Anthropogenic noise disrupts acoustic cues for recruitment.

Anthropogenic noise is rising and may interfere with natural acoustic cues used by organisms to recruit. Newly developed acoustic technology provides enriched settlement cues to boost recruitment of target organisms navigating to restoration sites, but can it boost recruitment in noise-polluted sites? To address this dilemma, we coupled replicated aquarium experiments with field experiments. Under controlled and replicated laboratory conditions, acoustic enrichment boosted recruitment by 2.57 times in the absence of anthropogenic noise, but yielded comparable recruitment in its presence (i.e. no boosting effect). Using the same technique, we then tested the replicability of these responses in real-world settings where independently replicated 'sites' are unfeasible owing to the inherent differences in soundscapes. Again, acoustic enrichment increased recruitment where anthropogenic noise was low (by 3.33 times), but had no effect at a site of noise pollution. Together, these coupled laboratory-to-field outcomes indicate that anthropogenic noise can mask the signal of acoustic enrichment. While noise pollution may reduce the effectiveness of acoustic enrichment, some of our reported observations suggest that anthropogenic noise per se might also provide an attractive cue for oyster larvae to recruit. These findings underscore the complexity of larval behavioural responses to acoustic stimuli during recruitment processes.

Ocean acidification boosts reproduction in fish via indirect effects.

Ocean acidification affects species populations and biodiversity through direct negative effects on physiology and behaviour. The indirect effects of elevated CO2 are less well known and can sometimes be counterintuitive. Reproduction lies at the crux of species population replenishment, but we do not know how ocean acidification affects reproduction in the wild. Here, we use natural CO2 vents at a temperate rocky reef and show that even though ocean acidification acts as a direct stressor, it can indirectly increase energy budgets of fish to stimulate reproduction at no cost to physiological homeostasis. Female fish maintained energy levels by compensation: They reduced activity (foraging and aggression) to increase reproduction. In male fish, increased reproductive investment was linked to increased energy intake as mediated by intensified foraging on more abundant prey. Greater biomass of prey at the vents was linked to greater biomass of algae, as mediated by a fertilisation effect of elevated CO2 on primary production. Additionally, the abundance and aggression of paternal carers were elevated at the CO2 vents, which may further boost reproductive success. These positive indirect effects of elevated CO2 were only observed for the species of fish that was generalistic and competitively dominant, but not for 3 species of subordinate and more specialised fishes. Hence, species that capitalise on future resource enrichment can accelerate their reproduction and increase their populations, thereby altering species communities in a future ocean.

Is Ocean Acidification Really a Threat to Marine Calcifiers? A Systematic Review and Meta-Analysis of 980+ Studies Spanning Two Decades.

Ocean acidification is considered detrimental to marine calcifiers, but mounting contradictory evidence suggests a need to revisit this concept. This systematic review and meta-analysis aim to critically re-evaluate the prevailing paradigm of negative effects of ocean acidification on calcifiers. Based on 5153 observations from 985 studies, many calcifiers (e.g., echinoderms, crustaceans, and cephalopods) are found to be tolerant to near-future ocean acidification (pH ≈ 7.8 by the year 2100), but coccolithophores, calcifying algae, and corals appear to be sensitive. Calcifiers are generally more sensitive at the larval stage than adult stage. Over 70% of the observations in growth and calcification are non-negative, implying the acclimation capacity of many calcifiers to ocean acidification. This capacity can be mediated by phenotypic plasticity (e.g., physiological, mineralogical, structural, and molecular adjustments), transgenerational plasticity, increased food availability, or species interactions. The results suggest that the impacts of ocean acidification on calcifiers are less deleterious than initially thought as their adaptability has been underestimated. Therefore, in the forthcoming era of ocean acidification research, it is advocated that studying how marine organisms persist is as important as studying how they perish, and that future hypotheses and experimental designs are not constrained within the paradigm of negative effects.

Ocean acidification drives global reshuffling of ecological communities.

