The role of preconditioning in ocean acidification experiments: a test with the intertidal isopod Paradella dianae

Environmental alterations are accelerating worldwide and the rate of change in ocean chemistry is predicted to happen so rapidly that it is unclear how marine ecosystems will respond. It is hypothesized that the phenotypic plasticity or acclimation capacity of an individual provides a buffer against environmental change; however, this plasticity depends on the speed at which the change occurs. Ocean acidification studies have found direct and acute responses from organisms exposed to elevated CO₂ levels. Now, the challenge lies in integrating acclimation into experimental design in short-term studies, requiring proper preconditioning setups. Here we experimentally show that different preconditioning approaches produce different physiological and behavioral responses in the intertidal isopod Paradella dianae. Isopods were impaired when immediately exposed to elevated CO₂ levels relative to individuals that were gradually acclimated to high CO₂ concentrations. Abruptly introducing organisms to severe changes in CO₂ conditions can produce confounding effects of short-term stress with acclimated responses to long-term shifts in ocean chemistry. By exposing organisms to sudden changes in CO₂ concentrations, we are forcing immediate physiological stress reactions that could be independent of exposure to specific CO₂ levels. We discuss how integrating acclimation in experimental design can help provide more accurate predictions about the impact of ocean acidification on marine ecosystems.

pH alters the swimming behaviors of the raphidophyte Heterosigma akashiwo: implications for bloom formation in an acidified ocean

We investigated the effects of pH on movement behaviors of the harmful algal bloom causing raphidophyte Heterosigma akashiwo. Motility parameters from >8000 swimming tracks of individual cells were quantified using 3D digital video analysis over a 6-h period in 3 pH treatments reflecting marine carbonate chemistry during the pre-industrial era, currently, and the year 2100. Movement behaviors were investigated in two different acclimation-to-target-pH conditions: instantaneous exposure and acclimation of cells for at least 11 generations. There was no negative impairment of cell motility when exposed to elevated PCO₂ (i.e., low pH) conditions but there were significant behavioral responses. Irrespective of acclimation condition, lower pH significantly increased downward velocity and frequency of downward swimming cells (p < 0.001). Rapid exposure to lower pH resulted in 9% faster downward vertical velocity and up to 19% more cells swimming downwards (p < 0.001). Compared to pH-shock experiments, pre-acclimation of cells to target pH resulted in ~30% faster swimming speed and up to 46% faster downward velocities (all p < 0.001). The effect of year 2100 PCO₂ levels on population diffusivity in pre-acclimated cultures was >2-fold greater than in pH-shock treatments (2.2 × 10⁵ μm² s⁻¹ vs. 8.4 × 10⁴ μm² s⁻¹). Predictions from an advection-diffusion model, suggest that as PCO₂ increased the fraction of the population aggregated at the surface declined, and moved deeper in the water column. Enhanced downward swimming of H. akashiwo at low pH suggests that these behavioral responses to elevated PCO₂ could reduce the likelihood of dense surface slick formation of H. akashiwo through reductions in light exposure or growth independent surface aggregations. We hypothesize that the HAB alga’s response to higher PCO₂ may exploit the signaling function of high PCO₂ as indicative of net heterotrophy in the system, thus indicative of high predation rates or depletion of nutrients.

Ocean acidification alters the otoliths of a pantropical fish species with implications for sensory function

