Levels Of Ocean Acidification Research Articles

Increased light intensity enhances photosynthesis and biochemical components of red macroalga of commercial importance, Kappaphycus alvarezii, in response to ocean acidification

The concentration of atmospheric carbon dioxide (CO2) has increased drastically over the past several decades, resulting in the pH of the ocean decreasing by 0.44 ± 0.005 units, known as ocean acidification (OA). The Kappaphycus alvarezii (Rhodophyta, Solieriaceae), is a commercially and ecologically important red macroalga with significant CO2 absorption potential from seawater. The K. alvarezii also experienced light variations from self-shading and varied cultivation depths. Thus, the aim of present study was to investigate the effects of two pCO2 levels (450 and 1200 ppmv) and three light intensities (50, 100, and 150 μmol photons·m−2·s−1) on photosynthesis and the biochemical components in K. alvarezii. The results of the present study showed that a light intensity of 50 μmol photons·m−2·s−1 was optimal for K. alvarezii photosynthesis with 0.663 ± 0.030 of Fv/Fm and 0.672 ± 0.025 of Fv’/Fm’. Phycoerythrin contents at two pCO2 levels decreased significantly with an increase in light intensity by 57.14–87.76%, while phycocyanin contents only decreased from 0.0069 ± 0.001 mg g−1 FW to 0.0047 ± 0.001 mg g−1 FW with an increase in light intensity at 1200 ppmv of pCO2. Moreover, moderate increases in light intensity and pCO2 had certain positive effects on the physiological performance of K. alvarezii, specifically in terms of increasing soluble carbohydrate production. Although OA and high light levels promoted total organic carbon accumulation (21.730 ± 0.205% DW) in K. alvarezii, they had a negative impact on total nitrogen accumulation (0.600 ± 0.017% DW).

Effects of ocean acidification on dopamine-mediated behavioral responses of a coral reef damselfish

We investigated whether CO2-induced ocean acidification (OA) affects dopamine receptor-dependent behavior in bicolor damselfish (Stegastes partitus). Damselfish were kept in aquaria receiving flow through control (pH ~ 8.03; pCO2 ~ 384 μatm) or OA (pH ~ 7.64; CO2 ~ 1100 μatm) seawater at a rate of 1 L min−1. Despite this relatively fast flow rate, fish respiration further acidified the seawater in both control (pH ~7.88; pCO2 ~ 595 μatm) and OA (pH ~7.55; pCO2 ~ 1450 μatm) fish-holding aquaria. After five days of exposure, damselfish locomotion, boldness, anxiety, and aggression were assessed using a battery of behavioral tests using automated video analysis. Two days later, these tests were repeated following application of the dopamine D1 receptor agonist SKF 38393. OA-exposure induced ceiling anxiety levels that were significantly higher than in control damselfish, and SKF 38393 increased anxiety in control damselfish to a level not significantly different than that of OA-exposed damselfish. Additionally, SKF 38393 decreased locomotion and increased boldness in control damselfish but had no effect in OA-exposed damselfish, suggesting an alteration in activity of dopaminergic pathways that regulate behavior under OA conditions. These results indicate that changes in dopamine D1 receptor function affects fish behavior during exposure to OA. However, subsequent measurements of seawater sampled using syringes during the daytime (~3–4 pm local time) from crevasses in coral reef colonies, which are used as shelter by damselfish, revealed an average pH of 7.73 ± 0.03 and pCO2 of 925.8 ± 62.2 μatm; levels which are comparable to Representative Concentration Pathway (RCP) 8.5 predicted end-of-century mean OA levels in the open ocean. Further studies considering the immediate environmental conditions experienced by fish as well as individual variability and effect size are required to understand potential implications of the observed OA-induced behavioral effects on damselfish fitness in the wild.

