Posts Tagged 'mesocosms'

Assessing the effects of ocean warming and acidification on the seagrass Thalassia hemprichii

Seagrass beds serve as important carbon sinks, and it is thought that increasing the quantity and quality of such sinks could help to slow the rate of global climate change. Therefore, it will be important to (1) gain a better understanding of seagrass bed metabolism and (2) document how these high-productivity ecosystems are impacted by climate change-associated factors, such as ocean acidification (OA) and ocean warming (OW). A mesocosm-based approach was taken herein in which a tropical, Western Pacific seagrass species Thalassia hemprichii was cultured under either control or OA-simulating conditions; the temperature was gradually increased from 25 to 31 °C for both CO2 enrichment treatments, and it was hypothesized that this species would respond positively to OA and elevated temperature. After 12 weeks of exposure, OA (~1200 ppm) led to (1) increases in underground biomass and root C:N ratios and (2) decreases in root nitrogen content. Rising temperatures (25 to 31 °C) increased the maximum quantum yield of photosystem II (Fv:Fm), productivity, leaf growth rate, decomposition rate, and carbon sequestration, but decreased the rate of shoot density increase and the carbon content of the leaves; this indicates that warming alone does not increase the short-term carbon sink capacity of this seagrass species. Under high CO2 and the highest temperature employed (31 °C), this seagrass demonstrated its highest productivity, Fv:Fm, leaf growth rate, and carbon sequestration. Collectively, then, it appears that high CO2 levels offset the negative effects of high temperature on this seagrass species. Whether this pattern is maintained at temperatures that actually induce marked seagrass stress (likely beginning at 33–34 °C in Southern Taiwan) should be the focus of future research.

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Enhanced silica export in a future ocean triggers global diatom decline

Diatoms account for up to 40% of marine primary production and require silicic acid to grow and build their opal shell3. On the physiological and ecological level, diatoms are thought to be resistant to, or even benefit from, ocean acidification. Yet, global-scale responses and implications for biogeochemical cycles in the future ocean remain largely unknown. Here we conducted five in situ mesocosm experiments with natural plankton communities in different biomes and find that ocean acidification increases the elemental ratio of silicon (Si) to nitrogen (N) of sinking biogenic matter by 17 ± 6 per cent under pCO2 conditions projected for the year 2100. This shift in Si:N seems to be caused by slower chemical dissolution of silica at decreasing seawater pH. We test this finding with global sediment trap data, which confirm a widespread influence of pH on Si:N in the oceanic water column. Earth system model simulations show that a future pH-driven decrease in silica dissolution of sinking material reduces the availability of silicic acid in the surface ocean, triggering a global decline of diatoms by 13–26 per cent due to ocean acidification by the year 2200. This outcome contrasts sharply with the conclusions of previous experimental studies, thereby illustrating how our current understanding of biological impacts of ocean change can be considerably altered at the global scale through unexpected feedback mechanisms in the Earth system.

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Sinking diatoms trap silicon in deep seawater of acidified oceans

The seas are acidifying as a result of carbon dioxide emissions. It now emerges that this will alter the solubility of the shells of marine organisms called diatoms — and thereby change the distribution of nutrients and plankton in the ocean.

The ecologically dominant phytoplankton in much of the ocean are a group of unicellular organisms known as diatoms. Writing in Nature, Taucher et al. present a study that uses a combination of experimental, observational and modelling approaches to examine how the diatom-driven effects of ocean acidification — a consequence of rising carbon dioxide concentrations in seawater — will affect biogeochemical cycles. The separate lines of evidence suggest that ocean acidification will have far-reaching effects on the export of elements to the deep ocean.

Diatoms are highly efficient at converting dissolved CO2 into organic carbon through photosynthesis, whereupon this organic carbon becomes incorporated into particles that sink rapidly to the deep ocean. Diatoms therefore serve as primary engines of a ‘biological pump’ that exports carbon to the deep ocean for sequestration. Each diatom cell is enclosed in a shell of silica (SiO2, where Si is silicon), and the solubility of the silicon in this biomineral is pH-sensitive — it becomes less soluble as seawater acidity rises. Although these features of diatoms are familiar to marine scientists, their combined implications for future biogeochemical cycles in the context of ocean acidification had not been explored.

