coastal protection and generate billions of dollars annually in tourism revenue. The underlying architecture of a reef is a biogenic carbonate structure that accretes over many years of active biomineralization by calcifying organisms, including corals and algae. Ocean acidification poses a chronic threat to coral reefs by reducing the saturation state of the aragonite mineral of which coral skeletons are primarily composed, and lowering the concentration of carbonate ions required to maintain the carbonate reef. Reduced calcification, coupled with increased bioerosion and dissolution, may drive reefs into a state of net loss this century. Our ability to predict changes in ecosystem function and associated services ultimately hinges on our understanding of community- and ecosystem-scale responses. Past research has primarily focused on the responses of individual species rather than evaluating more complex, community-level responses. Here we use an in situ carbon dioxide enrichment experiment to quantify the net calcification response of a coral reef flat to acidification. We present an estimate of community-scale calcification sensitivity to ocean acidification that is, to our knowledge, the first to be based on a controlled experiment in the natural environment. This estimate provides evidence that near-future reductions in the aragonite saturation state will compromise the ecosystem function of coral reefs.

High density populations of the crown-of-thorns seastar, Acanthaster planci, are a major contributor to the decline of coral reefs, however the causes behind periodic outbreaks of this species are not understood. The enhanced nutrients hypothesis posits that pulses of enhanced larval food in eutrophic waters facilitate metamorphic success with a flow-on effect for population growth. The larval resilience hypothesis suggests that A. planci larvae naturally thrive in tropical oligotrophic waters. Both hypotheses remain to be tested empirically. We raised A. planci larvae in a range of food regimes from starvation (no food) to satiation (excess food). Algal cell concentration and chlorophyll levels were used to reflect phytoplankton conditions in nature for oligotrophic waters (0-100 cells ml-1; 0-0.01 μg chl a L-1), natural background levels of nutrients on the Great Barrier Reef (GBR) (1,000-10,000 cells ml-1; 0.1-1.0 μg chl a L-1), and enhanced eutrophic conditions following runoff events (100,000 cells ml-1; 10 μg chl a L-1). We determine how these food levels affected larval growth and survival, and the metamorphic link between larval experience and juvenile quality (size) in experiments where food ration per larvae was carefully controlled. Phytoplankton levels of 1 μg chl a L-1, close to background levels for some reefs on the GBR and following flood events, were optimal for larval success. Development was less successful above and below this food treatment. Enhanced larval performance at 1 μg chl a L-1 provides empirical support for the enhanced nutrients hypothesis, but up to a limit, and emphasizes the need for appropriate mitigation strategies to reduce eutrophication and the consequent risk of A. planci outbreaks.

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Co-occurring ocean warming, acidification and reduced carbonate mineral saturation have significant impacts on marine biota, especially calcifying organisms. The effects of these stressors on development and calcification in newly metamorphosed juveniles (ca. 0.5 mm test diameter) of the intertidal sea urchin Heliocidaris erythrogramma, an ecologically important species in temperate Australia, were investigated in context with present and projected future conditions. Habitat temperature and pH/pCO2 were documented to place experiments in a biologically and ecologically relevant context. These parameters fluctuated diurnally up to 10 °C and 0.45 pH units. The juveniles were exposed to three temperature (21, 23 and 25 °C) and four pH (8.1, 7.8, 7.6 and 7.4) treatments in all combinations, representing ambient sea surface conditions (21 °C, pH 8.1; pCO2 397; ΩCa 4.7; ΩAr 3.1), near-future projected change (+2-4 °C, -0.3-0.5 pH units; pCO2 400-1820; ΩCa 5.0-1.6; ΩAr 3.3-1.1), and extreme conditions experienced at low tide (+4 °C, -0.3-0.7 pH units; pCO2 2850-2967; ΩCa 1.1-1.0; ΩAr 0.7-0.6). The lowest pH treatment (pH 7.4) was used to assess tolerance levels. Juvenile survival and test growth were resilient to current and near-future warming and acidification. Spine development, however, was negatively affected by near-future increased temperature (+2-4 °C) and extreme acidification (pH 7.4), with a complex interaction between stressors. Near-future warming was the more significant stressor. Spine tips were dissolved in the pH 7.4 treatments. Adaptation to fluctuating temperature-pH conditions in the intertidal may convey resilience to juvenile H. erythrogramma to changing ocean conditions, however, ocean warming and acidification may shift baseline intertidal temperature and pH/pCO2 to levels that exceed tolerance limits.

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Microorganisms are fundamental drivers of biogeochemical cycling, though their contribution to coral reef ecosystem functioning is poorly understood. Here, we infer predictors of bacterioplankton community dynamics across surface-waters of the Great Barrier Reef (GBR) through a meta-analysis, combining microbial with environmental data from the eReefs platform. Nutrient dynamics and temperature explained 41.4% of inter-seasonal and crossshelf variation in bacterial assemblages. Bacterial families OCS155, Cryomorphaceae, Flavobacteriaceae, Synechococcaceae and Rhodobacteraceae dominated inshore reefs and their relative abundances positively correlated with nutrient loads. In contrast, Prochlorococcaceae negatively correlated with nutrients and became increasingly dominant towards outershelf reefs. Cyanobacteria in Prochlorococcaceae and Synechococcaceae families occupy complementary cross-shelf biogeochemical niches; their abundance ratios representing a potential indicator of GBR nutrient levels. One Flavobacteriaceae-affiliated taxa was putatively identified as diagnostic for ecosystem degradation. Establishing microbial observatories along GBR environmental gradients will facilitate robust assessments of microbial contributions to reef health and inform tipping-points in reef condition.

Due to climatic warming, Asterias amurensis, a keystone boreal predatory seastar that has established extensive invasive populations in southern Australia, is a potential high-risk invader of the sub-Antarctic and Antarctic. To assess the potential range expansion of A. amurensis to the Southern Ocean as it warms, we investigated the bioclimatic envelope of the adult and larval life stages. We analysed the distribution of adult A. amurensis with respect to present-day and future climate scenarios using habitat temperature data to construct species distribution models (SDMs). To integrate the physiological response of the dispersive phase, we determined the thermal envelope of larval development to assess their performance in present-day and future thermal regimes and the potential for success of A. amurensis in poleward latitudes. The SDM indicated that the thermal 'niche' of the adult stage correlates with a 0-17 °C and 1-22.5 °C range, in winter and summer, respectively. As the ocean warms, the range of A. amurensis in Australia will contract, while more southern latitudes will have conditions favourable for range expansion. Successful fertilization occurred from 3 to 23.8 °C. By day 12, development to the early larval stage was successful from 5.5 to 18 °C. Although embryos were able to reach the blastula stage at 2 °C, they had arrested development and high mortality. The optimal thermal range for survival of pelagic stages was 3.5-19.2 °C with a lower and upper critical limit of 2.6 and 20.3 °C, respectively. Our data predict that A. amurensis faces demise in its current invasive range while more favourable conditions at higher latitudes would facilitate invasion of both larval and adult stages to the Southern Ocean. Our results show that vigilance is needed to reduce the risk that this ecologically important Arctic carnivore may invade the Southern Ocean and Antarctica.