Ocean acidification from 1997 to 2011 in the subarctic western North Pacific Ocean

Rising atmospheric CO₂ contents have led to greater CO₂ uptake by the oceans, lowering both pH due to increasing hydrogen ions and CaCO₃ saturation states due to declining carbonate ion (CO₃²⁻). Here, we used previously compiled data sets and new data collected in 2010 and 2011 to investigate ocean acidification of the North Pacific western subarctic gyre. In winter, the western subarctic gyre is a source of CO₂ to the atmosphere because of convective mixing of deep waters rich in dissolved inorganic carbon (DIC). We calculated pH in winter mixed layer from DIC and total alkalinity (TA), and found that it decreased at the rate of −0.001 ± 0.0004 yr⁻¹ from 1997 to 2011. This decrease rate is slower than that expected under condition of seawater/atmosphere equilibration, and it is also slower than the rate in the subtropical regions (−0.002 yr⁻¹). The slow rate is caused by a reduction of CO₂ emission in winter due to an increase in TA. Below the mixed layer, the calcite saturation horizon (~185 m depth) shoaled at the rate of 2.9 ± 0.9 m yr⁻¹ as the result of the declining CO₃²⁻ concentration (−0.03 ± 0.01 μmol k⁻¹yr⁻¹). Between 200 m and 300 m depth, pH decline during the study period (−0.0051 ± 0.0010 yr⁻¹) was larger than ever reported in the open North Pacific. This enhanced acidification rate below the calcite saturation horizon reflected not only the uptake of anthropogenic CO₂ but also the increase in the decomposition of organic matter evaluated from the increase in AOU, which suggests that the dissolution of CaCO₃ particles increased.

Natural variation, and the capacity to adapt to ocean acidification in the keystone sea urchin Strongylocentrotus purpuratus

A rapidly growing body of literature documents the potential negative effects of CO₂-driven ocean acidification (OA) on marine organisms. However, nearly all of this work has focused on the effects of future conditions on modern populations, neglecting the role of adaptation. Rapid evolution can alter demographic responses to environmental change, ultimately affecting the likelihood of population persistence, but the capacity for adaptation will differ among populations and species. Here, we measure the capacity of the ecologically important purple sea urchin Strongylocentrotus purpuratus to adapt to OA, using a breeding experiment to estimate additive genetic variance for larval size (an important component of fitness) under future high pCO₂/low pH conditions. Although larvae reared under future conditions were smaller than those reared under present-day conditions, we show that there is also abundant genetic variation for body size under elevated pCO₂, indicating that this trait can evolve. The observed heritability of size was 0.40±0.32 (95% CI) under low pCO₂, and 0.50±0.30 under high pCO₂ conditions. Accounting for the observed genetic variation in models of future larval size and demographic rates substantially alters projections of performance for this species in the future ocean. Importantly, our model shows that after incorporating the effects of adaptation, the OA-driven decrease in population growth rate is up to 50% smaller, than that predicted by the “no-adaptation” scenario. Adults used in the experiment were collected from two sites on the coast of the Northeast Pacific that are characterized by different pH regimes, as measured by autonomous sensors. Comparing results between sites, we also found subtle differences in larval size under high pCO₂ rearing conditions, consistent with local adaptation to carbonate chemistry in the field. These results suggest that spatially varying selection may help to maintain genetic variation necessary for adaptation to future ocean acidification.

Food availability and pCO2 impacts on planulation, juvenile survival, and calcification of the azooxanthellate scleractinian coral, Balanophyllia elegans

Ocean acidification, the assimilation of atmospheric CO₂ by the oceans that decreases the pH and CaCO₃ saturation state (Ω) of seawater, is projected to have severe consequences for calcifying organisms. Strong evidence suggests that tropical reef-building corals containing algal symbionts (zooxanthellae) will experience dramatic declines in calcification over the next century. The responses of azooxanthellate corals to ocean acidification are less well understood, and because they cannot obtain extra photosynthetic energy from symbionts, they provide a system for studying the direct effects of acidification on the energy available for calcification. The orange cup coral Balanophyllia elegans is a solitary, azooxanthellate scleractinian species common on the California coast where it thrives in the low pH waters of an upwelling regime. During an 8 month study, we addressed the effects of three pCO₂ treatments (410, 770, and 1230 μatm) and two feeding frequencies (High Food and Low Food) on adult Balanophyllia elegans planulation (larval release) rates, and on the survival, growth, and calcification of their juvenile offspring. Planulation rates were affected by food level but not pCO₂, while juvenile survival was highest under 410 μatm and High Food conditions. Our results suggest that feeding rate has a greater impact on calcification of B. elegans than pCO₂. Net calcification was positive even at 1230 μatm (~ 3 times current atmospheric pCO₂), although the increase from 410 to 1230 μatm reduced overall calcification by ~ 25–45%, and reduced skeletal density by ~ 35–45%. Higher pCO₂ also altered aragonite crystal morphology significantly. We discuss how feeding frequency affects azooxanthellate coral calcification, and how B. elegans may respond to ocean acidification in coastal upwelling waters.