The paradigm that climate change will alter global marine biodiversity is one of the most widely accepted. Yet, its predictions remain difficult to test because laboratory systems are inadequate at incorporating ecological complexity, and common biodiversity metrics have varying sensitivity to detect change. Here, we test for the prevalence of global responses in biodiversity and community-level change to future climate (acidification and warming) from studies at volcanic CO2 vents across four major global coastal ecosystems and studies in laboratory mesocosms. We detected globally replicable patterns of species replacements and community reshuffling under ocean acidification in major natural ecosystems, yet species diversity and other common biodiversity metrics were often insensitive to detect such community change, even under significant habitat loss. Where there was a lack of consistent patterns of biodiversity change, these were a function of similar numbers of studies observing negative versus positive species responses to climate stress. Laboratory studies showed weaker sensitivity to detect species replacements and community reshuffling in general. We conclude that common biodiversity metrics can be insensitive in revealing the anticipated effects of climate stress on biodiversity-even under significant biogenic habitat loss-and can mask widespread reshuffling of ecological communities in a future ocean. Although the influence of ocean acidification on community restructuring can be less evident than species loss, such changes can drive the dynamics of ecosystem stability or their functional change. Importantly, species identity matters, representing a substantial influence of future oceans.

Shark teeth can resist ocean acidification.

Ocean acidification can cause dissolution of calcium carbonate minerals in biological structures of many marine organisms, which can be exacerbated by warming. However, it is still unclear whether this also affects organisms that have body parts made of calcium phosphate minerals (e.g. shark teeth), which may also be impacted by the 'corrosive' effect of acidified seawater. Thus, we examined the effect of ocean acidification and warming on the mechanical properties of shark teeth (Port Jackson shark, Heterodontus portusjacksoni), and assessed whether their mineralogical properties can be modified in response to predicted near-future seawater pH (-0.3 units) and temperature (+3°C) changes. We found that warming resulted in the production of more brittle teeth (higher elastic modulus and lower mechanical resilience) that were more vulnerable to physical damage. Yet, when combined with ocean acidification, the durability of teeth increased (i.e. less prone to physical damage due to the production of more elastic teeth) so that they did not differ from those raised under ambient conditions. The teeth were chiefly made of fluorapatite (Ca5 (PO4 )3 F), with increased fluoride content under ocean acidification that was associated with increased crystallinity. The increased precipitation of this highly insoluble mineral under ocean acidification suggests that the sharks could modulate and enhance biomineralization to produce teeth which are more resistant to corrosion. This adaptive mineralogical adjustment could allow some shark species to maintain durability and functionality of their teeth, which underpins a fundamental component of predation and sustenance of the trophic dynamics of future oceans.

Turning a lost reef ecosystem into a national restoration program.

Achieving a sustainable socioecological future now requires large-scale environmental repair across legislative borders. Yet, enabling large-scale conservation is complicated by policy-making processes that are disconnected from socioeconomic interests, multiple sources of knowledge, and differing applications of policy. We considered how a multidisciplinary approach to marine habitat restoration generated the scientific evidence base, community support, and funding needed to begin the restoration of a forgotten, functionally extinct shellfish reef ecosystem. The key actors came together as a multidisciplinary community of researchers, conservation practitioners, recreational fisher communities, and government bodies that collaborated across sectors to rediscover Australia's lost shellfish reefs and communicate the value of its restoration. Actions undertaken to build a case for large-scale marine restoration included synthesizing current knowledge on Australian shellfish reefs and their historical decline, using this history to tell a compelling story to spark public and political interest, integrating restoration into government policy, and rallying local support through community engagement. Clearly articulating the social, economic, and environmental business case for restoration led to state and national funding for reef restoration to meet diverse sustainability goals (e.g., enhanced biodiversity and fisheries productivity) and socioeconomic goals (e.g., job creation and recreational opportunities). A key lesson learned was the importance of aligning project goals with public and industry interests so that projects could address multiple political obligations. This process culminated in Australia's largest marine restoration initiative and shows that solutions for large-scale ecosystem repair can rapidly occur when socially valued science acts on political opportunities.