Enter Taucher and colleagues. They carried out a series of five experiments in various parts of the ocean in which natural phytoplankton communities were grown in large enclosures (with volumes of 35–75 cubic metres) known as mesocosms, which simulated future ocean acidification. When the authors measured the elemental composition of the diatom-derived debris at the bottom of the mesocosms, they observed much higher ratios of silicon to nitrogen than the ratios of particles suspended near the surface. This suggested that, at low seawater pH, diatom silica shells were dissolving much more slowly than nitrogen-containing compounds in the same sinking material. In other words, silicon was being exported from the surface to deeper waters preferentially to nitrogen. The authors validated this finding using records of silicon-to-nitrogen ratios in sinking biological detritus in the open ocean, measured as a function of seawater pH, and obtained from particle-collecting sediment traps deployed by research vessels.

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Ocean acidification alters the predator – prey relationship between hydrozoa and fish larvae

Anthropogenic CO2 emissions cause a drop in seawater pH and shift the inorganic carbon speciation. Collectively, the term ocean acidification (OA) summarizes these changes. Few studies have examined OA effects on predatory plankton, e.g. Hydrozoa and fish larvae as well as their interaction in complex natural communities. Because Hydrozoa can seriously compete with and prey on other higher-level predators such as fish, changes in their abundances may have significant consequences for marine food webs and ecosystem services. To investigate the interaction between Hydrozoa and fish larvae influenced by OA, we enclosed a natural plankton community in Raunefjord, Norway, for 53 days in eight ≈ 58 m³ pelagic mesocosms. CO2 levels in four mesocosms were increased to ≈ 2000 µatm pCO2, whereas the other four served as untreated controls. We studied OA-induced changes at the top of the food web by following ≈2000 larvae of Atlantic herring (Clupea harengus) hatched inside each mesocosm during the first week of the experiment, and a Hydrozoa population that had already established inside the mesocosms. Under OA, we detected 20% higher abundance of hydromedusae staged jellyfish, but 25% lower biomass. At the same time, survival rates of Atlantic herring larvae were higher under OA (control pCO2: 0.1%, high pCO2: 1.7%) in the final phase of the study. These results indicate that a decrease in predation pressure shortly after hatch likely shaped higher herring larvae survival, when hydromedusae abundance was lower in the OA treatment compared to control conditions. We conclude that indirect food-web mediated OA effects drove the observed changes in the Hydrozoa – fish relationship, based on significant changes in the phyto-, micro-, and mesoplankton community under high pCO2. Ultimately, the observed immediate consequences of these changes for fish larvae survival and the balance of the Hydrozoa – fish larvae predator – prey relationship has important implications for the functioning of oceanic food webs.

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Dimethylsulfoniopropionate (DMSP) and dimethyl sulfide (DMS) dynamics in the surface ocean

Dimethyl sulfide (DMS) is a trace gas produced in the ocean that plays an important role in climate and contributes to the Earths energy balance. DMS is a product of the enzymatic cleavage of dimethyl sulfoniopropionate (DMSP), which is produced by certain phytoplankton species and bacteria. Processes within the DMS/P cycles in the surface ocean are complex and vary with time and space. In the sea surface microlayer (SML), which is the interface between the ocean and the atmosphere, DMS concentration may be altered relative to subsurface water (SSW), by elevated biological activity, light intensity, and gas exchange. The aim of this thesis is to determine the importance of the SML in DMS/P dynamics and air-sea exchange by developing a more robust technique for SML sampling to better understand the dynamics of DMSP and DMS, and comparing their dynamics in the SML and SSW in coastal waters and the open ocean. In addition, the impact of warming and ocean acidification on DMS/P dynamics is investigated to determine how they will be impacted by future climate change.

To characterize DMS dynamics in the SML, a more effective method for sampling trace gases in the SML was developed (Chapter 2). The method is reliant on diffusion through a gas-permeable tube due to the concentration gradient. The floating tube was tested and calibrated under semi-controlled conditions using coastal water, where its reproducibility, accuracy and effectiveness were established. The potential benefits of this new technique for sampling trace gases in the SML include reduced loss of DMS to air. The higher reproducibility and accuracy compared to other techniques confirmed the potential of the floating tube technique for trace gas measurement in the SML.