Ocean acidification increases the impact of typhoons on algal communities

Long-term environmental change, sudden pulses of extreme perturbation, or a combination of both can trigger regime shifts by changing the processes and feedbacks which determine community assembly, structure, and function, altering the state of ecosystems. Our understanding of the mechanisms that stabilise against regime shifts or lock communities into altered states is limited, yet also critical to anticipating future states, preventing regime shifts, and reversing unwanted state change. Ocean acidification contributes to the restructuring and simplification of algal systems, however the mechanisms through which this occurs and whether additional drivers are involved requires further study. Using monthly surveys over three years at a shallow-water volcanic seep we examined how the composition of algal communities change seasonally and following periods of significant physical disturbance by typhoons at three levels of ocean acidification (equivalent to means of contemporary ∼350 and future ∼500 and 900 μatm pCO2). Sites exposed to acidification were increasingly monopolised by structurally simple, fast-growing turf algae, and were clearly different to structurally complex macrophyte-dominated reference sites. The distinct contemporary and acidified community states were stabilised and maintained at their respective sites by different mechanisms following seasonal typhoon disturbance. Macroalgal-dominated sites were resistant to typhoon damage. In contrast, significant losses of algal biomass represented a near total ecosystem reset by typhoons for the turf-dominated communities at the elevated pCO2 sites (i.e. negligible resistance). A combination of disturbance and subsequent turf recovery maintained the same simplified state between years (elevated CO2 levels promote turf growth following algal removal, inhibiting macroalgal recruitment). Thus, ocean acidification may promote shifts in algal systems towards degraded ecosystem states, and short-term disturbances which reset successional trajectories may ‘lock-in’ these alternative states of low structural and functional diversity.

Ocean acidification does not overlook sex: Review of understudied effects and implications of low pH on marine invertebrate sexual reproduction

Sexual reproduction is a fundamental process essential for species persistence, evolution, and diversity. However, unprecedented oceanographic shifts due to climate change can impact physiological processes, with important implications for sexual reproduction. Identifying bottlenecks and vulnerable stages in reproductive cycles will enable better prediction of the organism, population, community, and global-level consequences of ocean change. This article reviews how ocean acidification impacts sexual reproductive processes in marine invertebrates and highlights current research gaps. We focus on five economically and ecologically important taxonomic groups: cnidarians, crustaceans, echinoderms, molluscs and ascidians. We discuss the spatial and temporal variability of experimental designs, identify trends of performance in acidified conditions in the context of early reproductive traits (gametogenesis, fertilization, and reproductive resource allocation), and provide a quantitative meta-analysis of the published literature to assess the effects of low pH on fertilization rates across taxa. A total of 129 published studies investigated the effects of ocean acidification on 122 species in selected taxa. The impact of ocean acidification is dependent on taxa, the specific reproductive process examined, and study location. Our meta-analysis reveals that fertilization rate decreases as pH decreases, but effects are taxa-specific. Echinoderm fertilization appears more sensitive than molluscs to pH changes, and while data are limited, fertilization in cnidarians may be the most sensitive. Studies with echinoderms and bivalve molluscs are prevalent, while crustaceans and cephalopods are among the least studied species even though they constitute some of the largest fisheries worldwide. This lack of information has important implications for commercial aquaculture, wild fisheries, and conservation and restoration of wild populations. We recommend that studies expose organisms to different ocean acidification levels during the entire gametogenic cycle, and not only during the final stages before gametes or larvae are released. We argue for increased focus on fundamental reproductive processes and associated molecular mechanisms that may be vulnerable to shifts in ocean chemistry. Our recommendations for future research will allow for a better understanding of how reproduction in invertebrates will be affected in the context of a rapidly changing environment.

Elevated CO2 does not alter behavioural lateralization in free‐swimming juvenile European sea bass (Dicentrarchus labrax) tested in groups

The authors investigated left-right turning preferences of n=260 juvenile European sea bass (Dicentrarchus labrax) reared in ambient conditions and ocean acidification (OA) conditions or in ambient conditions but tested in OA water. Groups of 10 individuals were observed alone in a circular tank, and individuals' left and right turning during free-swimming was quantified using trajectory data from the video. The authors showed that near-future OA levels do not affect the number of turns made, or behavioural lateralization (turning preference), in juvenile D. labrax tested in groups.

