Ocean acidification threatens coral reefs by reducing seawater pH and carbonate saturation state. Crustose coralline algae are particularly vulnerable because their high-magnesium calcite skeletons dissolve more readily than coral aragonite skeletons. However, this dissolution may increase alkalinity and buffer reef-water chemistry. Here we show, using repeated low-tide observations and in situ incubations in a shallow reef system in the southern Great Barrier Reef, that reef-water pH varies by more than one unit over the day (7.47 – 8.61), reaching levels comparable to those projected for the end of this century. Nighttime respiration promotes dissolution of high-magnesium calcite produced by crustose coralline algae, increasing alkalinity and helping maintain seawater supersaturated with respect to aragonite. At the same time, isolated coral incubations experience a greater decline in pH and aragonite saturation state in the absence of this buffering effect. These findings suggest that high-magnesium calcite-producing communities may help partially buffer reefs against future acidification. Crustose coralline algae overlying reef structures can reduce coral dissolution, especially at night-time, according to in-situ carbonate chemistry data from the Great Barrier Reef.

Abstract Seagrass meadows are natural carbon sinks, yet the effect of ocean acidification on their carbon burial capacity remains poorly understood. Here we investigated natural carbon dioxide vents in Ischia, Italy to assess how seawater pH influences carbon burial in an area dominated by the seagrass Posidonia oceanica. Organic carbon burial rates (mean ± standard error) between 1954 – 2021 were low under ambient conditions (1.5 ± 0.5 g m-2 yr-1) but increased sharply under acidified conditions (7 ± 1 g m-2 yr-1), reaching sevenfold higher values under extreme acidification (10 ± 3 g m-2 yr-1). Stable isotopes suggest that these patterns reflect changes in the relative contribution of seagrass, macroalgae, and epiphytes to buried carbon. These findings reveal that ocean acidification can substantially alter coastal carbon cycling, potentially through shifts in community composition, with important implications for understanding past and future feedbacks between seagrass ecosystems and the marine carbon cycle.

Past coral range expansions suggest that high-latitude environments may serve as refugia, potentially buffering coral biodiversity loss due to climate change. We explore this possibility for corals globally, using a dynamic metacommunity model incorporating temperature, photosynthetically available radiation, pH, and four distinct, interacting coral assemblages. This model reasonably reproduces the observed distribution and recent decline of corals across the Indo-Pacific and Caribbean. Our simulations suggest that there is a mismatch between the timescales of coral reef decline and range expansion under future predicted climate change. Whereas the most severe declines in coral cover will likely occur within 40 to 80 years, large-scale coral reef expansion requires centuries. The absence of large-scale coral refugia in the face of rapid anthropogenic climate change emphasizes the urgent need to reduce greenhouse gas emissions and mitigate nonthermal stressors for corals, both in the tropics and in higher latitudes.

Coastal regions are complex habitats, where multiple natural and anthropogenic drivers can interact to affect the survival and growth of marine organisms. The giant kelp Macrocystis pyrifera is sensitive to increasing seawater temperatures and susceptible to marine heatwaves. Light availability and hydrodynamics can also affect the growth, morphology, and resilience of this species. In this experiment, juvenile sporophytes of M. pyrifera from Scorching Bay, Wellington, Aotearoa, New Zealand, a were exposed to a combination of simulated marine heatwaves at one of four different temperatures (20, 22, and 24°C compared to a 16°C control), one of two irradiance levels (shaded: 0.9 mol photons · m −2 · d −1 or ambient: 1.4 mol photons · m −2 · d −1 ), and one of two flow speeds (5.3 cm · s −1 or 6.1 cm · s −1 ) in a fully factorial design. Simulated heatwaves lasted for 21 days, with temperatures ramped by 2°C · d −1 , followed by a 21‐day recovery phase. The heatwave treatments represented severe heatwaves in present day or hypothetical future conditions, whereas the control represented historical average summer sea temperatures in Wellington, and 21 days represented a realistic duration for heatwaves in this region. Temperature was the main driver of negative physiological impacts, with 100% of sporophytes dying within 42 days of exposure to a 24°C heatwave. Sporophytes experienced 44% mortality at 20°C and 81% mortality at 22°C, and growth rates declined significantly with increasing temperature. However, survival rates were modified by light and water velocity, with 56% of sporophytes surviving under a combination of ambient light and fast water velocity, compared with less than 50% under each of the other light‐velocity combinations. Light limitation also reduced sporophyte survival, growth rates, and effective quantum yield. Water velocity alone did not significantly affect sporophytes, but flow speeds had interactive effects with temperature and light. The findings of this experiment suggest that M. pyrifera at sites with optimal environmental conditions, including low sediment loads and fast tidal flows, could be more resilient to marine heatwaves, as long as temperatures do not exceed critical thresholds for survival.