Ocean acidification affects a wide diversity of marine organisms and is of particular concern for vulnerable larval stages critical to population replenishment and connectivity. Whereas it is well known that ocean acidification will negatively affect a range of calcareous taxa, the study of fishes is more limited in both depth of understanding and diversity of study species. We used new 3D microcomputed tomography to conduct in situ analysis of the impact of ocean acidification on otolith (ear stone) size and density of larval cobia (Rachycentron canadum), a large, economically important, pantropical fish species that shares many life history traits with a diversity of high-value, tropical pelagic fishes. We show that 2,100 μatm partial pressure of carbon dioxide (pCO₂) significantly increased not only otolith size (up to 49% greater volume and 58% greater relative mass) but also otolith density (6% higher). Estimated relative mass in 800 μatm pCO₂ treatments was 14% greater, and there was a similar but nonsignificant trend for otolith size. Using a modeling approach, we demonstrate that these changes could affect auditory sensitivity including a ∼50% increase in hearing range at 2,100 μatm pCO₂, which may alter the perception of auditory information by larval cobia in a high-CO₂ ocean. Our results indicate that ocean acidification has a graded effect on cobia otoliths, with the potential to substantially influence the dispersal, survival, and recruitment of a pelagic fish species. These results have important implications for population maintenance/replenishment, connectivity, and conservation efforts for other valuable fish stocks that are already being deleteriously impacted by overfishing.

Predicting the response of molluscs to the impact of ocean acidification

Elevations in atmospheric carbon dioxide (CO₂) are anticipated to acidify oceans because of fundamental changes in ocean chemistry created by CO₂ absorption from the atmosphere. Over the next century, these elevated concentrations of atmospheric CO₂ are expected to result in a reduction of the surface ocean waters from 8.1 to 7.7 units as well as a reduction in carbonate ion (CO₃²⁻) concentration. The potential impact that this change in ocean chemistry will have on marine and estuarine organisms and ecosystems is a growing concern for scientists worldwide. While species-specific responses to ocean acidification are widespread across a number of marine taxa, molluscs are one animal phylum with many species which are particularly vulnerable across a number of life-history stages. Molluscs make up the second largest animal phylum on earth with 30,000 species and are a major producer of CaCO₃. Molluscs also provide essential ecosystem services including habitat structure and food for benthic organisms (i.e., mussel and oyster beds), purification of water through filtration and are economically valuable. Even sub lethal impacts on molluscs due to climate changed oceans will have serious consequences for global protein sources and marine ecosystems.

Elevated CO2 affects predator-prey interactions through altered performance

Recent research has shown that exposure to elevated carbon dioxide (CO₂) affects how fishes perceive their environment, affecting behavioral and cognitive processes leading to increased prey mortality. However, it is unclear if increased mortality results from changes in the dynamics of predator-prey interactions or due to prey increasing activity levels. Here we demonstrate that ocean pCO₂ projected to occur by 2100 significantly effects the interactions of a predator-prey pair of common reef fish: the planktivorous damselfish Pomacentrus amboinensis and the piscivorous dottyback Pseudochromis fuscus. Prey exposed to elevated CO₂ (880 µatm) or a present-day control (440 µatm) interacted with similarly exposed predators in a cross-factored design. Predators had the lowest capture success when exposed to elevated CO₂ and interacting with prey exposed to present-day CO₂. Prey exposed to elevated CO₂ had reduced escape distances and longer reaction distances compared to prey exposed to present-day CO₂ conditions, but this was dependent on whether the prey was paired with a CO₂ exposed predator or not. This suggests that the dynamics of predator-prey interactions under future CO₂ environments will depend on the extent to which the interacting species are affected and can adapt to the adverse effects of elevated CO₂.

Consequences of ocean change for ecological function: observational and modeling case studies of larval echinoderms