Effects of one-year exposure to ocean acidification on two species of abalone

Ocean acidification (OA) resulting from the absorption of excess atmospheric CO2 by the ocean threatens the survival of marine calcareous organisms, including mollusks. This study investigated the effects of OA on adults of two abalone species (Haliotis diversicolor, a subtropical species, and Haliotis discus hannai, a temperate species). Abalone were exposed to three pCO2 conditions for 1 year (ambient, ~ 880, and ~ 1600 μatm), and parameters, including mortality, physiology, immune system, biochemistry, and carry-over effects, were measured. Survival decreased significantly at ~ 800 μatm pCO2 for H. diversicolor, while H. discus hannai survival was negatively affected only at a higher OA level (~ 1600 μatm pCO2). H. diversicolor exhibited depressed metabolic and excretion rates and a higher O:N ratio under OA, indicating a shift to lipids as a metabolism substrate, while these physiological parameters in H. discus hannai were robust to OA. Both abalone failed to compensate for the pH decrease of their internal fluids because of the lowered hemolymph pH under OA. However, the reduced hemolymph pH did not affect total hemocyte counts or tested biomarkers. Additionally, H. discus hannai increased its hemolymph protein content under OA, which could indicate enhanced immunity. Larvae produced by adults exposed to the three pCO2 levels were cultured in the same pCO2 conditions and larval deformation and shell length were measured to observe carry-over effects. Enhanced OA tolerance was observed for H. discus hannai exposed under both of the OA treatments, while that was only observed following parental pCO2 ~ 880 μatm exposure for H. diversicolor. Following pCO2 ~ 1600 μatm parental exposure, H. diversicolor offspring exhibited higher deformation and lower shell growth in all pCO2 treatments. In general, H. diversicolor were more susceptible to OA compared with H. discus hannai, suggesting that H. diversicolor could be unable to adapt to acidified oceans in the future.

A Systematic Review of the Behavioural Changes and Physiological Adjustments of Elasmobranchs and Teleost’s to Ocean Acidification with a Focus on Sharks

In recent years, much attention has been focused on the impact of climate change, particularly via ocean acidification (OA), on marine organisms. Studying the impact of OA on long-living organisms, such as sharks, is especially challenging. When the ocean waters absorb anthropogenic carbon dioxide (CO2), slow-growing shark species with long generation times may be subjected to stress, leading to a decrease in functionality. Our goal was to examine the behavioral and physiological responses of sharks to OA and the possible impacts on their fitness and resilience. We conducted a systematic review in line with PRISMA-Analyses, of previously reported scientific experiments. We found that most studies used CO2 partial pressures (pCO2) that reflect representative concentration pathways for the year 2100 (e.g., pH ~7.8, pCO2 ~1000 μatm). Since there is a considerable knowledge gap on the effect of OA on sharks, we utilized existing data on bony fish to synthesize the available knowledge. Given the similarities between the behaviors and physiology of these two superclasses’ to changes in CO2 and pH levels, there is merit in including the available information on bony fish as well. Several studies indicated a decrease in shark fitness in relation to increased OA and CO2 levels. However, the decrease was species-specific and influenced by the intensity of the change in atmospheric CO2 concentration and other anthropogenic and environmental factors (e.g., fishing, temperature). Most studies involved only limited exposure to future environmental conditions and were conducted on benthic shark species studied in the laboratory rather than on apex predator species. While knowledge gaps exist, and more research is required, we conclude that anthropogenic factors are likely contributing to shark species’ vulnerability worldwide. However, the impact of OA on the long-term stability of shark populations is not unequivocal.

Ocean acidification alters anti-predator responses in a competitive dominant intertidal mussel

Widespread intertidal mussels are exposed to a variety of natural and anthropogenic stressors. Even so, our understanding of the combined influence of stressors such as predation risk and ocean acidification (OA) on these species remains limited. This study examined the response of the purple mussel (Perumytilus purpuratus), a species distributed along Pacific southeastern rocky shores, to the effects of predation risk and OA. Using a laboratory 2 × 2 cross design, purple mussels were either devoid or exposed to predator cues from the muricid snail Acanthina monodon, while simultaneously exposing them to current (500 ppm) or projected OA conditions (1500 ppm). The response of purple mussels to these factors was assessed using growth, calcification, clearance, and metabolic rates, in addition to byssus production. After 60 d, the presence of predator cues reduced mussel growth in width and length, and in the latter case, OA enhanced this response making the effects of predator cues more severe. Calcification rates were driven by the interaction between the two stressors, whereas clearance rates increased only in response to OA, likely explaining some of the growth results. Mussel byssus production also increased with pCO2 but interacted with predation risk: in the absence of predator cues, byssus production increased with OA. These results suggest that projected levels of OA may alter and in some cases prevail over the natural response of purple mussels to predation risk. Considering the role played by this mussel as a dominant competitor and ecosystem engineer in rocky shores, these results have community-wide implications.