Transformación de un Ecosistema Arrecifal Perdido en un Programa Nacional de Restauración Resumen Actualmente se requiere una reparación ambiental a gran escala que atraviese fronteras legislativas para lograr un futuro socio-ecológico sustentable. Aun así, habilitar la conservación a gran escala es complicado debido a los procesos de elaboración de políticas que están desconectadas de los intereses socio-económicos, las múltiples fuentes de conocimiento y las diferentes aplicaciones de las políticas. Consideramos cómo una estrategia multidisciplinaria para la restauración de hábitats marinos generó una base de evidencia científica, apoyo comunitario y el financiamiento necesario para así iniciar la restauración de un ecosistema arrecifal de conchas funcionalmente extinto. Los actores clave formaron una comunidad multidisciplinaria de investigadores, practicantes de la conservación, comunidades de pescadores recreativos y órganos gubernamentales que colaboró con varios sectores para redescubrir los arrecifes perdidos de Australia y comunicó el valor de su restauración. Las acciones realizadas para armar el caso para la restauración marina a gran escala incluyeron la síntesis del conocimiento actual sobre los arrecifes de conchas en Australia y su declinación histórica, el uso de esta historia para contar una narración convincente que active el interés público y político, la integración de la restauración a la política gubernamental y la movilización del apoyo local por medio de la participación comunitaria. Claramente, la articulación del caso del negocio social, económico y ambiental para la restauración llevó al financiamiento estatal y nacional para la restauración arrecifal a cumplir diversos objetivos socio-económicos (p. ej.: creación de empleos, oportunidades recreativas) y de restauración (p. ej.: una productividad realzada de la biodiversidad y las pesquerías). Una lección clave que aprendimos fue lo importante que es alinear los objetivos del proyecto con los intereses públicos y de la industria, de tal manera que los proyectos aborden las múltiples obligaciones políticas. Este proceso culminó con la iniciativa de restauración marina más grande en Australia y demuestra que las soluciones para la reparación de los ecosistemas a gran escala pueden ocurrir rápidamente cuando la ciencia con valor social actúa sobre las oportunidades políticas.

Positive species interactions strengthen in a high-CO2 ocean.

Negative interactions among species are a major force shaping natural communities and are predicted to strengthen as climate change intensifies. Similarly, positive interactions are anticipated to intensify and could buffer the consequences of climate-driven disturbances. We used in situ experiments at volcanic CO2 vents within a temperate rocky reef to show that ocean acidification can drive community reorganization through indirect and direct positive pathways. A keystone species, the algal-farming damselfish Parma alboscapularis, enhanced primary productivity through its weeding of algae whose productivity was also boosted by elevated CO2. The accelerated primary productivity was associated with increased densities of primary consumers (herbivorous invertebrates), which indirectly supported increased secondary consumers densities (predatory fish) (i.e. strengthening of bottom-up fuelling). However, this keystone species also reduced predatory fish densities through behavioural interference, releasing invertebrate prey from predation pressure and enabling a further boost in prey densities (i.e. weakening of top-down control). We uncover a novel mechanism where a keystone herbivore mediates bottom-up and top-down processes simultaneously to boost populations of a coexisting herbivore, resulting in altered food web interactions and predator populations under future ocean acidification.

Repairing recruitment processes with sound technology to accelerate habitat restoration.

Humanity's ambitions to revive ecosystems at large scales require solutions to move restoration efforts beyond the small scale. There are increasing calls for technological solutions to reduce costs and facilitate large-scale restoration through the use of emerging technologies using an adaptive process of research and development. We show how technological enrichment of marine soundscapes may provide a solution that repairs the recruitment process to accelerate the recovery of lost marine habitats. This solution would solve the problems of current practice that largely relies upon natural recruitment processes, which carries considerable risk where recruitment is variable or eroded. By combining the literature with laboratory experiments, we describe evidence for "highways of sound" that convey navigable information for dispersing life stages in search for adult habitat. We show that these navigational cues tend to be silenced as their habitat is lost, creating negative feedbacks that hinders restoration. We suggest that reprovisioning soundscapes using underwater technology offers the potential to reverse this feedback and entice target organisms to recruit in greater densities. Collective evidence indicates that the application of soundscape theory and technology may unlock the recruitment potential needed to trigger the recruitment of target organisms and the natural soundscapes they create at large scales.

