Framework of barrier reefs threatened by ocean acidification

To date, studies of ocean acidification (OA) on coral reefs have focused on organisms rather than communities, and the few community effects that have been addressed have focused on shallow back reef habitats. The effects of OA on outer barrier reefs, which are the most striking of coral reef habitats and are functionally and physically different from back reefs, are unknown. Using 5‐m long outdoor flumes to create treatment conditions, we constructed coral reef communities comprised of calcified algae, corals, and reef pavement that were assembled to match the community structure at 17 m depth on the outer barrier reef of Moorea, French Polynesia. Communities were maintained under ambient and 1200 μatm pCO₂ for 7 weeks, and net calcification rates were measured at different flow speeds. Community net calcification was significantly affected by OA, especially at night when net calcification was depressed ~78% compared to ambient pCO₂. Flow speed (2–14 cm s^?1) enhanced net calcification only at night under elevated pCO₂. Reef pavement also was affected by OA, with dissolution ~86% higher under elevated pCO₂ compared to ambient pCO₂. These results suggest that net accretion of outer barrier reef communities will decline under OA conditions predicted within the next 100 years, largely because of increased dissolution of reef pavement. Such extensive dissolution poses a threat to the carbonate foundation of barrier reef communities.     

One‐year experiment on the physiological response of the Mediterranean crustose coralline alga,Lithophyllum cabiochae,to elevated pCO2 and temperature

The response of respiration, photosynthesis, and calcification to elevated pCO₂ and temperature was investigated in isolation and in combination in the Mediterranean crustose coralline alga Lithophyllum cabiochae. Algae were maintained in aquaria during 1 year at near‐ambient conditions of irradiance, at ambient or elevated temperature (+3°C), and at ambient (ca. 400 μatm) or elevated pCO₂ (ca. 700 μatm). Respiration, photosynthesis, and net calcification showed a strong seasonal pattern following the seasonal variations of temperature and irradiance, with higher rates in summer than in winter. Respiration was unaffected by pCO₂ but showed a general trend of increase at elevated temperature at all seasons, except in summer under elevated pCO₂. Conversely, photosynthesis was strongly affected by pCO₂ with a decline under elevated pCO₂ in summer, autumn, and winter. In particular, photosynthetic efficiency was reduced under elevated pCO₂. Net calcification showed different responses depending on the season. In summer, net calcification increased with rising temperature under ambient pCO₂ but decreased with rising temperature under elevated pCO₂. Surprisingly, the highest rates in summer were found under elevated pCO₂ and ambient temperature. In autumn, winter, and spring, net calcification exhibited a positive or no response at elevated temperature but was unaffected by pCO₂. The rate of calcification of L. cabiochae was thus maintained or even enhanced under increased pCO₂. However, there is likely a trade‐off with other physiological processes. For example, photosynthesis declines in response to increased pCO₂ under ambient irradiance. The present study reports only on the physiological response of healthy specimens to ocean warming and acidification, however, these environmental changes may affect the vulnerability of coralline algae to other stresses such as pathogens and necroses that can cause major dissolution, which would have critical consequence for the sustainability of coralligenous habitats and the budgets of carbon and calcium carbonate in coastal Mediterranean ecosystems.     

Productivity gains do not compensate for reduced calcification under near‐future ocean acidification in the photosynthetic benthic foraminifer species Marginopora vertebralis