Nest guarding behaviour of a temperate wrasse differs between sites off Mediterranean CO2 seeps

Organisms may respond to changing environmental conditions by adjusting their behaviour (i.e., behavioural plasticity). Ocean acidification (OA), resulting from anthropogenic emissions of carbon dioxide (CO2), is predicted to impair sensory function and behaviour of fish. However, reproductive behaviours, and parental care in particular, and their role in mediating responses to OA are presently overlooked. Here, we assessed whether the nesting male ocellated wrasse Symphodus ocellatus from sites with different CO2 concentrations showed different behaviours during their breeding season. We also investigated potential re-allocation of the time-budget towards different behavioural activities between sites. We measured the time period that the nesting male spent carrying out parental care, mating and exploring activities, as well as changes in the time allocation between sites at ambient (~400 μatm) and high CO2 concentrations (~1000 μatm). Whilst the behavioural connectance (i.e., the number of linkages among different behaviours relative to the total amount of linkages) was unaffected, we observed a significant reduction in the time spent on parental care behaviour, and a significant decrease in the guarding activity of fish at the high CO2 sites, with a proportional re-allocation of the time budget in favour of courting and wandering around, which however did not change between sites. This study shows behavioural differences in wild fish living off volcanic CO2 seeps that could be linked to different OA levels, suggesting that behavioural plasticity may potentially act as a mechanism for buffering the effects of ongoing environmental change. A reallocation of the time budget between key behaviours may play a fundamental role in determining which marine organisms are thriving under projected OA.

Chemical characterization of the Punta de Fuencaliente CO<sub>2</sub>-enriched system (La Palma, NE Atlantic Ocean): a new natural laboratory for ocean acidification studies

Abstract. We present a new natural carbon dioxide (CO2) system located off the southern coast of the island of La Palma (Canary Islands, Spain). Like CO2 seeps, these CO2 submarine groundwater discharges (SGDs) can be used as an analogue to study the effects of ocean acidification (OA) on the marine realm. With this aim, we present the chemical characterization of the area, describing the carbon system dynamics, by measuring pH, AT and CT and calculating Ω aragonite and calcite. Our explorations of the area have found several emission points with similar chemical features. Here, the CT varies from 2120.10 to 10 784.84 µmol kg−1, AT from 2415.20 to 10 817.12 µmol kg−1, pH from 7.12 to 8.07, Ω aragonite from 0.71 to 4.15 and Ω calcite from 1.09 to 6.49 units. Also, the CO2 emission flux varies between 2.8 and 28 kg CO2 d−1, becoming a significant source of carbon. These CO2 emissions, which are of volcanic origin, acidify the brackish groundwater that is discharged to the coast and alter the local seawater chemistry. Although this kind of acidified system is not a perfect image of future oceans, this area of La Palma is an exceptional spot to perform studies aimed at understanding the effect of different levels of OA on the functioning of marine ecosystems. These studies can then be used to comprehend how life has persisted through past eras, with higher atmospheric CO2, or to predict the consequences of present fossil fuel usage on the marine ecosystem of the future oceans.

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.

Changes in fish communities due to benthic habitat shifts under ocean acidification conditions

Ocean acidification will likely change the structure and function of coastal marine ecosystems over coming decades. Volcanic carbon dioxide seeps generate dissolved CO2 and pH gradients that provide realistic insights into the direction and magnitude of these changes. Here, we used fish and benthic community surveys to assess the spatio-temporal dynamics of fish community properties off CO2 seeps in Japan. Adding to previous evidence from ocean acidification ecosystem studies conducted elsewhere, our findings documented shifts from calcified to non-calcified habitats with reduced benthic complexity. In addition, we found that such habitat transition led to decreased diversity of associated fish and to selection of those fish species better adapted to simplified ecosystems dominated by algae. Our data suggest that near-future projected ocean acidification levels will oppose the ongoing range expansion of coral reef-associated fish due to global warming.

Antagonistic interplay between pH and food resources affects copepod traits and performance in a year-round upwelling system