Calcifiers can Adjust Shell Building at the Nanoscale to Resist Ocean Acidification.

Ocean acidification is considered detrimental to marine calcifiers based on laboratory studies showing that increased seawater acidity weakens their ability to build calcareous shells needed for growth and protection. In the natural environment, however, the effects of ocean acidification are subject to ecological and evolutionary processes that may allow calcifiers to buffer or reverse these short-term negative effects through adaptive mechanisms. Using marine snails inhabiting a naturally CO2 -enriched environment over multiple generations, it is discovered herein that they build more durable shells (i.e., mechanically more resilient) by adjusting the building blocks of their shells (i.e., calcium carbonate crystals), such as atomic rearrangement to reduce nanotwin thickness and increased incorporation of organic matter. However, these adaptive adjustments to future levels of ocean acidification (year 2100) are eroded at extreme CO2 concentrations, leading to construction of more fragile shells. The discovery of adaptive mechanisms of shell building at the nanoscale provides a new perspective on why some calcifiers may thrive and others collapse in acidifying oceans, and highlights the inherent adaptability that some species possess in adjusting to human-caused environmental change.

Environmental solutions sparked by environmental history.

Environmental solutions require a decision-making process that is ultimately political, in that they involve decisions with uncertain outcomes and stakeholders with conflicting viewpoints. If this process seeks broad alignment between the government and public, then reconciling conflicting viewpoints is a key to the legitimacy of these decisions. We show that ecological baselines can be particularly powerful tools for creating a common understanding for public support (legitimacy) and conformity to new rules or regulations (legality) that enable the solution. They are powerful because they move the discussion of solutions from the abstract to the concrete by providing a conceptual model for a common expectation (e.g., restoring habitat). They provide narratives of the past (ecological histories) that readjust the future expectations of individuals on how to perceive and respond to new policy. While ecological baselines offer scientists benchmarks for reinstating ecological functions, they also normalize public and government discussion of solutions. This social normalization of public issues may assist government policy and influence social views, practices, and behaviors that adopt the policy. For science to more effectively inform conservation, we encourage interdisciplinary thinking (science- and human-centered) because it can provide public support and government legitimacy for investing in environmental solutions.

Article impact statement: Ecological history provides legitimacy and confidence for government to collaborate on finding new policies for environmental conservation. Soluciones Ambientales Detonadas por la Historia Ambiental Resumen Las soluciones ambientales requieren un proceso de toma de decisiones que es básicamente político ya que involucra decisiones con resultados inciertos y actores con puntos de vista discrepantes. Si este proceso busca una alineación sólida entre el gobierno y el público, entonces la reconciliación de los puntos de vista discrepantes es de suma importancia para la legitimidad de estas decisiones. Demostramos que las líneas base ecológicas pueden ser herramientas particularmente poderosas para la creación de un entendimiento común para el apoyo público (legitimidad) y para crear una conformidad con las nuevas reglas o regulaciones (legalidad) que posibilitan las soluciones ambientales. Las líneas base ecológicas son poderosas porque trasladan la discusión sobre las soluciones desde lo abstracto hasta lo concreto al proporcionar un modelo conceptual para la expectativa común (p. ej.: la restauración de hábitats). Las líneas base ecológicas también proporcionan narrativas sobre el pasado (historias ecológicas) que sirven para reajustar las expectativas que tienen los individuos sobre cómo percibir y responder ante políticas nuevas. Mientras que las líneas base ecológicas les ofrecen a los científicos puntos de referencia para la reinstauración de las funciones ecológicas, también normalizan la discusión pública y gubernamental sobre las soluciones ambientales. Esta normalización social de los asuntos públicos puede ayudar con las políticas gubernamentales y puede influir sobre la opinión, las prácticas y los comportamientos sociales que adopten a las políticas. Para que la ciencia informe con mayor eficiencia a la conservación, incentivamos el pensamiento multidisciplinario (centrado en la ciencia y centrado en el humano) porque puede proporcionarles el apoyo público y la legitimidad gubernamental a las soluciones ambientales.

How calorie-rich food could help marine calcifiers in a CO2-rich future.