Changes in the seawater carbonate chemistry (ocean acidification) from increasing atmospheric carbon dioxide (CO₂) concentrations negatively affect many marine calcifying organisms, but may benefit primary producers under dissolved inorganic carbon (DIC) limitation. To improve predictions of the ecological effects of ocean acidification, the net gains and losses between the processes of photosynthesis and calcification need to be studied jointly on physiological and population levels. We studied productivity, respiration, and abundances of the symbiont‐bearing foraminifer species Marginopora vertebralis on natural CO₂ seeps in Papua New Guinea and conducted additional studies on production and calcification on the Great Barrier Reef (GBR) using artificially enhanced pCO₂. Net oxygen production increased up to 90% with increasing pCO₂; temperature, light, and pH together explaining 61% of the variance in production. Production increased with increasing light and increasing pCO₂ and declined at higher temperatures. Respiration was also significantly elevated (~25%), whereas calcification was reduced (16–39%) at low pH/high pCO₂ compared to present‐day conditions. In the field, M. vertebralis was absent at three CO₂ seep sites at pH_Total levels below ~7.9 (pCO₂ ~700 μatm), but it was found in densities of over 1000 m^?2 at all three control sites. The study showed that endosymbiotic algae in foraminifera benefit from increased DIC availability and may be naturally carbon limited. The observed reduction in calcification may have been caused either by increased energy demands for proton pumping (measured as elevated rates of respiration) or by stronger competition for DIC from the more productive symbionts. The net outcome of these two competing processes is that M. vertebralis cannot maintain populations under pCO₂ exceeding 700 μatm, thus are likely to be extinct in the next century.     

Stress-tolerant corals of Florida Bay are vulnerable to ocean acidification

In situ calcification measurements tested the hypothesis that corals from environments (Florida Bay, USA) that naturally experience large swings in pCO₂ and pH will be tolerant or less sensitive to ocean acidification than species from laboratory experiments with less variable carbonate chemistry. The pCO₂ in Florida Bay varies from summer to winter by several hundred ppm roughly comparable to the increase predicted by the end of the century. Rates of net photosynthesis and calcification of two stress-tolerant coral species, Siderastrea radians and Solenastrea hyades, were measured under the prevailing ambient chemical conditions and under conditions amended to simulate a pH drop of 0.1–0.2 units at bimonthly intervals over a 2-yr period. Net photosynthesis was not changed by the elevation in pCO₂ and drop in pH; however, calcification declined by 52 and 50 % per unit decrease in saturation state, respectively. These results indicate that the calcification rates of S. radians and S. hyades are just as sensitive to a reduction in saturation state as coral species that have been previously studied. In other words, stress tolerance to temperature and salinity extremes as well as regular exposure to large swings in pCO₂ and pH did not make them any less sensitive to ocean acidification. These two species likely survive in Florida Bay in part because they devote proportionately less energy to calcification than most other species and the average saturation state is elevated relative to that of nearby offshore water due to high rates of primary production by seagrasses.     

Relative sensitivity of five Hawaiian coral species to high temperature under high-pCO2 conditions

Coral reef ecosystems are presently undergoing decline due to anthropogenic climate change. The chief detrimental factors are increased temperature and increased pCO₂. The purpose of this study was to evaluate the effect of these two stressors operating independently and in unison on the biological response of common Hawaiian reef corals. Manipulative experiments were performed using five species (Porites compressa, Pocillopora damicornis, Fungia scutaria, Montipora capitata, and Leptastrea purpurea) in a continuous-flow mesocosm system under natural sunlight conditions. Corals were grown together as a community under treatments of high temperature (2 °C above normal maximum summer temperature), high pCO₂ (twice present-day conditions), and with both factors acting in unison. Control corals were grown under present-day pCO₂ and at normal summer temperatures. Leptastrea purpurea proved to be an extremely hardy coral. No change in calcification or mortality occurred under treatments of high temperature, high pCO₂, or combined high temperature–high pCO₂. The remaining four species showed reduced calcification in the high-temperature treatment. Two species (L. purpurea and M. capitata) showed no response to increased pCO₂. Also, high pCO₂ ameliorated the negative effect of high temperature on the calcification rates of P. damicornis. Mortality was driven primarily by high temperature, with a negative synergistic effect in P. compressa only in the high-pCO₂–high-temperature treatment. Results support the observation that biological response to temperature and pCO₂ elevation is highly species-specific, so generalizations based on response of a single species might not apply to a diverse and complex coral reef community.