The method developed in Chapter 2 was applied in sampling of DMS in the SML along a coastal-open ocean gradient (Chapter 3), and in various water masses of the open ocean (Chapter 4). In both chapters, DMSP and DMS dynamics were related to biological, biogeochemical, and physical properties of the SML and SSW. Sampling was conducted over 3 months at three different stations with different degrees of coastal and open water influence around Wellington, New Zealand in Chapter 3. DMSP was significantly enriched in the SML in most sampling events and DMSP and DMS enrichments were influenced by biological production and bacterial consumption. Overall, there was no temporal trends or coastal-offshore gradient in DMS or related biogeochemical variables in the SML. However, DMS concentration, and also DMS to DMSP ratio, were significantly correlated with solar radiation indicating a role for light as a determinant of DMSP and DMS in the SML. In open ocean waters around the Chatham Rise, east of New Zealand, the SML and SSW in water masses of different phytoplankton composition and biomass were sampled (Chapter 4). There was no chlorophyll a enrichment in the SML, and bacterial and DMSP enrichment were only apparent at one station, despite sampling within a phytoplankton bloom. Furthermore, there were no relationships between DMSP and phytoplankton biomass or community composition in the SML, although DMSP was negatively correlated with PAR. DMS was only significantly enriched in the SML at one station. DMS and DMSP concentrations were correlated in both SML and SSW, with the differing slopes attributed to DMS loss in the SML. Daily deck incubations were carried out to quantify DMSP and DMS processes in the SML, including the net effect of light on DMS/P, bacterial consumption of DMS/P and DMS production, and DMS air-sea flux. Air-sea flux was the main pathway with a DMS flux of 1.0-11.0 µmol m-2 d-1 that concurs with climatological predictions for the region. Excluding air-sea emission, biological DMS production was the dominant process in the SML relative to biological consumption and the net effect of light. SML DMS yield was not significantly different to that in the SSW, and consequently processes within the SML do not significantly affect regional DMS emissions.

The impact of ocean acidification and warming on DMSP and DMS concentrations was established for New Zealand coastal waters. Four mesocosm experiments, in which temperature and pH were manipulated to values projected for the years 2100 and 2150, were carried out over three years with the initial phytoplankton community differing in composition and bloom status (Chapter 6:). Temporal changes in DMSP and DMS were established and linked to changes in community composition and biogeochemistry. Results indicate that future warming may have greater influence on DMS production than ocean acidification. The observed reduction in DMSP at warmer temperatures was associated with changes in phytoplankton community, and in particular with a decrease in small flagellates. As nutrient availability also influenced the response this should also be considered in models of future DMS. Although DMS concentration decreased under future conditions of ocean acidification and higher temperature, this decrease was not as significant as reported by other studies.

The results in this thesis contribute to a better understanding of DMSP and DMS dynamics in the surface ocean. The floating tube method developed to sample DMS in the SML, will permit the study of DMS in the SML in various oceanic regions with improved accuracy. This technique may also have potential for measuring other trace gases in the SML. Application of this technique in coastal and open ocean waters demonstrated differences in DMS dynamics in the SML between these regions. DMS enrichment in the SML was rarely found, and DMS enrichment does not affect DMS air-sea flux significantly. Biological and biogeochemical variables and DMS/P process rates need to be established to further understanding of DMS/P dynamics in the SML and near surface water. Finally, results suggest that impacts of future climate change on DMS emissions may not be as significant as reported elsewhere, but that phytoplankton community composition plays a role and must be considered in future scenario models to better predict future DMS emissions. 

