Rebuilding relationships on coral reefs: Coral bleaching knowledge-sharing to aid adaptation planning for reef users: Bleaching emergence on reefs demonstrates the need to consider reef scale and accessibility when preparing for, and responding to, coral bleaching

Coral bleaching has impacted reefs worldwide and the predictions of near-annual bleaching from over two decades ago have now been realized. While technology currently provides the means to predict large-scale bleaching, predicting reef-scale and within-reef patterns in real-time for all reef users is limited. In 2020, heat stress across the Great Barrier Reef underpinned the region's third bleaching event in 5 years. Here we review the heterogeneous emergence of bleaching across Heron Island reef habitats and discuss the oceanographic drivers that underpinned variable bleaching emergence. We do so as a case study to highlight how reef end-user groups who engage with coral reefs in different ways require targeted guidance for how, and when, to alter their use of coral reefs in response to bleaching events. Our case study of coral bleaching emergence demonstrates how within-reef scale nowcasting of coral bleaching could aid the development of accessible and equitable bleaching response strategies on coral reefs. Also see the video abstract here: https://youtu.be/N9Tgb8N-vN0

Global declines in coral reef calcium carbonate production under ocean acidification and warming

Ocean warming and acidification threaten the future growth of coral reefs. This is because the calcifying coral reef taxa that construct the calcium carbonate frameworks and cement the reef together are highly sensitive to ocean warming and acidification. However, the global-scale effects of ocean warming and acidification on rates of coral reef net carbonate production remain poorly constrained despite a wealth of studies assessing their effects on the calcification of individual organisms. Here, we present global estimates of projected future changes in coral reef net carbonate production under ocean warming and acidification. We apply a meta-analysis of responses of coral reef taxa calcification and bioerosion rates to predicted changes in coral cover driven by climate change to estimate the net carbonate production rates of 183 reefs worldwide by 2050 and 2100. We forecast mean global reef net carbonate production under representative concentration pathways (RCP) 2.6, 4.5, and 8.5 will decline by 76, 149, and 156%, respectively, by 2100. While 63% of reefs are projected to continue to accrete by 2100 under RCP2.6, 94% will be eroding by 2050 under RCP8.5, and no reefs will continue to accrete at rates matching projected sea level rise under RCP4.5 or 8.5 by 2100. Projected reduced coral cover due to bleaching events predominately drives these declines rather than the direct physiological impacts of ocean warming and acidification on calcification or bioerosion. Presently degraded reefs were also more sensitive in our analysis. These findings highlight the low likelihood that the world’s coral reefs will maintain their functional roles without near-term stabilization of atmospheric CO2 emissions

Taking the Metabolic Pulse of the World\u27s Coral Reefs

Worldwide, coral reef ecosystems are experiencing increasing pressure from a variety of anthropogenic perturbations including ocean warming and acidification, increased sedimentation, eutrophication, and overfishing, which could shift reefs to a condition of net calcium carbonate (CaCO3) dissolution and erosion. Herein, we determine the net calcification potential and the relative balance of net organic carbon metabolism (net community production; NCP) and net inorganic carbon metabolism (net community calcification; NCC) within 23 coral reef locations across the globe. In light of these results, we consider the suitability of using these two metrics developed from total alkalinity (TA) and dissolved inorganic carbon (DIC) measurements collected on different spatiotemporal scales to monitor coral reef biogeochemistry under anthropogenic change. All reefs in this study were net calcifying for the majority of observations as inferred from alkalinity depletion relative to offshore, although occasional observations of net dissolution occurred at most locations. However, reefs with lower net calcification potential (i.e., lower TA depletion) could shift towards net dissolution sooner than reefs with a higher potential. The percent influence of organic carbon fluxes on total changes in dissolved inorganic carbon (DIC) (i.e., NCP compared to the sum of NCP and NCC) ranged from 32% to 88% and reflected inherent biogeochemical differences between reefs. Reefs with the largest relative percentage of NCP experienced the largest variability in seawater pH for a given change in DIC, which is directly related to the reefs ability to elevate or suppress local pH relative to the open ocean. This work highlights the value of measuring coral reef carbonate chemistry when evaluating their susceptibility to ongoing global environmental change and offers a baseline from which to guide future conservation efforts aimed at preserving these valuable ecosystems