     

End of the Century pCO2 Levels Do Not Impact Calcification in Mediterranean Cold-Water Corals

Ocean acidification caused by anthropogenic uptake of CO₂ is perceived to be a major threat to calcifying organisms. Cold-water corals were thought to be strongly affected by a decrease in ocean pH due to their abundance in deep and cold waters which, in contrast to tropical coral reef waters, will soon become corrosive to calcium carbonate. Calcification rates of two Mediterranean cold-water coral species, Lophelia pertusa and Madrepora oculata, were measured under variable partial pressure of CO₂ (pCO₂) that ranged between 380 µatm for present-day conditions and 930 µatm for the end of the century. The present study addressed both short- and long-term responses by repeatedly determining calcification rates on the same specimens over a period of 9 months. Besides studying the direct, short-term response to elevated pCO₂ levels, the study aimed to elucidate the potential for acclimation of calcification of cold-water corals to ocean acidification. Net calcification of both species was unaffected by the levels of pCO₂ investigated and revealed no short-term shock and, therefore, no long-term acclimation in calcification to changes in the carbonate chemistry. There was an effect of time during repeated experiments with increasing net calcification rates for both species, however, as this pattern was found in all treatments, there is no indication that acclimation of calcification to ocean acidification occurred. The use of controls (initial and ambient net calcification rates) indicated that this increase was not caused by acclimation in calcification response to higher pCO₂. An extrapolation of these data suggests that calcification of these two cold-water corals will not be affected by the pCO₂ level projected at the end of the century.     

Coral reefs modify their seawater carbon chemistry – implications for impacts of ocean acidification

Reviews suggest that that the biogeochemical threshold for sustained coral reef growth will be reached during this century due to ocean acidification caused by increased uptake of atmospheric CO₂. Projections of ocean acidification, however, are based on air‐sea fluxes in the open ocean, and not for shallow‐water systems such as coral reefs. Like the open ocean, reef waters are subject to the chemical forcing of increasing atmospheric pCO₂. However, for reefs with long water residence times, we illustrate that benthic carbon fluxes can drive spatial variation in pH, pCO₂ and aragonite saturation state (Ω_a) that can mask the effects of ocean acidification in some downstream habitats. We use a carbon flux model for photosynthesis, respiration, calcification and dissolution coupled with Lagrangian transport to examine how key groups of calcifiers (zooxanthellate corals) and primary producers (macroalgae) on coral reefs contribute to changes in the seawater carbonate system as a function of water residence time. Analyses based on flume data showed that the carbon fluxes of corals and macroalgae drive Ω_ain opposing directions. Areas dominated by corals elevate pCO₂ and reduce Ω_a, thereby compounding ocean acidification effects in downstream habitats, whereas algal beds draw CO₂ down and elevate Ω_a, potentially offsetting ocean acidification impacts at the local scale. Simulations for two CO₂ scenarios (600 and 900 ppm CO₂) suggested that a potential shift from coral to algal abundance under ocean acidification can lead to improved conditions for calcification in downstream habitats, depending on reef size, water residence time and circulation patterns. Although the carbon fluxes of benthic reef communities cannot significantly counter changes in carbon chemistry at the scale of oceans, they provide a significant mechanism of buffering ocean acidification impacts at the scale of habitat to reef.     