Warm-water coral reefs are facing unprecedented human-driven threats to their continued existence as biodiverse functional ecosystems upon which hundreds of millions of people rely. These impacts may drive coral ecosystems past critical thresholds, beyond which the system reorganises, often abruptly and potentially irreversibly; this is what the Intergovernmental Panel on Climate Change (IPCC, 2022) define as a tipping point. Determining tipping point thresholds for coral reef ecosystems requires a robust assessment of multiple stressors and their interactive effects. In this perspective piece, we draw upon the recent global tipping point revision initiative (Lenton et al., 2023a) and a literature search to identify and summarise the diverse range of interacting stressors that need to be considered for determining tipping point thresholds for warm-water coral reef ecosystems. Considering observed and projected stressor impacts, we endorse the global tipping point revision's conclusion of a global mean surface temperature (relative to pre-industrial) tipping point threshold of 1.2 °C (range 1–1.5 °C) and the long-term impacts of atmospheric CO2 concentrations above 350 ppm, while acknowledging that comprehensive assessment of stressors, including ocean warming response dynamics, overshoot, and cascading impacts, have yet to be sufficiently realised. These tipping point thresholds have already been exceeded, and therefore these systems are in an overshoot state and are reliant on policy actions to bring stressor levels back within tipping point limits. A fuller assessment of interacting stressors is likely to further lower the tipping point thresholds in most cases. Uncertainties around tipping points for such crucially important ecosystems underline the imperative of robust assessment and, in the case of knowledge gaps, employing a precautionary principle favouring lower-range tipping point values.

Coral reefs form complex physical structures that can help to mitigate coastal flooding risk1,2. This function will be reduced by sea-level rise (SLR) and impaired reef growth caused by climate change and local anthropogenic stressors3. Water depths above reef surfaces are projected to increase as a result, but the magnitudes and timescales of this increase are poorly constrained, which limits modelling of coastal vulnerability4,5. Here we analyse fossil reef deposits to constrain links between reef ecology and growth potential across more than 400 tropical western Atlantic sites, and assess the magnitudes of resultant above-reef increases in water depth through to 2100 under various shared socioeconomic pathway (SSP) emission scenarios. Our analysis predicts that more than 70% of tropical western Atlantic reefs will transition into net erosional states by 2040, but that if warming exceeds 2 °C (SSP2-4.5 and higher), nearly all reefs (at least 99%) will be eroding by 2100. The divergent trajectories of reef growth and SLR will thus magnify the effects of SLR; increases in water depth of around 0.3-0.5 m above the present are projected under all warming scenarios by 2060, but depth increases of 0.7-1.2 m are predicted by 2100 under scenarios in which warming surpasses 2 °C. This would increase the risk of flooding along vulnerable reef-fronted coasts and modify nearshore hydrodynamics and ecosystems. Reef restoration offers one pathway back to higher reef growth6,7, but would dampen the effects of SLR in 2100 only by around 0.3-0.4 m, and only when combined with aggressive climate mitigation.