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The potential of kelp Saccharina japonica in shielding Pacific oyster Crassostrea gigas from elevated seawater pCO2 stress

Ocean acidification (OA) caused by elevated atmospheric CO2 concentration is predicted to have negative impacts on marine bivalves in aquaculture. However, to date, most of our knowledge is derived from short-term laboratory-based experiments, which are difficult to scale to real-world production. Therefore, field experiments, such as this study, are critical for improving ecological relevance. Due to the ability of seaweed to absorb dissolved carbon dioxide from the surrounding seawater through photosynthesis, seaweed has gained theoretical attention as a potential partner of bivalves in integrated aquaculture to help mitigate the adverse effects of OA. Consequently, this study investigates the impact of elevated pCO2 on the physiological responses of the Pacific oyster Crassostrea gigas in the presence and absence of kelp (Saccharina japonica) using in situ mesocosms. For 30 days, mesocosms were exposed to six treatments, consisting of two pCO2 treatments (500 and 900 μatm) combined with three biotic treatments (oyster alone, kelp alone, and integrated kelp and oyster aquaculture). Results showed that the clearance rate (CR) and scope for growth (SfG) of C. gigas were significantly reduced by elevated pCO2, whereas respiration rates (MO2) and ammonium excretion rates (ER) were significantly increased. However, food absorption efficiency (AE) was not significantly affected by elevated pCO2. The presence of S. japonica changed the daytime pHNBS of experimental units by ~0.16 units in the elevated pCO2 treatment. As a consequence, CR and SfG significantly increased and MO2 and ER decreased compared to C. gigas exposed to elevated pCO2 without S. japonica. These findings indicate that the presence of S. japonica in integrated aquaculture may help shield C. gigas from the negative effects of elevated seawater pCO2.

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Effect of temperature and pH on the Millepora alcicornis and Mussismilia harttii corals in light of a spectral reflectance response

The increase in carbon dioxide (CO2) atmospheric levels contributes to the rise in temperature and ocean acidification; consequently, it directly impacts coral reefs. The increase in seawater temperature is the primary factor that causes the collapse of coral-algal symbiosis, which can be followed by coral death and, generally, ocean acidification impairs biogenic calcification and promotes dissolution of carbonate substrata. These harmful effects on corals associated with the continuous increase in CO2 atmospheric levels raise widespread concerns about the coral reef decline, intensifying the efforts to understand/monitor their effects on these organisms. The objective of this study was to evaluate the physiological effect of temperature increase, water acidification (i.e. decrease in pH), and their effects combined (temperature increase with water acidification), through the reflectance analyses and maximum photosynthetic capacity of zooxanthellae (Fv/Fm) in two coral species: Millepora alcicornis and Mussismilia harttii. Fragments of four large colonies of each specie were collected, fragmented, and submitted to four different treatments for 15 days: (i) control treatment (under identical temperature and pH conditions observed in the sampling seawater site), (ii) temperature treatment (with an increase temperature of around ≅2ºC); (iii) water acidification treatment (with a decrease of nearly 0.3 in pH); and (iv) a treatment of combined effects from water temperature rising and acidification. Spectral reflectance and Fv/Fm were measured from samples of these species in a marine mesocosm. Data of reflectance, first and second-order derivative, area under the curve, full width at half maximum (FWHM), depth values and the Fv/Fm were used to classify the coral species and treatments through the linear discriminant analysis (LDA). Coral samples were exposed to the increased temperature bleached, whilst decreased pH caused a slight reduction in reflectance albedo with minimal effects on Fv/Fm. The combined factors (treatment iv) triggered a bleaching response, presenting spectral reflectance and colouring patterns similar to those observed in bleached corals, especially for M. alcicornis. The two-way ANOVA indicated statistically meaningful spectral differences between treatments for the second-order derivatives at 634 nm and for Fv/Fm values. However, there was no statistically meaningful interaction effect due to the treatment type and coral species response for the second-order derivative at 670 nm and to the Fv/Fm values. LDA classified the corals’ species and the corals in different treatment, using their spectral responses and Fv/Fm results, with high accuracy (96.7% and 73.3%, respectively), reinforcing its application for coral physiology evaluation and species classification. The control and combined groups achieved the best classification scores, with only one misclassification.