Linking pH/pCO2 natural variation to phenotypic traits and performance of foundational species provides essential information for assessing and predicting the impact of ocean acidification (OA) on marine ecosystems. Yet, evidence of such linkage for copepods, the most abundant metazoans in the oceans, remains scarce, particularly for naturally corrosive Eastern Boundary Upwelling systems (EBUs). This study assessed the relationship between pH levels and traits (body and egg size) and performance (ingestion rate (IR) and egg reproduction rate (EPR)) of the numerically dominant neritic copepod Acartia tonsa, in a year-round upwelling system of the northern (23° S) Humboldt EBUs. The study revealed decreases in chlorophyll (Chl) ingestion rate, egg production rate and egg size with decreasing pH as well as egg production efficiency, but the opposite for copepod body size. Further, ingestion rate increased hyperbolically with Chl, and saturated at ~1 µg Chl. L−1. Food resources categorized as high (H, >1 µg L−1) and low (L, <1 µg L−1) levels, and pH-values categorized as equivalent to present day (≤400 µatm pCO2, pH > 7.89) and future (>400 µatm pCO2, pH < 7.89) were used to compare our observations to values globally employed to experimentally test copepod sensitivity to OA. A comparison (PERMANOVA) test with Chl/pH (2*2) design showed that partially overlapping OA levels expected for the year 2100 in other ocean regions, low-pH conditions in this system negatively impacted traits and performance associated with copepod fitness. However, interacting antagonistically with pH, food resource (Chl) maintained copepod production in spite of low pH levels. Thus, the deleterious effects of ocean acidification are modulated by resource availability in this system.

Ocean acidification does not impair the behaviour of coral reef fishes.

The partial pressure of CO2 in the oceans has increased rapidly over the past century, driving ocean acidification and raising concern for the stability of marine ecosystems1-3. Coral reef fishes are predicted to be especially susceptible to end-of-century ocean acidification on the basis of several high-profile papers4,5 that have reported profound behavioural and sensory impairments-for example, complete attraction to the chemical cues of predators under conditions of ocean acidification. Here, we comprehensively and transparently show that-in contrast to previous studies-end-of-century ocean acidification levels have negligible effects on important behaviours of coral reef fishes, such as the avoidance of chemical cues from predators, fish activity levels and behavioural lateralization (left-right turning preference). Using data simulations, we additionally show that the large effect sizes and small within-group variances that have been reported in several previous studies are highly improbable. Together, our findings indicate that the reported effects of ocean acidification on the behaviour of coral reef fishes are not reproducible, suggesting that behavioural perturbations will not be a major consequence for coral reef fishes in high CO2 oceans.

Elevated CO2 and food ration affect growth but not the size-based hierarchy of a reef fish

Under projected levels of ocean acidification, shifts in energetic demands and food availability could interact to effect the growth and development of marine organisms. Changes to individual growth rates could then flow on to influence emergent properties of social groups, particularly in species that form size-based hierarchies. To test the potential interactive effects of (1) food availability, (2) elevated CO2 during juvenile development, and (3) parental experience of elevated CO2 on the growth, condition and size-based hierarchy of juvenile fish, we reared orange clownfish (Amphiprion percula) for 50 days post-hatching in a fully orthogonal design. Development in elevated CO2 reduced standard length and weight of juveniles, by 9% and 11% respectively, compared to ambient. Development under low food availability reduced length and weight of juveniles by 7% and 15% respectively, compared to high food. Parental exposure to elevated CO2 restored the length of juveniles to that of controls, but it did not restore weight, resulting in juveniles from elevated CO2 parents exhibiting 33% lower body condition when reared in elevated CO2. The body size ratios (relative size of a fish from the rank above) within juvenile groups were not affected by any treatment, suggesting relative robustness of group-level structure despite alterations in individual size and condition. This study demonstrates that both food availability and elevated CO2 can influence the physical attributes of juvenile reef fish, but these changes may not disrupt the emergent group structure of this social species, at least amongst juveniles.

Transcriptome profiling reveals exposure to predicted end-of-century ocean acidification as a stealth stressor for Atlantic cod larvae

Ocean acidification (OA), a direct consequence of increasing atmospheric CO2 concentration dissolving in ocean waters, is impacting many fish species. Little is known about the molecular mechanisms underlying the observed physiological impacts in fish. We used RNAseq to characterize the transcriptome of 3 different larval stages of Atlantic cod (Gadus morhua) exposed to simulated OA at levels (1179 µatm CO2) representing end-of-century predictions compared to controls (503 µatm CO2), which were shown to induce tissue damage and elevated mortality in G. morhua. Only few genes were differentially expressed in 6 and 13 days-post-hatching (dph) (3 and 16 genes, respectively), during a period when maximal mortality as a response to elevated pCO2 occurred. At 36 dph, 1413 genes were differentially expressed, most likely caused by developmental asynchrony between the treatment groups, with individuals under OA growing faster. A target gene analysis revealed only few genes of the universal and well-defined cellular stress response to be differentially expressed. We thus suggest that predicted ocean acidification levels constitute a “stealth stress” for early Atlantic cod larvae, with a rapid breakdown of cellular homeostasis leading to organismal death that was missed even with an 8-fold replication implemented in this study.