Planktonic larvae of many marine invertebrates play important roles in connecting and sustaining disjunct adult populations. Most larvae are denser than seawater and rely on swimming to regulate their vertical positions. Because environmental variables including direction and strength of advective currents and prey and predator concentrations vary with depth, larval swimming behaviors can significantly impact larval survival and transport. Quantification of larval movement is therefore essential for understanding population dynamics, especially in the face of global climate change because of the need to predict possible shifts in ecosystems. Larval swimming is physically constrained by their morphologies, which are often complex and highly variable. Behavioral responses to surrounding environmental variables modulate the actual swimming performance within physical limits. This study took a two-pronged approach to understand larval swimming through 1) quantifying larval behaviors under changing environmental conditions and 2) modeling larval morphology-flow interactions. This study applied novel non-invasive video motion analysis techniques to quantify effects of environmental variations. Ocean acidification is considered one of the major threats to marine ecosystems and larvae are suggested to be particularly vulnerable. When reared under elevated pCO2 level, larval sand dollars Dendraster excentricus maintained their swimming performance but had lower feeding success. By combining feeding and respiration experiments with motion analysis, we observed similar tradeoffs among larval purple urchins, Strongylocentrotus purpuratus, and heart urchins, Brissopsis lyrifera. These two echinoids also underwent budding under acidified conditions, an asexual reproduction strategy that has not been previously reported. These results suggest that sublethal OA impacts could be carried over from planktonic stages to later development stages and affect population dynamics. Previous studies suggest that larval swimming performance peaks within a tight morphospace. Larval sand dollars are phenotypically plastic and develop longer arms when starved. Starved individuals swam with higher oscillatory speeds than their fed counterpart. To distinguish the biomechanical constraints associated with morphological changes from behavioral adjustments, we developed a detailed 3-dimensional model of individual larvae using laser confocal microscopy and finite-element mesh generation. This novel modeling approach can easily be adapted for other taxa to help understand constraints that swimming imposes on the evolution of larval form.

Mussel byssus attachment weakened by ocean acidification

Biomaterials connect organisms to their environments. Their function depends on biological, chemical and environmental factors, both at the time of creation and throughout the life of the material. Shifts in the chemistry of the oceans driven by anthropogenic CO₂ (termed ocean acidification) have profound implications for the function of critical materials formed under these altered conditions. Most ocean acidification studies have focused on one biomaterial (secreted calcium carbonate), frequently using a single assay (net rate of calcification) to quantify whether reductions in environmental pH alter how organisms create biomaterials¹. Here, we examine biological structures critical for the success of ecologically and economically important bivalve molluscs. One non-calcified material, the proteinaceous byssal threads that anchor mytilid mussels to hard substrates, exhibited reduced mechanical performance when secreted under elevated p_CO2 conditions, whereas shell and tissue growth were unaffected. Threads made under high p_CO2 (>1,200 μatm) were weaker and less extensible owing to compromised attachment to the substratum. According to a mathematical model, this reduced byssal fibre performance, decreasing individual tenacity by 40%. In the face of ocean acidification, weakened attachment presents a potential challenge for suspension-culture mussel farms and for intertidal communities anchored by mussel beds.

Benthic interactions in a high CO2-world

The Baltic Sea is a physically diverse habitat with a generally low species diversity. The blue mussel Mytilus edulis is widely distributed in the benthic macrozoobenthos of the Western Baltic, the main predators of which are the common sea star Asterias rubens and the shore crab Carcinus maenas. Any environmental stress influencing the predator-prey interactions between these species has the potential to shape the entire ecosystem. The current increase in atmospheric pCO2 causes a concurrent increase in the acidification of seawater and can thus pose such an environmental stress. In coastal habitats and specifically the Baltic Sea, the decrease of seawater pH can be much more pronounced than in the open ocean. In order to estimate possible interaction shifts in the macrozoobenthos under conditions of seawater acidification, this work investigates the effect of an increase in water pCO2 on the predators A. rubens and C. maenas and their consumption of M. edulis. The results of three different own studies show an impact of increased seawater pCO2 around 3500 μatm on growth and mussel consumption in adult sea stars and a seawater pCO2 of around 1200 μatm to impact growth, mussel consumption, scope for growth and righting response of juvenile A. rubens. Mussel sizes consumed, metabolism, NH4+-excretion and calcification were, however, not impacted by an increase in seawater acidification and coelomic pH not regulated by means of active bicarbonate accumulation. Crabs were impacted in metabolism, NH4+-excretion, O:N-ratio and metabolic energy loss at a seawater pCO2 of about 3500 μatm. Mussel consumption was only impacted at a pCO2 of 3500 μatm, over intermediate time spans (10 weeks), but not over a longer (six month) time span. Hemolyph pH was regulated to remain at control levels by active bicarbonate accumulation at intermediate (10 week) time spans over all levels of seawater acidification, while hemolymph pH followed the non-bicarbonate buffer line at the intermediate (around 1200 μatm) level and was only regulated at the high (around 3500 μatm) treatment level over the long (six month) time span. Moulting intervals, growth, mussel sizes consumed, carapace thickness, stability, dry weight and calcification were not influenced by seawater acidification These results indicate a change in feeding pressure on the blue mussel M. edulis under future high levels of seawater acidification. Further, A. rubens appears stronger impacted by seawater acidification than C. maenas and juvenile A. rubens even stronger than adult specimen. I conclude that seawater acidification has the potential to reshape the benthic ecosystem of the Western Baltic. This work therefore helps to understand ecosystem responses to environmental stress and contributes to making predictions on future species distributions in the Baltic Sea.