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Effects of ocean acidification on Lythrypnus dalli reproductive output and behavior

Reproduction in fishes is an energetically costly but vital process that is important to nearshore fisheries and proper ecosystem functioning. Successful fish reproduction is generally limited to a narrow breadth of specific environmental conditions, and variation in these conditions may affect the ability of fish to allocate energy towards reproduction. In particular, ocean acidification (OA) is generally assumed to be a major threat to fish reproduction, but past studies on the effects of OA have produced variable results. To examine how OA affects bluebanded goby (Lythrypnus dalli) reproduction, female reproductive output and male reproductive behaviors were quantified under two experimental treatments that represent differences between present-day (ambient) and future OA (decreased by 0.2 pH units) conditions. To do this, sexually mature bluebanded gobies were placed in laboratory mesocosms for continuous seven-day trials and allowed to reproduce in artificial nests. Four artificial nests were placed in each of the four mesocosms to provide fish with similar nesting habitats to encourage reproduction. Each mesocosm included similar fish size structures and numbers of female gobies to control for any size- or sex-dependent responses. Male reproductive behavior was quantified daily through visual assessment of their movement patterns within each mesocosm. Female reproductive output was quantified by checking the nests for the presence of eggs, which were photographed to evaluate egg quantity, size, and development. Results indicate that future OA conditions did not significantly affect any of the reproducti on metrics examined in this study. These results suggest that future changes in environmental factors such as seawater pH may not have dramatic effects on the reproductive output and behavior of bluebanded gobies.

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No evidence of altered relationship between diet and consumer fatty acid composition in a natural plankton community under combined climate drivers

Fatty acids (FA), especially polyunsaturated fatty acids (PUFA), are key biomolecules involved in immune responses, reproduction, and membrane fluidity. PUFA in marine environments are synthesized exclusively by primary producers. Therefore the FA composition of these organisms at the base of the food web (i.e., phytoplankton) and their primary consumers (i.e., zooplankton) are important determinants of the health and productivity of entire ecosystems as they are transferred to higher trophic levels. However, environmental conditions such as seawater pH and temperature, which are already changing in response to climate change and predicted to continue to change in the future, can affect the FA composition of phytoplankton and zooplankton at both the organismal and community level. During a 20 day mesocosm experiment, we tested the effect of ocean acidification alone and in combination with ocean warming on 1) the fatty acid composition of a natural prey community for zooplankton (i.e. phytoplankton and microzooplankton), 2) the fatty acid composition of zooplankton, and 3) the relationship between prey and consumer fatty acid compositions in coastal waters. Significant effects of the climate stressors were not detected in the fatty acid composition of the prey or the relationship between diet and consumer fatty acids. A significant decrease in C18:4n-3 (stearidonic acid) was observed in the zooplankton but not their diet, but understanding the mechanism behind this decrease and its potential biological implications requires further investigation. These results highlight the importance of multi-stressor investigations on dynamics and variability contained within natural coastal plankton communities.

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Phenotypic responses in fish behaviour narrow as climate ramps up

Natural selection alters the distribution of phenotypes as animals adjust their behaviour and physiology to environmental change. We have little understanding of the magnitude and direction of environmental filtering of phenotypes, and therefore how species might adapt to future climate, as trait selection under future conditions is challenging to study. Here, we test whether climate stressors drive shifts in the frequency distribution of behavioural and physiological phenotypic traits (17 fish species) at natural analogues of climate change (CO2 vents and warming hotspots) and controlled laboratory analogues (mesocosms and aquaria). We discovered that fish from natural populations (4 out of 6 species) narrowed their phenotypic distribution towards behaviourally bolder individuals as oceans acidify, representing loss of shyer phenotypes. In contrast, ocean warming drove both a loss (2/11 species) and gain (2/11 species) of bolder phenotypes in natural and laboratory conditions. The phenotypic variance within populations was reduced at CO2 vents and warming hotspots compared to control conditions, but this pattern was absent from laboratory systems. Fishes that experienced bolder behaviour generally showed increased densities in the wild. Yet, phenotypic alterations did not affect body condition, as all 17 species generally maintained their physiological homeostasis (measured across 5 different traits). Boldness is a highly heritable trait that is related to both loss (increased mortality risk) and gain (increased growth, reproduction) of fitness. Hence, climate conditions that mediate the relative occurrence of shy and bold phenotypes may reshape the strength of species interactions and consequently alter fish population and community dynamics in a future ocean.