Interactive Effects of CO2, Temperature,Irradiance, and Nutrient Limitation on the Growth and Physiology of the Marine Diatom Thalassiosira pseudonana (Coscinodiscophyceae)

The marine diatom Thalassiosira pseudonana was grown in continuous culture systems to study the interactive effects of temperature, irradiance, nutrient limitation, and the partial pressure of CO₂ (pCO₂) on its growth and physiological characteristics. The cells were able to grow at all combinations of low and high irradiance (50 and 300 μmol photons · m⁻² · s⁻¹, respectively, of visible light), low and high pCO₂ (400 and 1,000 μatm, respectively), nutrient limitation (nitrate-limited and nutrient-replete conditions), and temperatures of 10–32°C. Under nutrient-replete conditions, there was no adverse effect of high pCO₂ on growth rates at temperatures of 10–25°C. The response of the cells to high pCO₂ was similar at low and high irradiance. At supraoptimal temperatures of 30°C or higher, high pCO₂ depressed growth rates at both low and high irradiance. Under nitrate-limited conditions, cells were grown at 38 ± 2.4% of their nutrient-saturated rates at the same temperature, irradiance, and pCO₂. Dark respiration rates consistently removed a higher percentage of production under nitrate-limited versus nutrient-replete conditions. The percentages of production lost to dark respiration were positively correlated with temperature under nitrate-limited conditions, but there was no analogous correlation under nutrient-replete conditions. The results suggest that warmer temperatures and associated more intense thermal stratification of ocean surface waters could lower net photosynthetic rates if the stratification leads to a reduction in the relative growth rates of marine phytoplankton, and at truly supraoptimal temperatures there would likely be a synergistic interaction between the stresses from temperature and high pCO₂ (lower pH).     

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 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 OA.     

Seasonality Affects Macroalgal Community Response to Increases in pCO2

Ocean acidification is expected to alter marine systems, but there is uncertainty about its effects due to the logistical difficulties of testing its large-scale and long-term effects. Responses of biological communities to increases in carbon dioxide can be assessed at CO₂ seeps that cause chronic exposure to lower seawater pH over localised areas of seabed. Shifts in macroalgal communities have been described at temperate and tropical pCO₂ seeps, but temporal and spatial replication of these observations is needed to strengthen confidence our predictions, especially because very few studies have been replicated between seasons. Here we describe the seawater chemistry and seasonal variability of macroalgal communities at CO₂ seeps off Methana (Aegean Sea). Monitoring from 2011 to 2013 showed that seawater pH decreased to levels predicted for the end of this century at the seep site with no confounding gradients in Total Alkalinity, salinity, temperature or wave exposure. Most nutrient levels were similar along the pH gradient; silicate increased significantly with decreasing pH, but it was not limiting for algal growth at all sites. Metal concentrations in seaweed tissues varied between sites but did not consistently increase with pCO₂. Our data on the flora are consistent with results from laboratory experiments and observations at Mediterranean CO₂ seep sites in that benthic communities decreased in calcifying algal cover and increased in brown algal cover with increasing pCO₂. This differs from the typical macroalgal community response to stress, which is a decrease in perennial brown algae and proliferation of opportunistic green algae. Cystoseira corniculata was more abundant in autumn and Sargassum vulgare in spring, whereas the articulated coralline alga Jania rubens was more abundant at reference sites in autumn. Diversity decreased with increasing CO₂ regardless of season. Our results show that benthic community responses to ocean acidification are strongly affected by season.     

Is the response of coral calcification to seawater acidification related to nutrient loading?