Increasing carbon emissions not only enrich oceans with CO2 but also make them more acidic. This acidifying process has caused considerable concern because laboratory studies show that ocean acidification impairs calcification (or shell building) and survival of calcifiers by the end of this century. Whether this impairment in shell building also occurs in natural communities remains largely unexplored, but requires re-examination because of the recent counterintuitive finding that populations of calcifiers can be boosted by CO2 enrichment. Using natural CO2 vents, we found that ocean acidification resulted in the production of thicker, more crystalline and more mechanically resilient shells of a herbivorous gastropod, which was associated with the consumption of energy-enriched food (i.e. algae). This discovery suggests that boosted energy transfer may not only compensate for the energetic burden of ocean acidification but also enable calcifiers to build energetically costly shells that are robust to acidified conditions. We unlock a possible mechanism underlying the persistence of calcifiers in acidifying oceans.

A triple trophic boost: How carbon emissions indirectly change a marine food chain.

The pervasive enrichment of CO2 in our oceans is a well-documented stressor to marine life. Yet, there is little understanding about how CO2 affects species indirectly in naturally complex communities. Using natural CO2 vents, we investigated the indirect effects of CO2 enrichment through a marine food chain. We show how CO2 boosted the biomass of three trophic levels: from the primary producers (algae), through to their grazers (gastropods), and finally through to their predators (fish). We also found that consumption by both grazers and predators intensified under CO2 enrichment, but, ultimately, this top-down control failed to compensate for the boosted biomass of both primary producers and herbivores (bottom-up control). Our study suggests that indirect effects can buffer the ubiquitous and direct, negative effects of CO2 enrichment by allowing the upward propagation of resources through the food chain. Maintaining the natural complexity of food webs in our ocean communities could, therefore, help minimize the future impacts of CO2 enrichment.

Global patterns of kelp forest change over the past half-century.

Kelp forests (Order Laminariales) form key biogenic habitats in coastal regions of temperate and Arctic seas worldwide, providing ecosystem services valued in the range of billions of dollars annually. Although local evidence suggests that kelp forests are increasingly threatened by a variety of stressors, no comprehensive global analysis of change in kelp abundances currently exists. Here, we build and analyze a global database of kelp time series spanning the past half-century to assess regional and global trends in kelp abundances. We detected a high degree of geographic variation in trends, with regional variability in the direction and magnitude of change far exceeding a small global average decline (instantaneous rate of change = -0.018 y-1). Our analysis identified declines in 38% of ecoregions for which there are data (-0.015 to -0.18 y-1), increases in 27% of ecoregions (0.015 to 0.11 y-1), and no detectable change in 35% of ecoregions. These spatially variable trajectories reflected regional differences in the drivers of change, uncertainty in some regions owing to poor spatial and temporal data coverage, and the dynamic nature of kelp populations. We conclude that although global drivers could be affecting kelp forests at multiple scales, local stressors and regional variation in the effects of these drivers dominate kelp dynamics, in contrast to many other marine and terrestrial foundation species.

The duality of ocean acidification as a resource and a stressor.

Ecologically dominant species often define ecosystem states, but as human disturbances intensify, their subordinate counterparts increasingly displace them. We consider the duality of disturbance by examining how environmental drivers can simultaneously act as a stressor to dominant species and as a resource to subordinates. Using a model ecosystem, we demonstrate that CO2 -driven interactions between species can account for such reversals in dominance; i.e., the displacement of dominants (kelp forests) by subordinates (turf algae). We established that CO2 enrichment had a direct positive effect on productivity of turfs, but a negligible effect on kelp. CO2 enrichment further suppressed the abundance and feeding rate of the primary grazer of turfs (sea urchins), but had an opposite effect on the minor grazer (gastropods). Thus, boosted production of subordinate producers, exacerbated by a net reduction in its consumption by primary grazers, accounts for community change (i.e., turf displacing kelp). Ecosystem collapse, therefore, is more likely when resource enrichment alters competitive dominance of producers, and consumers fail to compensate. By recognizing such duality in the responses of interacting species to disturbance, which may stabilize or exacerbate change, we can begin to understand how intensifying human disturbances determine whether or not ecosystems undergo phase shifts.