Kelp forests are productive and biodiverse ecosystems with high ecological, cultural, and economic importance. However, the high sensitivity of kelp to water temperature means that these ecosystems are vulnerable to marine heatwaves (MHWs), especially at the equatorward edge of their range. To date, few laboratory studies have compared the effects of MHWs of different durations or intensities on kelp, and it is difficult to determine these effects from naturally occurring MHWs in the field. We exposed juvenile sporophytes of the giant kelp Macrocystis pyrifera from Wellington, Aotearoa New Zealand to simulated MHWs three or six weeks in duration, at temperatures of 18°C, 20°C, and 22°C, corresponding to 2, 4, and 6°C above local mean summer temperatures. While all MHW treatments reduced mean kelp growth rates by over 30% relative to 16°C controls, the 22°C treatments had much more severe and wide-ranging effects, including rapid blade erosion, reduced chlorophyll fluorescence, tissue bleaching, increased δ13C values, and mortality. Nonetheless, sporophytes had some ability to recover from heat stress; within the 18°C treatment, mean relative growth rates neared or exceeded those within the control treatment within three weeks after MHWs concluded. These results support the findings of previous studies which indicate that M. pyrifera sporophytes experience a key physiological tipping point around 20°C. Additionally, our findings suggest that juvenile M. pyrifera from the Wellington population could be relatively resilient to MHWs if temperatures remain at sub-lethal levels. However, if average MHW intensities and durations continue to increase over time, survival and recruitment of juvenile kelp could be adversely affected, thus threatening the long-term persistence of giant kelp forests near the warm edge of their range in New Zealand.

Ocean acidification can negatively affect a broad range of physiological processes in marine shelled molluscs. Marine bioeroding organisms could, in contrast, benefit from ocean acidification due to reduced energetic costs of bioerosion. Ocean acidification could thus exacerbate negative effects (e.g. reduced growth) of ocean acidification and shell borers on oysters. The aim of this study was to assess the impact of ocean acidification on the oyster Ostrea chilensis, the boring sponge Cliona sp., and their host-parasite relationship. We exposed three sets of organisms 1) O. chilensis, 2) Cliona sp., and 3) O. chilensis infested with Cliona sp. to pHT 8.03, 7.83, and 7.63. Reduced pH had no significant effect on calcification, respiration and clearance rate of uninfested O. chilensis. Low pH significantly reduced calcification leading to net dissolution of oyster shells at pHT 7.63 in sponge infested oysters. Net dissolution was likely caused by increased bioerosion by Cliona sp. at pHT 7.63. Additionally, declining pH and sponge infestation had a significant negative antagonistic effect (less negative than predicted additively) on clearance rate. This interaction suggests that sponge infested oysters increase clearance rates to cope with higher energy demand of increased shell repair resulting from higher boring activity of Cliona sp. at low seawater pH. O. chilensis body condition was unaffected by sponge infestation, pH, and the interaction of the two. The reduction in calcification rate suggests sponge infestation and ocean acidification together would exacerbate direct (reduced growth) and indirect (e.g., increased predation) negative effects on oyster health and survival. Our results indicate that ocean acidification by the end of the century could have severe consequences for marine molluscs with boring organisms.

Marine kelp forests cover 1/3 of our world's coastlines, are heralded as a nature-based solution to address socio-environmental issues, connect hundreds of millions of people with the ocean, and support a rich web of biodiversity throughout our oceans. But they are increasingly threatened with some areas reporting over 90% declines in kelp forest cover in living memory. Despite their importance and the threats they face, kelp forests are entirely absent from the international conservation dialogue. No international laws, policies, or targets focus on kelp forests and very few countries consider them in their national policy. The Kelp Forest Challenge addresses that gap. Together with 252 kelp experts, professionals, and citizens from 25 countries, the Kelp Forest Challenge was developed as a grassroots vision of what the world can achieve for kelp forest conservation. It is a global call to restore 1 million and protect 3 million hectares of kelp forests by 2040. This is a monumental challenge, that will require coordination across multiple levels of society and the mobilization of immense resources. Pledges may therefore include area for protection or restoration, enabling pledges which assist in conservation (funding, equipment, professional expertise, capacity building), or awareness-based pledges which increase awareness or education about kelp forests. Correspondingly, participants may be from government, scientific institutions, private sector, NGOs, community groups, or individuals. This challenge is the beginning of a 17-year mission to save our kelp forests and anyone and any organisation is invited to participate.