Recoverable impacts of ocean acidification on the tubeworm, Hydroides elegans: implication for biofouling in future coastal oceans

Ocean uptake of anthropogenic CO2 causes ocean acidification (OA), which not only decreases the calcification rate, but also impairs the formation of calcareous shells or tubes in marine invertebrates such as the dominant biofouling tubeworm species, Hydroides elegans. This study examined the ability of tubeworms to resume normal tube calcification when returned to ambient pH 8.1 from a projected near-future OA level of pH 7.8. Tubeworms produced structurally impaired and mechanically weaker calcareous tubes at pH 7.8 compared to at pH 8.1, but were able to recover when the pH was restored to ambient levels. This suggests that tubeworms can physiologically recover from the impacts of OA on tube calcification, composition, density, hardness and stiffness when returned to optimal conditions. These results help understanding of the progression of biofouling communities dominated by tubeworms in future oceans with low pH induced by OA.

Ocean acidification impact on ascidian Ciona robusta spermatozoa: New evidence for stress resilience

Rising atmospheric CO2 is causing a progressive decrease of seawater pH, termed ocean acidification. Predicting its impact on marine invertebrate reproduction is essential to anticipate the consequences of future climate change on species fitness and survival. Ocean acidification may affect reproductive success either in terms of gamete or progeny quality threating species survival. Despite an increasing number of studies focusing on the effects of ocean acidification on the early life history of marine organisms, very few have investigated the effects on invertebrate gamete quality. In this study, we set up two experimental approaches simulating the ocean conditions predicted for the end of this century, in situ transplant experiments at a naturally acidified volcanic vent area along the Ischia island coast and microcosm experiments, to evaluate the short-term effects of the predicted near-future levels of ocean acidification on sperm quality of the ascidian Ciona robusta after parental exposure. In the first days of exposure to acidified conditions, we detected alteration of sperm motility, morphology and physiology, followed by a rapid recovery of physiological conditions that provide a new evidence of resilience of ascidian spermatozoa in response to ocean acidification. Overall, the short-term tolerance to adverse conditions opens a new scenario on the marine species capacity to continue to reproduce and persist in changing oceans.

Climate change erodes competitive hierarchies among native, alien and range-extending crabs

Global warming and ocean acidification alter a wide range of animal behaviours, yet the effect on resource competition among species is poorly understood. We tested whether the combination of moderate levels of ocean acidification and warming altered the feeding success of co-occurring native, alien, and range-extending crab species, and how these changes affected their hierarchical dominance. Under contemporary conditions the range-extending species spent more time feeding, than the alien and the native species. Under conditions simulating future climate there was no difference in the proportion of time spent feeding among the three species. These behavioural changes translated to alterations in their dominance hierarchy (based on feeding success) with the most dominant species under present day conditions becoming less dominant under future conditions, and vice versa for the least dominant species. While empirical studies have predicted either reversal or strengthening of hierarchical dominance in animal species, we suggest that even moderate increases in ocean temperature and acidification can drive a homogenisation in behavioural competitiveness, eroding dominance differences among species that are linked to fitness-related traits in nature and hence important for their population persistence.

Divergent responses of Atlantic cod to ocean acidification and food limitation.

In order to understand the effect of global change on marine fishes, it is imperative to quantify the effects on fundamental parameters such as survival and growth. Larval survival and recruitment of the Atlantic cod (Gadus morhua) were found to be heavily impaired by end-of-century levels of ocean acidification. Here, we analysed larval growth among 35-36days old surviving larvae, along with organ development and ossification of the skeleton. We combined CO2 treatments (ambient: 503µatm, elevated: 1,179µatm) with food availability in order to evaluate the effect of energy limitation in addition to the ocean acidification stressor. As expected, larval size (as a proxy for growth) and skeletogenesis were positively affected by high food availability. We found significant interactions between acidification and food availability. Larvae fed ad libitum showed little difference in growth and skeletogenesis due to the CO2 treatment. Larvae under energy limitation were significantly larger and had further developed skeletal structures in the elevated CO2 treatment compared to the ambient CO2 treatment. However, the elevated CO2 group revealed impairments in critically important organs, such as the liver, and had comparatively smaller functional gills indicating a mismatch between size and function. It is therefore likely that individual larvae that had survived acidification treatments will suffer from impairments later during ontogeny. Our study highlights important allocation trade-off between growth and organ development, which is critically important to interpret acidification effects on early life stages of fish.