Global alteration of ocean ecosystem functioning due to increasing human CO2 emissions.

Rising anthropogenic CO2 emissions are anticipated to drive change to ocean ecosystems, but a conceptualization of biological change derived from quantitative analyses is lacking. Derived from multiple ecosystems and latitudes, our metaanalysis of 632 published experiments quantified the direction and magnitude of ecological change resulting from ocean acidification and warming to conceptualize broadly based change. Primary production by temperate noncalcifying plankton increases with elevated temperature and CO2, whereas tropical plankton decreases productivity because of acidification. Temperature increases consumption by and metabolic rates of herbivores, but this response does not translate into greater secondary production, which instead decreases with acidification in calcifying and noncalcifying species. This effect creates a mismatch with carnivores whose metabolic and foraging costs increase with temperature. Species diversity and abundances of tropical as well as temperate species decline with acidification, with shifts favoring novel community compositions dominated by noncalcifiers and microorganisms. Both warming and acidification instigate reduced calcification in tropical and temperate reef-building species. Acidification leads to a decline in dimethylsulfide production by ocean plankton, which as a climate gas, contributes to cloud formation and maintenance of the Earth's heat budget. Analysis of responses in short- and long-term experiments and of studies at natural CO2 vents reveals little evidence of acclimation to acidification or temperature changes, except for microbes. This conceptualization of change across whole communities and their trophic linkages forecast a reduction in diversity and abundances of various key species that underpin current functioning of marine ecosystems.

Boosted food web productivity through ocean acidification collapses under warming.

Future climate is forecast to drive bottom-up (resource driven) and top-down (consumer driven) change to food web dynamics and community structure. Yet, our predictive understanding of these changes is hampered by an over-reliance on simplified laboratory systems centred on single trophic levels. Using a large mesocosm experiment, we reveal how future ocean acidification and warming modify trophic linkages across a three-level food web: that is, primary (algae), secondary (herbivorous invertebrates) and tertiary (predatory fish) producers. Both elevated CO2 and elevated temperature boosted primary production. Under elevated CO2 , the enhanced bottom-up forcing propagated through all trophic levels. Elevated temperature, however, negated the benefits of elevated CO2 by stalling secondary production. This imbalance caused secondary producer populations to decline as elevated temperature drove predators to consume their prey more rapidly in the face of higher metabolic demand. Our findings demonstrate how anthropogenic CO2 can function as a resource that boosts productivity throughout food webs, and how warming can reverse this effect by acting as a stressor to trophic interactions. Understanding the shifting balance between the propagation of resource enrichment and its consumption across trophic levels provides a predictive understanding of future dynamics of stability and collapse in food webs and fisheries production.

Climate-driven disparities among ecological interactions threaten kelp forest persistence.

The combination of ocean warming and acidification brings an uncertain future to kelp forests that occupy the warmest parts of their range. These forests are not only subject to the direct negative effects of ocean climate change, but also to a combination of unknown indirect effects associated with changing ecological landscapes. Here, we used mesocosm experiments to test the direct effects of ocean warming and acidification on kelp biomass and photosynthetic health, as well as climate-driven disparities in indirect effects involving key consumers (urchins and rock lobsters) and competitors (algal turf). Elevated water temperature directly reduced kelp biomass, while their turf-forming competitors expanded in response to ocean acidification and declining kelp canopy. Elevated temperatures also increased growth of urchins and, concurrently, the rate at which they thinned kelp canopy. Rock lobsters, which are renowned for keeping urchin populations in check, indirectly intensified negative pressures on kelp by reducing their consumption of urchins in response to elevated temperature. Overall, these results suggest that kelp forests situated towards the low-latitude margins of their distribution will need to adapt to ocean warming in order to persist in the future. What is less certain is how such adaptation in kelps can occur in the face of intensifying consumptive (via ocean warming) and competitive (via ocean acidification) pressures that affect key ecological interactions associated with their persistence. If such indirect effects counter adaptation to changing climate, they may erode the stability of kelp forests and increase the probability of regime shifts from complex habitat-forming species to more simple habitats dominated by algal turfs.