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Coral calcification mechanisms in a warming ocean and the interactive effects of temperature and light

Ocean warming is transforming the world’s coral reefs, which are governed by the growth of marine calcifiers, most notably branching corals. Critical to skeletal growth is the corals’ regulation of their internal chemistry to promote calcification. Here we investigate the effects of temperature and light on the calcifying fluid chemistry (using boron isotope systematics), calcification rates, metabolic rates and photo-physiology of Acropora nasuta during two mesocosm experiments simulating seasonal and static temperature and light regimes. Under the seasonal regime, coral calcification rates, calcifying fluid carbonate chemistry, photo-physiology and metabolic productivity responded to both changes in temperature and light. However, under static conditions the artificially prolonged exposure to summer temperatures resulted in heat stress and a heightened sensitivity to light. Our results indicate that temperature and light effects on coral physiology and calcification mechanisms are interactive and context-specific, making it essential to conduct realistic multi-variate dynamic experiments in order to predict how coral calcification will respond to ocean warming.

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Can seagrass modify the effects of ocean acidification on oysters?

Highlights

  • Effect of seagrass on oyster shell growth and extracellular pH at ambient pCO2
  • No effects of seagrass on oyster growth or extracellular pH at elevated pCO2
  • Microbiome of oysters was not affected by elevated pCO2.
  • Microbiome of seagrass was altered by elevated pCO2.
  • Seagrass may not modify the impacts of climate change.

Abstract

Solutions are being sought to ameliorate the impacts of anthropogenic climate change. Seagrass may be a solution to provide refugia from climate change for marine organisms. This study aimed to determine if the seagrass Zostera muelleri sub spp. capricorni benefits the Sydney rock oyster Saccostrea glomerata, and if these benefits can modify any anticipated negative impacts of ocean acidification. Future and ambient ocean acidification conditions were simulated in 52 L mesocosms at control (381 μatm) and elevated (848 μatm) CO2 with and without Z. muelleri. Oyster growth, physiology and microbiomes of oysters and seagrass were measured. Seagrass was beneficial to oyster growth at ambient pCO2, but did not positively modify the impacts of ocean acidification on oysters at elevated pCO2. Oyster microbiomes were altered by the presence of seagrass but not by elevated pCO2. Our results indicate seagrasses may not be a panacea for the impacts of climate change.

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An integrated multiple driver mesocosm experiment reveals the effect of global change on planktonic food web structure

Global change puts coastal marine systems under pressure, affecting community structure and functioning. Here, we conducted a mesocosm experiment with an integrated multiple driver design to assess the impact of future global change scenarios on plankton, a key component of marine food webs. The experimental treatments were based on the RCP 6.0 and 8.5 scenarios developed by the IPCC, which were Extended (ERCP) to integrate the future predicted changing nutrient inputs into coastal waters. We show that simultaneous influence of warming, acidification, and increased N:P ratios alter plankton dynamics, favours smaller phytoplankton species, benefits microzooplankton, and impairs mesozooplankton. We observed that future environmental conditions may lead to the rise of Emiliania huxleyi and demise of Noctiluca scintillans, key species for coastal planktonic food webs. In this study, we identified a tipping point between ERCP 6.0 and ERCP 8.5 scenarios, beyond which alterations of food web structure and dynamics are substantial.

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Influence of ocean warming and acidification on habitat-forming coralline algae and their associated molluscan assemblages

Highlights

  • We assessed whether ocean warming and acidification impacts habitat-forming coralline algal turfs and their associated molluscan assemblages.
  • Ocean warming negatively impacted the cover and photosynthetic efficiency of coralline fronds.
  • Ocean acidification caused a 56% and a 59% reduction in the biomass and frond density of coralline turfs, respectively.
  • Ocean acidification caused a decrease in the richness and abundance of molluscs in coralline turfs by 43% and 61%, respectively.

