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Abstract Fjord systems in the Norwegian Arctic are experiencing an increasing frequency and magnitude of marine heatwaves. These episodic heat stress events can have varying degrees of acute impacts on primary production and nutrient uptake of mixed kelp communities, as well as modifying the biogeochemical cycling in nearshore systems where vast areas of kelp create structural habitat. To assess the impact of future marine heatwaves on kelp communities, we conducted a 23 day mesocosm experiment exposing mixed kelp communities to warming and heatwave scenarios projected for the year 2100. Three treatments were considered: a constant warming (+1.8°C from the control), a medium magnitude and long duration heatwave event (+2.8°C from the control for 13 days), and two short‐term, more intense, heatwaves(5 day long scenarios with temperature peaks at +3.9°C from the control). The results show that both marine heatwave treatments reduced net community production, whereas the constant warm temperature treatment displayed no difference from the control. The long marine heatwave scenario resulted in reduced accumulated net community production, indicating that prolonged exposure had a greater severity than two high magnitude, short‐term heatwave events. We estimated an 11°C temperature threshold at which negative effects to primary production appeared present. We highlight that marine heatwaves can induce sublethal effects on kelp communities by depressing net community production. These results are placed in the context of potential physiological resilience of kelp communities and implications of reduced net community production to future Arctic fjord environmental conditions.

The Mediterranean Sea is a hotspot of global change, particularly exposed to ocean warming and the increasing occurrence of marine heatwaves (MHWs). However, experiments based on long-term temperature data from the field are scarce. Here, we investigate the response of the zooxanthellate coral Cladocora caespitosa and the azooxanthellate coral Astroides calycularis to future warming and MHWs based on 8 years of in situ data. Corals were maintained in the laboratory for five months under four temperature conditions: Warming (3.2°C above the in situ mean from 2012 to 2020), Heatwave (temperatures of 2018 with two heatwaves), Ambient (in situ mean) and Cool (deeper water temperatures). Under the Warming treatment, some C. caespitosa colonies severely bleached and A. calycularis colonies presented necrosis. Cladocora caespitosa symbiosis was impaired by temperature with a decrease in the density of endosymbiotic algae and an increase in per cent whiteness in all the treatments except for the coolest. Recovery for both species was observed through different mechanisms such as regrowth of polyps of A. calycularis and recovery of pigmentation for C. caespitosa. These results suggest that A. calycularis and C. caespitosa may be resilient to heat stress and can recover from physiological stresses caused by heatwaves in the laboratory.

The global increase in anthropogenic CO2 is leading to ocean warming and acidification, which is threatening corals. In Ischia, Italy, two species of Mediterranean scleractinian corals-the symbiotic Cladocora caespitosa and the asymbiotic Astroides calycularis-were collected from ambient pH sites (average pHT = 8.05) and adjacent CO2 vent sites (average pHT = 7.8) to evaluate their response to ocean acidification. Coral colonies from both sites were reared in a laboratory setting for six months at present day pH (pHT ~ 8.08) or low pH (pHT ~7.72). Previous work showed that these corals were tolerant of low pH and maintained positive calcification rates throughout the experiment. We hypothesized that these corals cope with low pH by increasing their heterotrophic capacity (i.e., feeding and/or proportion of heterotrophically derived compounds incorporated in their tissues), irrespective of site of origin, which was quantified indirectly by measuring δ13C, δ15N, and sterols. To further characterize coral health, we quantified energy reserves by measuring biomass, total lipids, and lipid classes. Additional analysis for C. caespitosa included carbohydrates (an energy reserve) and chlorophyll a (an indicator of photosynthetic capacity). Isotopic evidence shows that ambient-sourced Mediterranean corals, of both species, decreased heterotrophy in response to six months of low pH. Despite maintaining energy reserves, lower net photosynthesis (C. caespitosa) and a trend of declining calcification (A. calycularis) suggest a long-term cost to low heterotrophy under ocean acidification conditions. Conversely, vent-sourced corals maintained moderate (C. caespitosa) or high (A. calycularis) heterotrophic capacity and increased photosynthesis rates (C. caespitosa) in response to six months at low pH, allowing them to sustain themselves physiologically. Provided there is sufficient zooplankton and/or organic matter to meet their heterotrophic needs, vent-sourced corals are more likely to persist this century and potentially be a source for new corals in the Mediterranean.

Crustose coralline algae can contribute equal or even larger amounts of calcium carbonate to coral reefs than corals, yet their contribution is often underrepresented or omitted in coral reef carbonate budgets, according to a conceptual model developed from a meta-analysis of calcification rates and a case study in French Polynesia.