Testing for thresholds of ecosystem collapse in seagrass meadows.

Although the public desire for healthy environments is clear-cut, the science and management of ecosystem health has not been as simple. Ecological systems can be dynamic and can shift abruptly from one ecosystem state to another. Such unpredictable shifts result when ecological thresholds are crossed; that is, small cumulative increases in an environmental stressor drive a much greater change than could be predicted from linear effects, suggesting an unforeseen tipping point is crossed. In coastal waters, broad-scale seagrass loss often occurs as a sudden event associated with human-driven nutrient enrichment (eutrophication). We tested whether the response of seagrass ecosystems to coastal nutrient enrichment is subject to a threshold effect. We exposed seagrass plots to different levels of nutrient enrichment (dissolved inorganic nitrogen) for 10 months and measured net production. Seagrass response exhibited a threshold pattern when nutrient enrichment exceeded moderate levels: there was an abrupt and large shift from positive to negative net leaf production (from approximately 0.04 leaf production to 0.02 leaf loss per day). Epiphyte load also increased as nutrient enrichment increased, which may have driven the shift in leaf production. Inadvertently crossing such thresholds, as can occur through ineffective management of land-derived inputs such as wastewater and stormwater runoff along urbanized coasts, may account for the widely observed sudden loss of seagrass meadows. Identification of tipping points may improve not only adaptive-management monitoring that seeks to avoid threshold effects, but also restoration approaches in systems that have crossed them.

Mineralogical Plasticity Acts as a Compensatory Mechanism to the Impacts of Ocean Acidification.

Calcifying organisms are considered particularly susceptible to the future impacts of ocean acidification (OA), but recent evidence suggests that they may be able to maintain calcification and overall fitness. The underlying mechanism remains unclear but may be attributed to mineralogical plasticity, which modifies the energetic cost of calcification. To test the hypothesis that mineralogical plasticity enables the maintenance of shell growth and functionality under OA conditions, we assessed the biological performance of a gastropod (respiration rate, feeding rate, somatic growth, and shell growth of Austrocochlea constricta) and analyzed its shell mechanical and geochemical properties (shell hardness, elastic modulus, amorphous calcium carbonate, calcite to aragonite ratio, and magnesium to calcium ratio). Despite minor metabolic depression and no increase in feeding rate, shell growth was faster under OA conditions, probably due to increased precipitation of calcite and trade-offs against inner shell density. In addition, the resulting shell was functionally suitable for increasingly "corrosive" oceans, i.e., harder and less soluble shells. We conclude that mineralogical plasticity may act as a compensatory mechanism to maintain overall performance of calcifying organisms under OA conditions and could be a cornerstone of calcifying organisms to acclimate to and maintain their ecological functions in acidifying oceans.

Impacts of Near-Future Ocean Acidification and Warming on the Shell Mechanical and Geochemical Properties of Gastropods from Intertidal to Subtidal Zones.

Many marine organisms produce calcareous shells as the key structure for defense, but the functionality of shells may be compromised by ocean acidification and warming. Nevertheless, calcifying organisms may adaptively modify their shell properties in response to these impacts. Here, we examined how reduced pH and elevated temperature affect shell mechanical and geochemical properties of common grazing gastropods from intertidal to subtidal zones. Given the greater environmental fluctuations in the intertidal zone, we hypothesized that intertidal gastropods would exhibit more plastic responses in shell properties than subtidal gastropods. Overall, three out of five subtidal gastropods produced softer shells at elevated temperature, while intertidal gastropods maintained their shell hardness at both elevated pCO2 (i.e., reduced pH) and temperature. Regardless of pH and temperature, degree of crystallization was maintained (except one subtidal gastropod) and carbonate polymorph remained unchanged in all tested species. One intertidal gastropod produced less soluble shells (e.g., higher calcite/aragonite) in response to reduced pH. In contrast, subtidal gastropods produced only aragonite which has higher solubility than calcite. Overall, subtidal gastropods are expected to be more susceptible than intertidal gastropods to shell dissolution and physical damage under future seawater conditions. The increased vulnerability to shell dissolution and predation could have serious repercussions for their survival and ecological contributions in the future subtidal environment.