Seawater carbonate chemistry and Calcium carbonate (CaCO3) sediment dissolution

Ocean acidification (OA), attributed to the sequestration of atmospheric carbon dioxide (CO2) into the surface ocean, and coastal eutrophication, attributed in part to land-use change and terrestrial runoff of fertilizers, have received recent attention in an experimental framework examining the effects of each on coral reef net ecosystem calcification (Gnet). However, OA and eutrophication in conjunction have yet to receive attention from the perspective of coral reef sediment dissolution. To address this omission, CO2 and nitrate (NO3-) addition experiments were performed in Mo'orea, French Polynesia. Incubation chambers were used to measure sediment Gnet during the day and night under three different [NO3-] (0, 9.8, and 19.7 M) that were nested within four separate constructed coral reef communities maintained at different PCO2 levels (417, 721, 1030, and 1333 ?atm, respectively). PCO2 negatively affected sediment Gnetduring the day and night, resulting in a shift to diel net dissolution at a PCO2 of 1030 µatm. Elevated NO3- alone, and the combination of NO3- and PCO2, both negatively affected sediment Gnet at night. However, the response of Gnet to NO3- was less clear during the day, where diurnal sediment Gnet was enhanced under the combined treatment of elevated NO3- and PCO2, resulting in no net effect of NO3- on sediment Gnet on diel timescales. Overall, these results show that ocean acidification represents a greater threat to the balance of calcification and dissolution in Mo'orea's back reef sediment communities than the potential impact of NO3- enrichment on relatively short timescales

The effect of warming and benthic community acclimation on coral reef carbonate sediment metabolism and dissolution

Global warming (and the consequent increase in sea surface temperature) is expected to modify rates of gross primary production (GPP), respiration (R), and net calcium carbonate (CaCO3) dissolution in permeable coral reef carbonate sediments. Previous simulations of seawater warming on coral reef sediments found a decline in the GPP/R ratio and an associated increase in CaCO3 dissolution but were only conducted over a short timescale ( 24 h) effect of seawater warming on coral reef CaCO3 sediment metabolism and dissolution, which may allow the benthic community to acclimatise. This study used 600-L flume aquaria to examine the effect of seawater warming on GPP, R, and CaCO3 dissolution in the permeable coral reef CaCO3 sediments of Mo'orea, French Polynesia over a period of 15 d. On average, when exposed to warmed seawater (+ 2.8 ºC), R in the CaCO3 sediments was enhanced (+ 58 %) to a greater extent than GPP (+19 %), resulting in a decline in GPP/R (- 23 %) and an associated increase in net CaCO3 dissolution (+ 126 %). The magnitude of these warming-mediated metabolic changes increased each day until reaching a plateau after about 8 days, indicating that 24-h experiments may be underestimating the effect of warming over longer timescales. Interestingly, the increase in dissolution relative to control treatments was more striking during the day (+ 163 %) than at night (+ 89 %), suggesting that warming acted to both enhance geochemical dissolution and reduce biogenic calcification or inorganic precipitation. Together, these data indicate that, over the timescale observed here, photosynthesis and associated inorganic and biogenic CaCO3 precipitation do not exhibit the ability to counterbalance the warming-mediated increase in sediment heterotrophy and CaCO3 dissolution

Ocean Acidification and Organic Matter Enrichment Alter Carbonate Sediment Metabolism Through Different Pathways

Ocean acidification (OA) and organic matter enrichment (due to coastal eutrophication) could act in concert to shift coral reef carbonate sediments from a present state of net calcification to a future state of net dissolution, but no studies have examined the combined effect of these stressors on sediment metabolism and dissolution. This study used 22-hour incubations in flume aquaria with captive sediment communities to measure the combined effect of OA and organic matter (OM) enrichment, on coral reef sediment gross primary productivity (GPP), respiration (R), and net calcification (Gnet). Relative to control sediment communities, both OA ( 1000 µatm) and OM enrichment (+ 40 µmol C/L) significantly decreased rates of sediment Gnet by 98% and 15% mmol CaCO3/m**2/h, respectively , but the mechanism behind this decrease differed. The OA-mediated transition to net dissolution was geochemical, as rates of GPP and R remained unaffected and dissolution was solely enhanced by a decline in the aragonite saturation state (Omega arg) of the overlying water column. In contrast, the OM-mediated decline in Gnet was due to a decline in GPP/R, thereby biologically reducing overlying seawater Ωarg due to the increased respiratory addition of CO2. The decrease in Gnet in response to a combination of both stressors was additive (- 10% relative to OA alone) but this decrease did not significantly differ from the effect of OA alone. In this study OA was the primary driver of future carbonate sediment dissolution, but longer-term experiments with chronic organic matter enrichment are required