Abstract. Warm-water coral reefs are facing unprecedented Anthropogenic driven threats to their continued existence as biodiverse, functional ecosystems upon which hundreds of millions of people rely. Determining the tipping point thresholds of coral reef ecosystems requires robust assessment of multiple stressors and their interactive effects. We draw upon a literature search and the recent Global Tipping Points Revision initiative to consider warm-water coral reef ecosystem tipping point threshold sensitivity. Considering observed and projected stressor impacts we recognise a global mean surface temperature (relative to pre-industrial) tipping point threshold of 1.2 °C (range 0.7–1.5 °C) and an atmospheric CO2 warming threshold of 350 ppm (range 326–400 ppm), whilst acknowledging that interacting stressors, ocean warming response time, overshoot and cascading impacts have yet to be sufficiently assessed but are likely to lower this threshold. These uncertainties around tipping point sensitivities for such a crucially important ecosystem underlines the imperative of robust assessment and, in the case of knowledge gaps, employing a precautionary principle favouring the lower range tipping point values.

Ocean acidification (OA) is predicted to cause profound shifts in many marine ecosystems by impairing the ability of calcareous taxa to calcify and grow and by influencing the physiology of many others. In both calcifying and non-calcifying taxa, ocean acidification could further impair the ability of marine life to regulate internal pH and thus metabolic function and/or behaviour. Identifying tipping points at which these effects will occur for different taxa due to the direct impacts of ocean acidification on organism physiology is difficult because they have not adequately been determined for most taxa nor for ecosystems at higher levels. This is due to the presence of both resistant and sensitive species within most taxa. However, calcifying taxa such as coralline algae, corals, molluscs, and sea urchins appear to be most sensitive to ocean acidification. Conversely, non-calcareous seaweeds, seagrasses, diatoms, cephalopods, and fish tend to be more resistant or even benefit from the direct effects of ocean acidification, though the effects of ocean acidification are more subtle for these taxa. While physiological tipping points of the effects of ocean acidification either do not exist or are not well defined, their direct effects on organism physiology will have flow-on indirect effects. These indirect effects will cause ecological tipping points in the future through changes in competition, herbivory, and predation. Evidence for indirect effects and ecological change is mostly taken from benthic ecosystems in warm temperate–tropical locations in situ that have elevated CO2. Species abundances at these locations indicate a shift away from calcifying taxa and towards non-calcareous taxa at high-CO2 concentrations. For example, lower abundance of corals and coralline algae and higher covers of non-calcareous macroalgae, often turfing species, are often found at elevated CO2. However, there are some locations where only minor changes or no detectable changes occur. Where ecological tipping points do occur, it is usually at locations with naturally elevated mean pCO2 concentrations of 500 µatm or more, which also corresponds to just under that concentration where the direct physiological impacts of ocean acidification are detectable in the most sensitive taxa in laboratory research (coralline algae and corals). Collectively, the available data support the concern that ocean acidification will most likely cause ecological change in the near future in most benthic marine ecosystems, with tipping points in some ecosystems as low as 500 µatm pCO2. However, further research is required to more adequately quantify and model the extent of these impacts in order to accurately project future marine ecosystem tipping points under ocean acidification.