Non-lethal effects of ocean acidification on the symbiont-bearing benthic foraminifer Amphistegina gibbosa

The responses of marine taxa to ocean acidification are varied, with, for example, some exhibiting decreased and some increased calcification rates. Experiments were conducted to assess the effect of elevated atmospheric carbon dioxide concentrations on the survival, fitness, shell microfabric and growth of Amphistegina gibbosa, a symbiont-bearing, coral-reef dwelling, benthic foraminiferal species that precipitates low-Mg calcite tests, using CO₂ partial pressure ( pCO₂) levels similar to those likely to occur in shallow marine pore waters in the decades ahead. Specimens were cultured at constant temperature and controlled pCO₂ (ambient, 1000 parts per million by volume [ppmv], and 2000 ppmv) for 6 wk, and total alkalinity and dissolved inorganic carbon were measured every 2 wk to characterize the carbonate chemistry of the incubations. Foraminiferal survival and cellular energy levels were assessed using adenosine triphosphate analyses, and test microstructure and growth were evaluated using high resolution scanning electron microscopy and image analysis. Fitness and survival were not directly affected by elevated pCO₂ and the concomitant decrease in pH and calcite saturation states (Ω_c). Test growth was not affected by elevated pCO₂. However, areas of dissolution were observed after 6 wk, even though Ω_c was >1 in all treatments; the fraction of test area dissolved increased with decreasing Ω_c. Test dissolution occurred only in small, well defined patches that appeared to be distributed randomly over the whole test surface. Similar dissolution was observed in offspring produced in the 2000 ppmv pCO₂ treatments. The long-term ecological consequences of the effects observed are not yet known.

Elevated temperature elicits greater effects than decreased pH on the development, feeding and metabolism of northern shrimp (Pandalus borealis) larvae

Climate models predict that the average temperature in the North Sea could increase 3–5 °C and surface-waters pH could decrease 0.3–0.5 pH units by the end of this century. Consequently, we investigated the combined effect of decreased pH (control pH 8.1; decreased pH 7.6) and temperature (control 6.7 °C; elevated 9.5 °C) on the hatching timing and success, and the zoeal development, survival, feeding, respiration and growth (up to stage IV zoea) of the northern shrimp, Pandalus borealis. At elevated temperature, embryos hatched 3 days earlier, but experienced 2–4 % reduced survival. Larvae developed 9 days faster until stage IV zoea under elevated temperature and exhibited an increase in metabolic rates (ca 20 %) and an increase in feeding rates (ca 15–20 %). Decreased pH increased the development time, but only at the low temperature. We conclude that warming will likely exert a greater effect on shrimp larval development than ocean acidification manifesting itself as accelerated developmental rates with greater maintenance costs and decreased recruitment in terms of number and size.