A developmental and energetic basis linking larval oyster shell formation to acidification sensitivity

Acidified waters are impacting commercial oyster production in the U.S. Pacific Northwest and favorable carbonate chemistry conditions are predicted to become less frequent. Within 48 hours of fertilization, unshelled Pacific oyster (Crassostrea gigas) larvae precipitate roughly 90% of their body weight as calcium carbonate. We measured stable carbon isotopes in larval shell and tissue and in algal food and seawater dissolved inorganic carbon in a longitudinal study of larval development and growth. Using these data and measured biochemical composition of larvae we show that sensitivity of initial shell formation to ocean acidification results from diminished ability to isolate calcifying fluid from surrounding seawater, a limited energy budget dependent, and a strong kinetic demand for calcium carbonate precipitation. Our results highlight an important link between organism physiology and mineral kinetics in larval bivalves and suggest the consideration of mineral kinetics may improve understanding winners and losers in a high CO₂ world.

Symbiosis increases coral tolerance to ocean acidification

Increasing the acidity of ocean waters will directly threaten calcifying marine organisms such as reef-building scleractinian corals, and the myriad of species that rely on corals for protection and sustenance. Ocean pH has already decreased by around 0.1 pH units since the beginning of the industrial revolution, and is expected to decrease by another 0.2–0.4 pH units by 2100. This study mimicked the pre-industrial, present, and near-future levels of pCO² using a precise control system (±5% pCO²), to assess the impact of ocean acidification on the calcification of recently-settled primary polyps of Acropora digitifera, both with and without symbionts, and adult fragments with symbionts. The increase in pCO² of 100 μatm between the pre-industrial period and the present had more effect on the calcification rate of adult A. digitifera than the anticipated future increases of several hundreds of micro-atmospheres of pCO². The primary polyps with symbionts showed higher calcification rates than primary polyps without symbionts, suggesting that (i) primary polyps housing symbionts are more tolerant to near-future ocean acidification than organisms without symbionts, and (ii) corals acquiring symbionts from the environment (i.e. broadcasting species) will be more vulnerable to ocean acidification than corals that maternally acquire symbionts.

Evolutionary change during experimental ocean acidification

Rising atmospheric carbon dioxide (CO₂) conditions are driving unprecedented changes in seawater chemistry, resulting in reduced pH and carbonate ion concentrations in the Earth’s oceans. This ocean acidification has negative but variable impacts on individual performance in many marine species. However, little is known about the adaptive capacity of species to respond to an acidified ocean, and, as a result, predictions regarding future ecosystem responses remain incomplete. Here we demonstrate that ocean acidification generates striking patterns of genome-wide selection in purple sea urchins (Strongylocentrotus purpuratus) cultured under different CO₂ levels. We examined genetic change at 19,493 loci in larvae from seven adult populations cultured under realistic future CO₂ levels. Although larval development and morphology showed little response to elevated CO₂, we found substantial allelic change in 40 functional classes of proteins involving hundreds of loci. Pronounced genetic changes, including excess amino acid replacements, were detected in all populations and occurred in genes for biomineralization, lipid metabolism, and ion homeostasis—gene classes that build skeletons and interact in pH regulation. Such genetic change represents a neglected and important impact of ocean acidification that may influence populations that show few outward signs of response to acidification. Our results demonstrate the capacity for rapid evolution in the face of ocean acidification and show that standing genetic variation could be a reservoir of resilience to climate change in this coastal upwelling ecosystem. However, effective response to strong natural selection demands large population sizes and may be limited in species impacted by other environmental stressors.