The worldwide decline of coral reefs has renewed interest in coral communities at the edge of environmental limits because they have the potential to serve as resilience hotspots and climate change refugia, and can provide insights into how coral reefs might function in future ocean conditions. These coral communities are often referred to as marginal or extreme but few definitions exist and usage of these terms has therefore been inconsistent. This creates significant challenges for categorising these often poorly studied communities and synthesising data across locations. Furthermore, this impedes our understanding of how coral communities can persist at the edge of their environmental limits and the lessons they provide for future coral reef survival. Here, we propose that marginal and extreme coral communities are related but distinct and provide a novel conceptual framework to redefine them. Specifically, we define coral reef extremeness solely based on environmental conditions (i.e., large deviations from optimal conditions in terms of mean and/or variance) and marginality solely based on ecological criteria (i.e., altered community composition and/or ecosystem functioning). This joint but independent assessment of environmental and ecological criteria is critical to avoid common pitfalls where coral communities existing outside the presumed optimal conditions for coral reef development are automatically considered inferior to coral reefs in more traditional settings. We further evaluate the differential potential of marginal and extreme coral communities to serve as natural laboratories, resilience hotspots and climate change refugia, and discuss strategies for their conservation and management as well as priorities for future research. Our new classification framework provides an important tool to improve our understanding of how corals can persist at the edge of their environmental limits and how we can leverage this knowledge to optimise strategies for coral reef conservation, restoration and management in a rapidly changing ocean.

Coralline algae play crucial roles in near-shore ecosystems but are susceptible to ocean acidification (OA). It has been hypothesized that low water velocity, allowing localized photosynthesis-driven pH-increases in the coralline surface boundary layer, could buffer against the negative impacts of OA. To test how water motion affected the sensitivity of coralline algae to OA, coralline communities (from 2 m and 10 m depth) were grown for 220 days at two pH levels (present-day: pH 8.03, OA: pH 7.65) under differing inflow rates (400, 200 and 100 ml min(-1)) providing water velocities of 2.7, 5.9 and 7.8 cm s(-1). Communities from both depths were grown together, photographed to assess growth, and the resulting recruitment was evaluated at the experiment's conclusion. Low seawater pH reduced growth by c. 11% (highest flow), further decreased by >23% under the lowest flow. This reduction resulted in differential outcomes for the two depths, with skeletal net-dissolution under the combination of low flow and pH 7.65 for 10 m communities. Furthermore, there was a synergistic interaction between the effects of flow and pH, whereby the negative effect of OA strengthened under low flow, with recruitment halved at pH 7.65. This demonstrates that OA impacts can be modulated by the flow environment. Surprisingly, increased flow rates/water velocities reduced negative impacts of low pH, thus further challenging the notion that slow flow habitats offer protection from OA. The observed interactions between water flow and OA on early successional communities and their recruits may hold implications for the future of rocky reef systems dominated by these communities.

Split-plot experimental data are often analyzed as if the data came from a completely randomized design. As is well known, ignoring the different levels of randomization and replication can lead to serious inferential errors. However, in some experiments, including many of the ocean global change experiments that motivated this research, variation between whole-plot experimental units may be small relative to variation between subplot units. Even though a factorial analysis will often perform poorly in general, in this special case it outperforms a split-plot analysis, providing narrower confidence intervals for treatment means and differences with coverage rates close to the desired level. The performance of the proposed model-averaged analysis was compared to a classical split-plot analysis via a simulation study, and its utility demonstrated for an ocean global change experiment examining growth and condition of a juvenile mussel species. In our simulation study, model-averaged confidence intervals for whole-plot treatment means or comparisons of means were up to 40% narrower than split-plot confidence intervals while maintaining close to nominal coverage rates. In our example experiment, we observed narrowing of up to 25%. We recommend model averaging as a preferred approach when variation between whole-plot experimental units is expected to be less than between subplot units, with a few caveats for studies with very few replicates.