The effect of decreasing aragonite saturation state (Ω_Arag) of seawater (elevated pCO₂) on calcification rates of Acropora muricata was studied using nubbins prepared from parent colonies located at two sites of La Saline reef (La Réunion Island, western Indian Ocean): a back-reef site (BR) affected by nutrient-enriched groundwater discharge (mainly nitrate), and a reef flat site (RF) with low terrigenous inputs. Protein and chlorophyll a content of the nubbins, as well as zooxanthellae abundance, were lower at RF than BR. Nubbins were incubated at ~27°C over 2 h under sunlight, in filtered seawater manipulated to get differing initial pCO₂ (1,440–340 μatm), Ω_Arag (1.4–4.0), and dissolved inorganic carbon (DIC) concentrations (2,100–1,850 μmol kg⁻¹). Increasing DIC concentrations at constant total alkalinity (A_T) resulted in a decrease in Ω_Arag and an increase in pCO₂. A_T at the beginning of the incubations was kept at a natural level of 2,193 ± 6 μmol kg⁻¹ (mean ± SD). Net photosynthesis (NP) and calcification were calculated from changes in pH and A_T during the incubations. Calcification decrease in response to doubling pCO₂ relative to preindustrial level was 22% for RF nubbins. When normalized to surface area of the nubbins, (1) NP and calcification were higher at BR than RF, (2) NP increased in high pCO₂ treatments at BR compared to low pCO₂ treatments, and (3) calcification was not related to Ω_Arag at BR. When normalized to NP, calcification was linearly related to Ω_Arag at both sites, and the slopes of the relationships were not significantly different. The increase in NP at BR in the high pCO₂ treatments may have increased calcification and thus masked the negative effect of low Ω_Arag on calcification. Removing the effect of NP variations at BR showed that calcification declined in a similar manner with decreased Ω_Arag (increased pCO₂) whatever the nutrient loading.     

Ocean acidification and calcifying reef organisms: a mesocosm investigation

A long-term (10 months) controlled experiment was conducted to test the impact of increased partial pressure of carbon dioxide (pCO₂) on common calcifying coral reef organisms. The experiment was conducted in replicate continuous flow coral reef mesocosms flushed with unfiltered sea water from Kaneohe Bay, Oahu, Hawaii. Mesocosms were located in full sunlight and experienced diurnal and seasonal fluctuations in temperature and sea water chemistry characteristic of the adjacent reef flat. Treatment mesocosms were manipulated to simulate an increase in pCO₂ to levels expected in this century [midday pCO₂ levels exceeding control mesocosms by 365 ± 130 μatm (mean ± sd)]. Acidification had a profound impact on the development and growth of crustose coralline algae (CCA) populations. During the experiment, CCA developed 25% cover in the control mesocosms and only 4% in the acidified mesocosms, representing an 86% relative reduction. Free-living associations of CCA known as rhodoliths living in the control mesocosms grew at a rate of 0.6 g buoyant weight year⁻¹ while those in the acidified experimental treatment decreased in weight at a rate of 0.9 g buoyant weight year⁻¹, representing a 250% difference. CCA play an important role in the growth and stabilization of carbonate reefs, so future changes of this magnitude could greatly impact coral reefs throughout the world. Coral calcification decreased between 15% and 20% under acidified conditions. Linear extension decreased by 14% under acidified conditions in one experiment. Larvae of the coral Pocillopora damicornis were able to recruit under the acidified conditions. In addition, there was no significant difference in production of gametes by the coral Montipora capitata after 6 months of exposure to the treatments.     

Differential modification of seawater carbonate chemistry by major coral reef benthic communities

Ocean acidification (OA) resulting from uptake of anthropogenic CO₂ may negatively affect coral reefs by causing decreased rates of biogenic calcification and increased rates of CaCO₃ dissolution and bioerosion. However, in addition to the gradual decrease in seawater pH and Ω _a resulting from anthropogenic activities, seawater carbonate chemistry in these coastal ecosystems is also strongly influenced by the benthic metabolism which can either exacerbate or alleviate OA through net community calcification (NCC = calcification – CaCO₃ dissolution) and net community organic carbon production (NCP = primary production ? respiration). Therefore, to project OA on coral reefs, it is necessary to understand how different benthic communities modify the reef seawater carbonate chemistry. In this study, we used flow-through mesocosms to investigate the modification of seawater carbonate chemistry by benthic metabolism of five distinct reef communities [carbonate sand, crustose coralline algae (CCA), corals, fleshy algae, and a mixed community] under ambient and acidified conditions during summer and winter. The results showed that different communities had distinct influences on carbonate chemistry related to the relative importance of NCC and NCP. Sand, CCA, and corals exerted relatively small influences on seawater pH and Ω _a over diel cycles due to closely balanced NCC and NCP rates, whereas fleshy algae and mixed communities strongly elevated daytime pH and Ω _a due to high NCP rates. Interestingly, the influence on seawater pH at night was relatively small and quite similar across communities. NCC and NCP rates were not significantly affected by short-term acidification, but larger diel variability in pH was observed due to decreased seawater buffering capacity. Except for corals, increased net dissolution was observed at night for all communities under OA, partially buffering against nighttime acidification. Thus, algal-dominated areas of coral reefs and increased net CaCO₃ dissolution may partially counteract reductions in seawater pH associated with anthropogenic OA at the local scale.     