Regional analysis on decadal variation in aspects of water quality in three contrasting coastal systems of Ishikawa Coast, Japan

This study examines drivers and trends of change in time and space in some coastal water quality parameters and management/cultural practices on the Ishikawa coast, Japan, between 1984 and 2009. Generally, chemical oxygen demand (COD) and dissolved oxygen (DO) concentrations varied significantly (P < 0.001) across the three coastal compartments (Kanazawa area (KNA), Noto area (NTA), and Nanao area (NNA)), and pH and DO concentrations varied significantly (P < 0.001) over the last three decades, suggesting that human perturbation within the last three decades varied along the coast and may be more relative than environmental change impact. Other water quality variables showed different spatial and temporal patterns and implications across the coastal compartments. Although there is a uniform approach to the development of water quality standards in Japan, these results suggest that setting stricter standards may be required by the prefectural government in collaboration with industry, stakeholders groups, and coastal community resource users.

Carbonate dissolution rates at the deep ocean floor

This paper reexamines experimental data on the seawater dissolution of CaCO₃-bearing sediment beds to establish that the dependence of the calcite dissolution rate is linearly dependent on the calcite saturation state of the overlying water. This linearity is inherent to the original data and is not the result of an error in the solubility product for calcite. A comparison between these linear kinetics and the rate of solute transport across the benthic boundary layer further reveals that the overall rate of dissolution at ocean depths below the saturation horizon is controlled by boundary layer transfer. A carbonate mass-balance model for the sediment-water interface, which includes both kinetics and boundary layer effects, predictively reproduces the currently observed CaCO₃ depth distribution for two test areas in the oceans. These findings allow important simplifications in modeling CO₂ neutralization in the oceans.

Effects of ocean acidification on juvenile red king crab (Paralithodes camtschaticus) and tanner crab (Chionoecetes bairdi) growth, condition, calcification, and survival

Ocean acidification, a decrease in the pH in marine waters associated with rising atmospheric CO₂ levels, is a serious threat to marine ecosystems. In this paper, we determine the effects of long-term exposure to near-future levels of ocean acidification on the growth, condition, calcification, and survival of juvenile red king crabs, Paralithodes camtschaticus, and Tanner crabs, Chionoecetes bairdi. Juveniles were reared in individual containers for nearly 200 days in flowing control (pH 8.0), pH 7.8, and pH 7.5 seawater at ambient temperatures (range 4.4–11.9 °C). In both species, survival decreased with pH, with 100% mortality of red king crabs occurring after 95 days in pH 7.5 water. Though the morphology of neither species was affected by acidification, both species grew slower in acidified water. At the end of the experiment, calcium concentration was measured in each crab and the dry mass and condition index of each crab were determined. Ocean acidification did not affect the calcium content of red king crab but did decrease the condition index, while it had the opposite effect on Tanner crabs, decreasing calcium content but leaving the condition index unchanged. This suggests that red king crab may be able to maintain calcification rates, but at a high energetic cost. The decrease in survival and growth of each species is likely to have a serious negative effect on their populations in the absence of evolutionary adaptation or acclimatization over the coming decades.

Temperature and CO2 additively regulate physiology, morphology and genomic responses of larval sea urchins, Strongylocentrotus purpuratus

Ocean warming and ocean acidification, both consequences of anthropogenic production of CO₂, will combine to influence the physiological performance of many species in the marine environment. In this study, we used an integrative approach to forecast the impact of future ocean conditions on larval purple sea urchins (Strongylocentrotus purpuratus) from the northeast Pacific Ocean. In laboratory experiments that simulated ocean warming and ocean acidification, we examined larval development, skeletal growth, metabolism and patterns of gene expression using an orthogonal comparison of two temperature (13°C and 18°C) and pCO₂ (400 and 1100 μatm) conditions. Simultaneous exposure to increased temperature and pCO₂ significantly reduced larval metabolism and triggered a widespread downregulation of histone encoding genes. pCO₂ but not temperature impaired skeletal growth and reduced the expression of a major spicule matrix protein, suggesting that skeletal growth will not be further inhibited by ocean warming. Importantly, shifts in skeletal growth were not associated with developmental delay. Collectively, our results indicate that global change variables will have additive effects that exceed thresholds for optimized physiological performance in this keystone marine species.