Understanding the drivers of net coral reef calcium carbonate production is increasingly important as ocean warming, acidification, and other anthropogenic stressors threaten the maintenance of coral reef structures and the services these ecosystems provide. Despite intense research effort on coral reef calcium carbonate production, the inclusion of a key reef forming/accreting calcifying group, the crustose coralline algae, remains challenging both from a theoretical and practical standpoint. While corals are typically the primary reef builders of contemporary reefs, crustose coralline algae can contribute equally. Here, we combine several sets of data with numerical and theoretical modelling to demonstrate that crustose coralline algae carbonate production can match or even exceed the contribution of corals to reef carbonate production. Despite their importance, crustose coralline algae are often inaccurately recorded in benthic surveys or even entirely missing from coral reef carbonate budgets. We outline several recommendations to improve the inclusion of crustose coralline algae into such carbonate budgets under the ongoing climate crisis.

Limnology and Oceanography BulletinVolume 32, Issue 1 p. 37-38 Meeting Highlights Developing a Multiple Driver Research Strategy Using Multiple Environmental Driver Design Lab for Experiments Christina M. McGraw, Christina M. McGraw orcid.org/0000-0001-5841-0748 Department of Chemistry and Coastal People: Southern Skies Centre of Research Excellence, University of Otago, Dunedin, New ZealandSearch for more papers by this authorErik C. Krieger, Erik C. Krieger orcid.org/0000-0002-3940-3785 School of Biological Sciences and Coastal People: Southern Skies Centre of Research Excellence, Victoria University of Wellington, Wellington, New ZealandSearch for more papers by this authorNatalie del Carmen Bravo Senmache, Natalie del Carmen Bravo Senmache Department of Fisheries Biology, Universidad Nacional Pedro Ruiz Gallo, Lambayeque, PeruSearch for more papers by this authorChristopher E. Cornwall, Christopher E. Cornwall orcid.org/0000-0002-6154-4082 School of Biological Sciences and Coastal People: Southern Skies Centre of Research Excellence, Victoria University of Wellington, Wellington, New ZealandSearch for more papers by this authorSam Dupont, Sam Dupont Department of Biological and Environmental Sciences, University of Gothenburg, Kristineberg, Sweden International Atomic Energy Agency Radioecology Laboratory, Monaco, MonacoSearch for more papers by this author Christina M. McGraw, Christina M. McGraw orcid.org/0000-0001-5841-0748 Department of Chemistry and Coastal People: Southern Skies Centre of Research Excellence, University of Otago, Dunedin, New ZealandSearch for more papers by this authorErik C. Krieger, Erik C. Krieger orcid.org/0000-0002-3940-3785 School of Biological Sciences and Coastal People: Southern Skies Centre of Research Excellence, Victoria University of Wellington, Wellington, New ZealandSearch for more papers by this authorNatalie del Carmen Bravo Senmache, Natalie del Carmen Bravo Senmache Department of Fisheries Biology, Universidad Nacional Pedro Ruiz Gallo, Lambayeque, PeruSearch for more papers by this authorChristopher E. Cornwall, Christopher E. Cornwall orcid.org/0000-0002-6154-4082 School of Biological Sciences and Coastal People: Southern Skies Centre of Research Excellence, Victoria University of Wellington, Wellington, New ZealandSearch for more papers by this authorSam Dupont, Sam Dupont Department of Biological and Environmental Sciences, University of Gothenburg, Kristineberg, Sweden International Atomic Energy Agency Radioecology Laboratory, Monaco, MonacoSearch for more papers by this author First published: 20 December 2022 https://doi.org/10.1002/lob.10535Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat No abstract is available for this article. References Boyd, P. W., and others. 2018. Experimental strategies to assess the biological ramifications of multiple drivers of global ocean change—A review. Global Change Biol. 24: 2239–2261. doi:10.1111/gcb.14102 10.1111/gcb.14102 PubMedWeb of Science®Google Scholar Boyd, P. W., and others. 2019. SCOR WG149 handbook to support the SCOR best practice guide for ‘multiple drivers’ marine research. University of Tasmania for Scientific Committee on Oceanic Research (SCOR). doi:10.25959/5c92fdf0d3c7a 10.25959/5c92fdf0d3c7a Google Scholar Volume32, Issue1February 2023Pages 37-38 ReferencesRelatedInformation