Effects of ocean acidification on growth and physiology of Ulva lactuca (Chlorophyta) in a rockpool‐scenario

Rising atmospheric CO₂‐concentrations will have severe consequences for a variety of biological processes. We investigated the responses of the green alga Ulva lactuca (Linnaeus) to rising CO₂‐concentrations in a rockpool scenario. U. lactuca was cultured under aeration with air containing either preindustrial pCO₂ (280 μatm) or the pCO₂ predicted by the end of the 21st century (700 μatm) for 31 days. We addressed the following question: Will elevated CO₂‐concentrations affect photosynthesis (net photosynthesis, maximum relative electron transport rate (rETR(max)), maximum quantum yield (Fv/Fm), pigment composition) and growth of U. lactuca in rockpools with limited water exchange? Two phases of the experiment were distinguished: In the initial phase (day 1–4) the Seawater Carbonate System (SWCS) of the culture medium could be adjusted to the selected atmospheric pCO₂ condition by continuous aeration with target pCO₂ values. In the second phase (day 4–31) the SWCS was largely determined by the metabolism of the growing U. lactuca biomass. In the initial phase, Fv/Fm and rETR(max) were only slightly elevated at high CO₂‐concentrations, whereas growth was significantly enhanced. After 31 days the Chl a content of the thalli was significantly lower under future conditions and the photosynthesis of thalli grown under preindustrial conditions was not dependent on external carbonic anhydrase. Biomass increased significantly at high CO₂‐concentrations. At low CO₂‐concentrations most adult thalli disintegrated between day 14 and 21, whereas at high CO₂‐concentrations most thalli remained integer until day 31. Thallus disintegration at low CO₂‐concentrations was mirrored by a drastic decline in seawater dissolved inorganic carbon and HCO₃^?. Accordingly, the SWCS differed significantly between the treatments. Our results indicated a slight enhancement of photosynthetic performance and significantly elevated growth of U. lactuca at future CO₂‐concentrations. The accelerated thallus disintegration at high CO₂‐concentrations under conditions of limited water exchange indicates additional CO₂ effects on the life cycle of U. lactuca when living in rockpools.     

Signals of resilience to ocean change: high thermal tolerance of early stage Antarctic sea urchins (Sterechinus neumayeri) reared under present-day and future pCO2 and temperature

We tested the hypothesis that development of the Antarctic urchin Sterechinus neumayeri under future ocean conditions of warming and acidification would incur physiological costs, reducing the tolerance of a secondary stressor. The aim of this study is twofold: (1) quantify current austral spring temperature and pH near sea urchin habitat at Cape Evans in McMurdo Sound, Antarctica and (2) spawn S. neumayeri in the laboratory and raise early developmental stages (EDSs) under ambient (?0.7 °C; 400 µatm pCO₂) and future (+2.6 °C; 650 and 1,000 µatm pCO₂) ocean conditions and expose four EDSs (blastula, gastrula, prism, and 4-arm echinopluteus) to a one hour acute heat stress and assess survivorship. Results of field data from 2011 to 2012 show extremely stable inter-annual pH conditions ranging from 7.99 to 8.08, suggesting that future ocean acidification will drastically alter the pH-seascape for S. neumayeri. In the laboratory, S. neumayeri EDSs appear to be tolerant of temperatures and pCO₂ levels above their current habitat conditions. EDSs survived acute heat exposures >20 °C above habitat temperatures of ?1.9 °C. No pCO₂ effect was observed for EDSs reared at ?0.7 °C. When reared at +2.6 °C, small but significant pCO₂ effects were observed at the blastula and prism stage, suggesting that multiple stressors are more detrimental than single stressors. While surprisingly tolerant overall, blastulae were the most sensitive stage to ocean warming and acidification. We conclude that S. neumayeri may be unexpectedly physiologically tolerant of future ocean conditions.     