Abstract Marine forests are shrinking globally due to several anthropogenic impacts including climate change. Forest-forming macroalgae, such as Cystoseira s.l. species, can be particularly sensitive to environmental conditions (e.g. temperature increase, pollution or sedimentation), especially during early life stages. However, not much is known about their response to the interactive effects of ocean warming (OW) and acidification (OA). These drivers can also affect the performance and survival of crustose coralline algae, which are associated understory species likely playing a role in the recruitment of later successional species such as forest-forming macroalgae. We tested the interactive effects of elevated temperature, low pH and species facilitation on the recruitment of Cystoseira compressa. We demonstrate that the interactive effects of OW and OA negatively affect the recruitment of C. compressa and its associated coralline algae Neogoniolithon brassica-florida. The density of recruits was lower under the combinations OW and OA, while the size was negatively affected by the temperature increase but positively affected by the low pH. The results from this study show that the interactive effects of climate change and the presence of crustose coralline algae can have a negative impact on the recruitment of Cystoseira s.l. species. While new restoration techniques recently opened the door to marine forest restoration, our results show that the interactions of multiple drivers and species interactions have to be considered to achieve long-term population sustainability.

Grazing is one of the most important biological factors controlling the abundance of early-life stages of fucoids and one of the major issues when restoring marine forests. Benthic macroinvertebrates (e.g., sea urchins) and fish shape and regulate benthic macroalgal communities from polar to tropical regions and can be responsible for regime shifts leading to the predominance of turfs and/or barren grounds. However, other herbivores (i.e., mesograzers) could also significantly participate in the grazing, especially on early-life stages, hampering the persistence and capacity of Cystoseira sensu lato populations to recover after major disturbances and being a cause of failure of restoration actions. We performed experiments in the field and in mesocosm in order to investigate the herbivory pressure and the effects of different grazers on recruits of Cystoseira compressa. The results highlight that non-strict herbivorous invertebrates, such as Clibanarius erythropus, Cerithium vulgatum, and Idotea balthica, graze on recruits of Cystoseira s.l. spp., with I. balthica showing the highest consumption rate. We concluded that biotic factors such as herbivory, which affect key life stages, can be crucial for the conservation of Cystoseira s.l. forests and need to be better understood and considered on a case-by-case basis when planning restoration actions.

Corals show a heightened sensitivity to a combination of light and thermal stress after prolonged exposure to high ocean temperatures, according to four months-long mesocosm experiments that disentangle the effects of light and ocean warming.

In corals, pH regulation of the extracellular calcifying medium (ECM) by the calcifying cell layer is a crucial step in the calcification process and is potentially important to influencing how corals respond to ocean acidification. Here, we analyzed the growing edge of the reef coral Stylophora pistillata to make the first characterization of the proton gradient across the coral calcifying epithelium. At seawater pH 8 we found that while the calcifying epithelium elevates pH in the ECM on its apical side above that of seawater, pH on its basal side in the mesoglea is markedly lower, highlighting that the calcifying cells are exposed to a microenvironment distinct from the external environment. Coral symbiont photosynthesis elevates pH in the mesoglea, but experimental ocean acidification and decreased seawater inorganic carbon concentration lead to large declines in mesoglea pH relative to the ECM, which is maintained relatively stable. Together, our results indicate that the coral calcifying epithelium is functionally polarized and that environmental variation impacts pHECM regulation through its effects on the basal side of the calcifying cells.

Ocean acidification (OA) is a serious consequence of climate change with complex organism-to-ecosystem effects that have been observed through field observations but are mainly derived from experimental studies. Although OA trends and the resulting biological impacts are likely exacerbated in the semi-enclosed and highly populated Mediterranean Sea, some fundamental knowledge gaps still exist. These gaps are at tributed to both the uneven capacity for OA research that exists between Mediterranean countries, as well as to the subtle and long-term biological, physical and chemical interactions that define OA impacts. In this paper, we systematically analyzed the different aspects of OA research in the Mediterranean region based on two sources: the United Nation’s International Atomic Energy Agency’s (IAEA) Ocean Acidification International Coordination Center (OA-ICC) database, and an extensive survey. Our analysis shows that 1) there is an uneven geographic capacity in OA research, and illustrates that both the Algero-Provencal and Ionian sub-basins are currently the least studied Mediterranean areas, 2) the carbonate system is still poorly quantified in coastal zones, and long-term time-series are still sparse across the Mediterranean Sea, which is a challenge for studying its variability and assessing coastal OA trends, 3) the most studied groups of organisms are autotrophs (algae, phanerogams, phytoplankton), mollusks, and corals, while microbes, small mollusks (mainly pteropods), and sponges are among the least studied, 4) there is an overall paucity in socio-economic, paleontological, and modeling studies in the Mediterranean Sea, and 5) in spite of general resource availability and the agreement for improved and coordinated OA governance, there is a lack of consistent OA policies in the Mediterranean Sea. In addition to highlighting the current status, trends and gaps of OA research, this work also provides recommendations, based on both our literature assessment and a survey that targeted the Mediterranean OA scientific community. In light of the ongoing 2021-2030 United Nations Decade of Ocean Science for Sustainable Development, this work might provide a guideline to close gaps of knowledge in the Mediterranean OA research.Systematic Review Registrationhttps://www.oceandecade.org/

