The world's tropical reef ecosystems, and the people who depend on them, are increasingly 60 impacted by climate change [1][2][3][4][5][6][7] Reef, as well as the potential influence of water quality and fishing pressure on the severity of 71 bleaching. 72The geographic footprints of mass bleaching of corals on the Great Barrier Reef have varied 73 strikingly during three major events in 1998 , 2002 and 2016). In 1998, bleaching was 74 primarily coastal and most severe in the central and southern regions. In 2002, bleaching was 75 more widespread, and affected offshore reefs in the central region that had escaped in 1998 8 . 76In 2016, bleaching was even more extensive and much more severe, especially in the 77 northern, and to a lesser extent the central regions, where many coastal, mid-shelf and 78 offshore reefs were affected (Fig. 1a, b). In 2016, the proportion of reefs experiencing 79 extreme bleaching (>60% of corals bleached) was over four times higher compared to 1998 80 or 2002 (Fig. 1f) The severity and distinctive geographic footprints of bleaching in each of the three 88 years can be explained by differences in the magnitude and spatial distribution of sea-surface 89 temperature anomalies (Fig. 1a, b 102The geographic pattern of bleaching also demonstrates how marine heatwaves can be (Fig. 2a) (Fig. 1g). largely escaped bleaching in the two earlier events (Fig. 1a). Thirty-five percent of the reefs (Fig. 1b, e). We conclude that the overlap of disparate geographic bleaching at the scale of both individual reefs and the entire Great Barrier Reef (Fig. 1a, b). 134We found a similar strong relationship between the amount of bleaching measured 135 underwater, and the satellite-based estimates of heat exposure on individual reefs (Fig. 3). 136Low levels of bleaching was observed at some locations when DHW values were only 2-3 137 o C-weeks. Typically, 30-40% of corals bleached on reefs exposed to 4 o C-weeks, whereas an 138 average of 70-90% of corals bleached on reefs that experience 8 o C-weeks or more (Fig. 3). 139Resistance and adaptation to bleaching 140 Once we account for the amount of heat stress experienced on each reef, adding 141 chlorophyll-a, a proxy for water quality, to our statistical model yielded no support for the 142 hypothesis that good water quality confers resistance to bleaching 13 . Rather, the estimated 143 effect of chlorophyll-a was to significantly reduce the DHW threshold for bleaching 144 (Extended Data Table 1). However, despite the statistical significance, the effect in real terms 145 beyond heat stress alone is very small (Extended Data Fig. 1). Similarly, we found no effect 146 of the level of protection (in fished or protected zones) on bleaching (P > 0.1: Extended Data 147 Table 1). These results are consistent with the broad-scale pattern of severe bleaching in the 148 northern Great Barrier Reef, which affected hundreds of reefs across inshore-offshore 149 gradients in water quality, and regardless of their zoning (protection) status (Fig. 1a, b). 150Simila...

Over the next century, elevated quantities of atmospheric CO 2 are expected to penetrate into the oceans, causing a reduction in pH (-0.3/-0.4 pH unit in the surface ocean) and in the concentration of carbonate ions (so-called ocean acidification). Of growing concern are the impacts that this will have on marine and estuarine organisms and ecosystems. Marine shelled molluscs, which colonized a large latitudinal gradient and can be found from intertidal to deep-sea habitats, are economically and ecologically important species providing essential ecosystem services including habitat structure for benthic organisms, water purification and a food source for other organisms. The effects of ocean acidification on the growth and shell production by juvenile and adult shelled molluscs are variable among species and even within the same species, precluding the drawing of a general picture. This is, however, not the case for pteropods, with all species tested so far, being negatively impacted by ocean acidification. The blood of shelled molluscs may exhibit lower pH with consequences for several physiological processes (e.g. respiration, excretion, etc.) and, in some cases, increased mortality in the long term. While fertilization may remain unaffected by elevated pCO 2 , embryonic and larval development will be highly sensitive with important reductions in size and decreased survival of larvae, increases in the number of abnormal larvae and an increase in the developmental time. There are big gaps in the current understanding of the biological consequences of an Communicated by S. Dupont.Frédéric Gazeau and Laura M. Parker have contributed equally to this work.Electronic supplementary material The online version of this article

The objective of this study was to investigate whether a tipping point exists in the calcification responses of coral reef calcifiers to CO 2 . We compared the effects of six partial pressures of CO 2 (P CO 2 ) from 28 Pa to 210 Pa on the net calcification of four corals (Acropora pulchra, Porites rus, Pocillopora damicornis, and Pavona cactus), and four calcified algae (Hydrolithon onkodes, Lithophyllum flavescens, Halimeda macroloba, and Halimeda minima). After 2 weeks of acclimation in a common environment, organisms were incubated in 12 aquaria for 2 weeks at the targeted P CO 2 levels and net calcification was quantified. All eight species calcified at the highest P CO 2 in which the calcium carbonate aragonite saturation state was , 1. Calcification decreased linearly as a function of increasing partial P CO 2 in three corals and three algae. Overall, the decrease in net calcification as a function of decreasing pH was , 10% when ambient P CO 2 (39 Pa) was doubled. The calcification responses of P. damicornis and H. macroloba were unaffected by increasing P CO 2 . These results are inconsistent with the notion that coral reefs will be affected by rising P CO 2 in a response characterized by a tipping point. Instead, our findings combined among taxa suggest a gradual decline in calcification will occur, but this general response includes specific cases of complete resistance to rising P CO 2 . Together our results suggest that the overall response of coral reef communities to ocean acidification will be monotonic and inversely proportional to P CO 2 , with reef-wide responses dependent on the species composition of calcifying taxa.Ocean acidification (OA), and its effects in synergy with rising temperature and a wide variety of local disturbances, is one of the major threats that coral reefs are facing in the next century (Erez et al. 2011 The projected changes in carbonate chemistry of seawater are expected to threaten the long-term survival of tropical and cold-water scleractinians corals, and it has been proposed that coral reefs will cease to deposit calcium carbonate (CaCO 3 ) when the saturation state of aragonite (V a , the mineral form of calcium carbonate used by corals) drops below 3.3 (Hoegh-Guldberg et al. 2007). It also has been proposed that coral reefs may exhibit a negative balance between precipitation and dissolution of CaCO 3 (i.e., they will lose more than they will deposit) when the concentration of atmospheric CO 2 relative to preindustrial levels doubles (Silverman et al. 2009). However, most studies of these effects are based on a limited set of empirical measurements establishing the functional relationship between the partial pressure of CO 2 (P CO 2 ; and hence, V arag and the saturation state of calcite, V calc ) and the calcification rates of important functional groups of calcifying taxa on tropical reefs (Erez et al. 2011;Edmunds et al. 2012). In addition, only one study (Marubini et al. 2008) to date has resolved precisely the shape of the relationship between calcifi...