Combined effects of ocean acidification and solar UV radiation on photosynthesis,growth, pigmentation and calcification of the coralline alga Corallina sessilis (Rhodophyta)

Previous studies have shown that increasing atmospheric CO₂ concentrations affect calcification in some planktonic and macroalgal calcifiers due to the changed carbonate chemistry of seawater. However, little is known regarding how calcifying algae respond to solar UV radiation (UVR, UVA+UVB, 280–400 nm). UVR may act synergistically, antagonistically or independently with ocean acidification (high CO₂/low pH of seawater) to affect their calcification processes. We cultured the articulated coralline alga Corallina sessilis Yendo at 380 ppmv (low) and 1000 ppmv (high) CO₂ levels while exposing the alga to solar radiation treatments with or without UVR. The presence of UVR inhibited the growth, photosynthetic O₂ evolution and calcification rates by13%, 6% and 3% in the low and by 47%, 20% and 8% in the high CO₂ concentrations, respectively, reflecting a synergistic effect of CO₂ enrichment with UVR. UVR induced significant decline of pH in the CO₂‐enriched cultures. The contents of key photosynthetic pigments, chlorophyll a and phycobiliproteins decreased, while UV‐absorptivity increased under the high pCO₂/low pH condition. Nevertheless, UV‐induced inhibition of photosynthesis increased when the ratio of particulate inorganic carbon/particulate organic carbon decreased under the influence of CO₂‐acidified seawater, suggesting that the calcified layer played a UV‐protective role. Both UVA and UVB negatively impacted photosynthesis and calcification, but the inhibition caused by UVB was about 2.5–2.6 times that caused by UVA. The results imply that coralline algae suffer from more damage caused by UVB as they calcify less and less with progressing ocean acidification.     

Differential Responses of Calcifying and Non-Calcifying Epibionts of a Brown Macroalga to Present-Day and Future Upwelling pCO2

Seaweeds are key species of the Baltic Sea benthic ecosystems. They are the substratum of numerous fouling epibionts like bryozoans and tubeworms. Several of these epibionts bear calcified structures and could be impacted by the high pCO₂ events of the late summer upwellings in the Baltic nearshores. Those events are expected to increase in strength and duration with global change and ocean acidification. If calcifying epibionts are impacted by transient acidification as driven by upwelling events, their increasing prevalence could cause a shift of the fouling communities toward fleshy species. The aim of the present study was to test the sensitivity of selected seaweed macrofoulers to transient elevation of pCO₂ in their natural microenvironment, i.e. the boundary layer covering the thallus surface of brown seaweeds. Fragments of the macroalga Fucus serratus bearing an epibiotic community composed of the calcifiers Spirorbis spirorbis (Annelida) and Electra pilosa (Bryozoa) and the non-calcifier Alcyonidium hirsutum (Bryozoa) were maintained for 30 days under three pCO₂ conditions: natural 460±59 µatm, present-day upwelling1193±166 µatm and future upwelling 3150±446 µatm. Only the highest pCO₂ caused a significant reduction of growth rates and settlement of S. spirorbis individuals. Additionally, S. spirorbis settled juveniles exhibited enhanced calcification of 40% during daylight hours compared to dark hours, possibly reflecting a day-night alternation of an acidification-modulating effect by algal photosynthesis as opposed to an acidification-enhancing effect of algal respiration. E. pilosa colonies showed significantly increased growth rates at intermediate pCO₂ (1193 µatm) but no response to higher pCO₂. No effect of acidification on A. hirsutum colonies growth rates was observed. The results suggest a remarkable resistance of the algal macro-epibionts to levels of acidification occurring at present day upwellings in the Baltic. Only extreme future upwelling conditions impacted the tubeworm S. spirorbis, but not the bryozoans.     