Coralline algae are foundation species in many hard-bottom ecosystems acting as a settlement substrate, and binding together and even creating reefs in some locations. Ocean acidification is known to be a major threat to coralline algae. However, the effects of ocean warming are less certain. Here we bring multiple lines of evidence together to discuss the potential impacts of ocean warming on these ecologically crucial taxa. We use a meta-analysis of 40 responses within 14 different studies available which assessed the effects of increasing temperature on coralline algal calcification in laboratory experiments. We find a net negative impact of increasing temperature on coralline algal calcification at 5.2°C above ambient conditions. Conversely, negative effects are observed when temperature drops below 2.0°C from ambient conditions. We propose that some coralline algae will be more capable of both acclimatizing and locally adapting to increasing ocean temperatures over the coming decades. This is because many species possess short generation times, the ability to opportunistically rapidly utilize open space, and relatively high phenotypic plasticity. However, less resistant and resilient species will be those that are long-lived, those with long generation times, or with narrow thermal tolerances (e.g., tropical taxa living close to their thermal maxima). Additionally, ocean warming will occur simultaneously with ocean acidification, a potentially greater threat to coralline algae, which could also reduce any tolerance to ocean warming for many species. To maximize the potential to accurately determine how coralline algae will respond to future ocean warming and marine heatwaves, future research should use environmentally relevant temperature treatments, use appropriate acclimation times and follow best practices in experimental design.

Current research on coral reefs seeks to link the responses to anthropogenic stressors (such as global warming and ocean acidification [OA]) among differing functional levels of biological organization. While experimental studies have identified ex situ taxon-specific responses to OA and global warming, isolating and connecting these effects in situ at the community-level has proved difficult. The difficulties arise from the large number of naturally varying parameters affecting corals reefs, such as light intensity and seawater residence time that affect net community production and calcification. To control variation in seawater residence time and allow light intensity to vary naturally, experimental outer reef (17-m depth) benthic communities composed of calcified algae, corals, and reef pavement were constructed in large outdoor flumes in Mo'orea, French Polynesia. Net community production (P), net community calcification (G), the ratio of P/G (P/Gratio), and slope of P regressed on G (P/Gslope) were calculated for the communities, and concurrently for the constituent members under the same temperature, light, and flow conditions. P and G, for both the communities and constituent members, were correlated positively with light intensity, whereas P/Gratio and P/Gslope were unaffected by light intensity. P/Gratios and P/Gslopes exhibited values that were specific to each community member. These results suggest that the P/Gratio and P/Gslope may be unaffected by natural variability in light intensity and could serve as useful metrics to relate responses at the taxon and community level, which is an important step in assessing the effects of environmental changes on coral reefs.

The Moorea Coral Reef Long Term Ecological Research program affords a unique opportunity to study the implications of ocean acidification (OA) for coral reefs, as ongoing ecological and physical monitoring there provides a context in which the patterns of community change attributed to OA can be detected. We used mesocosms to study the impacts of OA on corals and calcified algae, and conducted experiments to compare multiple species and evaluate how they are affected by changing concentrations of the different forms of dissolved inorganic carbon. Our results reveal taxonomic variation in the response to OA, with some corals and algae showing signs of resistance to OA conditions. This discovery is informing an urgent debate over the form in which coral reefs will survive (if at all) in the future.

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 CO(2) 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 pCO(2) levels of 280, 380, 550, 760 and 1020 microatm and at control (0 degrees C) and elevated (4 degrees C) temperatures. The respiration rate was unaffected by pCO(2) at control temperature, but significantly increased as a function of the pCO(2) level at elevated temperature. pCO(2) had no effect on the gut clearance rate at either temperature. Precipitation of CaCO(3), measured as the incorporation of (45)Ca, significantly declined as a function of pCO(2) 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.

Thecosome pteropods play a key role in the food web of various marine ecosystems and they calcify, secreting the unstable CaCO3 mineral aragonite to form their shell material. Here, we have estimated the effect of ocean acidification on pteropod calcification by exploiting empirical relationships between their gross calcification rates (CaCO3 precipitation) and aragonite saturation state Ωa, combined with model projections of future Ωa. These were corrected for modern model-data bias and taken over the depth range where pteropods are observed to migrate vertically. Results indicate large reductions in gross calcification at temperate and high latitudes. Over much of the Arctic, the pteropod Limacina helicina will become unable to precipitate CaCO3 by the end of the century under the IPCC SRES A2 scenario. These results emphasize concerns over the future of shelled pteropods, particularly L. helicina in high latitudes. Shell-less L. helicina are not known to have ever existed nor would we expect them to survive. Declines of pteropod populations could drive dramatic ecological changes in the various pelagic ecosystems in which they play a critical role. [ABSTRACT FROM AUTHOR]