Coralline algae provide important ecosystem services but are susceptible to the impacts of ocean acidification. However, the mechanisms are uncertain, and the magnitude is species specific. Here, we assess whether species-specific responses to ocean acidification of coralline algae are related to differences in pH at the site of calcification within the calcifying fluid/medium (pH ) using δ B as a proxy. Declines in δ B for all three species are consistent with shifts in δ B expected if B(OH) was incorporated during precipitation. In particular, the δ B ratio in Amphiroa anceps was too low to allow for reasonable pH values if B(OH) rather than B(OH) was directly incorporated from the calcifying fluid. This points towards δ B being a reliable proxy for pH for coralline algal calcite and that if B(OH) is present in detectable proportions, it can be attributed to secondary postincorporation transformation of B(OH) . We thus show that pH is elevated during calcification and that the extent is species specific. The net calcification of two species of coralline algae (Sporolithon durum, and Amphiroa anceps) declined under elevated CO , as did their pH . Neogoniolithon sp. had the highest pH , and most constant calcification rates, with the decrease in pH being ¼ that of seawater pH in the treatments, demonstrating a control of coralline algae on carbonate chemistry at their site of calcification. The discovery that coralline algae upregulate pH under ocean acidification is physiologically important and should be included in future models involving calcification.

Abstract. Thecosome pteropods (shelled pelagic molluscs) can play an important role in the food web of various ecosystems and play a key role in the cycling of carbon and carbonate. Since they harbor an aragonitic shell, they could be very sensitive to ocean acidification driven by the increase of anthropogenic CO2 emissions. The impact of changes in the carbonate chemistry was investigated on Limacina helicina, a key species of Arctic ecosystems. Pteropods were kept in culture under controlled pH conditions corresponding to pCO2 levels of 350 and 760 μatm. Calcification was estimated using a fluorochrome and the radioisotope 45Ca. It exhibits a 28% decrease at the pH value expected for 2100 compared to the present pH value. This result supports the concern for the future of pteropods in a high-CO2 world, as well as of those species dependent upon them as a food resource. A decline of their populations would likely cause dramatic changes to the structure, function and services of polar ecosystems.

Thecosome pteropods (pelagic mollusks) can play a key role in the food web of various marine ecosystems. They are a food source for zooplankton or higher predators such as fishes, whales and birds that is particularly important in high latitude areas. Since they harbor a highly soluble aragonitic shell, they could be very sensitive to ocean acidification driven by the increase of anthropogenic CO2 emissions. The effect of changes in the seawater chemistry was investigated on Limacina helicina, a key species of Arctic pelagic ecosystems. Individuals were kept in the laboratory under controlled pCO2 levels of 280, 380, 550, 760 and 1020 µatm and at control (0°C) and elevated (4°C) temperatures. The respiration rate was unaffected by pCO2 at control temperature, but significantly increased as a function of the pCO2 level at elevated temperature. pCO2 had no effect on the gut clearance rate at either temperature. Precipitation of CaCO3, measured as the incorporation of 45Ca, significantly declined as a function of pCO2 at both temperatures. The decrease in calcium carbonate precipitation was highly correlated to the aragonite saturation state. Even though this study demonstrates that pteropods are able to precipitate calcium carbonate at low aragonite saturation state, the results support the current concern for the future of Arctic pteropods, as the production of their shell appears to be very sensitive to decreased pH. A decline of pteropod populations would likely cause dramatic changes to various pelagic ecosystems.

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To identify the properties of taxa sensitive and resistant to ocean acidification (OA), we tested the hypothesis that coral reef calcifiers differ in their sensitivity to OA as predictable outcomes of functional group alliances determined by conspicuous traits. We contrasted functional groups of eight corals and eight calcifying algae defined by morphology in corals and algae, skeletal structure in corals, spatial location of calcification in algae, and growth rate in corals and algae. The responses of calcification to OA were unrelated to morphology and skeletal structure in corals; they were, however, affected by growth rate in corals and algae (fast calcifiers were more sensitive than slow calcifiers), and by the site of calcification and morphology in algae. Species assemblages characterized by fast growth, and for algae, also cell-wall calcification, are likely to be ecological losers in the future ocean. This shift in relative success will affect the relative and absolute species abundances as well as the goods and services provided by coral reefs.