Effects of elevated pCO2 on epilithic and endolithic metabolism of reef carbonates

We investigated effects of elevated partial pressure of CO₂ (pCO₂) on the metabolism of epilithic and endolithic phototrophic communities that colonized experimental coral blocks. Blocks of the massive coral Porites lobata were exposed to colonization by epilithic and endolithic organisms at an oceanic site in Kaneohe Bay (Hawaii) for 6 months, and then were transported to laboratory tanks. A bubbling system was used to maintain two treatments for 3 months, one at ambient pCO₂ (400 ppm) and the second at elevated pCO₂ (750 ppm). Net photosynthetic rates of epilithic communities in the high pCO₂ treatment, dominated by encrusting coralline algae, decreased by 35% while respiration rates remained constant. In contrast, metabolism of endolithic phototrophs, comprised of cyanobacteria and algae, was not significantly affected by the elevated pCO₂ even though endoliths contributed about 63% to block production.     

Ocean acidification stimulates particulate organic carbon accumulation in two Antarctic diatom species under moderate and high natural solar radiation

Impacts of rising atmospheric CO₂ concentrations and increased daily irradiances from enhanced surface water stratification on phytoplankton physiology in the coastal Southern Ocean remain still unclear. Therefore, in the two Antarctic diatoms Fragilariopsis curta and Odontella weissflogii, the effects of moderate and high natural solar radiation combined with either ambient or future pCO₂ on cellular particulate organic carbon (POC) contents and photophysiology were investigated. Results showed that increasing CO₂ concentrations had greater impacts on diatom physiology than exposure to increasing solar radiation. Irrespective of the applied solar radiation regime, cellular POC quotas increased with future pCO₂ in both diatoms. Lowered maximum quantum yields of photochemistry in PSII (F_v/F_m) indicated a higher photosensitivity under these conditions, being counteracted by increased cellular concentrations of functional photosynthetic reaction centers. Overall, our results suggest that both bloom‐forming Antarctic coastal diatoms might increase carbon contents under future pCO₂ conditions despite reduced physiological fitness. This indicates a higher potential for primary productivity by the two diatom species with important implications for the CO₂ sequestration potential of diatom communities in the future coastal Southern Ocean.     

Stoichiometric impacts of increased carbon dioxide on a planktonic herbivore

The partial pressure of carbon dioxide (pCO₂) in lake ecosystems varies over four orders of magnitude and is affected by local and global environmental perturbations associated with both natural and anthropogenic processes. Little is known, however, about how changes in pCO₂ extend into the function and structure of food webs in freshwater ecosystems. To fill this gap, we performed laboratory experiments using the ecologically important planktonic herbivore Daphnia and its algal prey under a natural range of pCO₂ with low light and phosphorus supplies. The experiment showed that increased pCO₂ stimulated algal growth but reduced algal P : C ratio. When feeding on algae grown under high pCO₂, herbivore growth decreased regardless of algal abundance. Thus, high CO₂‐raised algae were poor food for Daphnia. Short‐term experimental supplementation of PO₄ raised the P content of the high CO₂‐raised algae and improved Daphnia growth, indicating that low Daphnia growth rates under high pCO₂ conditions were due to lowered P content in the algal food. These results suggest that, in freshwater ecosystems with low nutrient supplies, natural processes as well as anthropogenic perturbations resulting in increased pCO₂ enhance algal production but reduce energy and mass transfer efficiency to herbivores by decreasing algal nutritional quality.