Abstract

When ocean warming and acidification impact habitat-forming species, substantial alterations to the supported ecological communities and associated ecosystems are likely to follow. Here, we used experimental manipulations to test the hypotheses that ocean warming and acidification would negatively affect habitat-forming coralline algal turfs and the diverse molluscan assemblages they support. Boulders covered in a turf of Amphiroa anceps with intact faunal assemblages were subjected to an orthogonal combination of current (~ 23 °C) and future (~ 26 °C) ocean temperatures, and current (~ 430 µatm) and future (~ 880 µatm) seawater pCO2. Ocean warming negatively impacted the cover and photosynthetic efficiency of Amphiroa fronds, whereas ocean acidification caused the biomass per unit area and the frond density of Amphiroa turf to be 56% and 59% less than current ocean conditions, respectively. Ocean acidification also caused a significant change in the structure of molluscan assemblages associated with Amphiroa turf, which included a 43% and a 61% reduction in the species richness and overall abundance of molluscs, respectively. The results demonstrate that coralline algal turfs are particularly vulnerable to ocean climate change, which has implications for the biodiversity and ecosystem functions supported by these globally distributed foundation species.

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Mussels repair shell damage despite limitations imposed by ocean acidification

Bivalves frequently withstand shell damage that must be quickly repaired to ensure survival. While the processes that underlie larval shell development have been extensively studied within the context of ocean acidification (OA), it remains unclear whether shell repair is impacted by elevated pCO2. To better understand the stereotypical shell repair process, we monitored mussels (Mytilus edulis) with sublethal shell damage that breached the mantle cavity within both field and laboratory conditions to characterize the deposition rate, composition, and integrity of repaired shell. Results were then compared with a laboratory experiment wherein mussels (Mytilus trossulus) repaired shell damage in one of seven pCO2 treatments (400–2500 µatm). Shell repair proceeded through distinct stages; an organic membrane first covered the damaged area (days 1–15), followed by the deposition of calcite crystals (days 22–43) and aragonite tablets (days 51–69). OA did not impact the ability of mussels to close drill holes, nor the microstructure, composition, or integrity of end-point repaired shell after 10 weeks, as measured by µCT and SEM imaging, energy-dispersive X-ray (EDX) analysis, and mechanical testing. However, significant interactions between pCO2, the length of exposure to treatment conditions, the strength and inorganic content of shell, and the physiological condition of mussels within OA treatments were observed. These results suggest that while OA does not prevent adult mussels from repairing or mineralizing shell, both OA and shell damage may elicit stress responses that impose energetic constraints on mussel physiology.

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Physiological acclimatization in Hawaiian corals following a 22-month shift in baseline seawater temperature and pH

Climate change poses a major threat to coral reefs. We conducted an outdoor 22-month experiment to investigate if coral could not just survive, but also physiologically cope, with chronic ocean warming and acidification conditions expected later this century under the Paris Climate Agreement. We recorded survivorship and measured eleven phenotypic traits to evaluate the holobiont responses of Hawaiian coral: color, Symbiodiniaceae density, calcification, photosynthesis, respiration, total organic carbon flux, carbon budget, biomass, lipids, protein, and maximum Artemia capture rate. Survivorship was lowest in Montipora capitata and only some survivors were able to meet metabolic demand and physiologically cope with future ocean conditions. Most M. capitata survivors bleached through loss of chlorophyll pigments and simultaneously experienced increased respiration rates and negative carbon budgets due to a 236% increase in total organic carbon losses under combined future ocean conditions. Porites compressa and Porites lobata had the highest survivorship and coped well under future ocean conditions with positive calcification and increased biomass, maintenance of lipids, and the capacity to exceed their metabolic demand through photosynthesis and heterotrophy. Thus, our findings show that significant biological diversity within resilient corals like Porites, and some genotypes of sensitive species, will persist this century provided atmospheric carbon dioxide levels are controlled. Since Porites corals are ubiquitous throughout the world’s oceans and often major reef builders, the persistence of this resilient genus provides hope for future reef ecosystem function globally.

Continue reading ‘Physiological acclimatization in Hawaiian corals following a 22-month shift in baseline seawater temperature and pH’

Temporal dynamics of surface ocean carbonate chemistry in response to natural and simulated upwelling events during the 2017 coastal El Niño near Callao, Peru

Oxygen minimum zones (OMZs) are characterized by enhanced carbon dioxide (CO2) levels and low pH and are being further acidified by uptake of anthropogenic atmospheric CO2. With ongoing intensification and expansion of OMZs due to global warming, carbonate chemistry conditions may become more variable and extreme, particularly in the eastern boundary upwelling systems. In austral summer (February–April) 2017, a large-scale mesocosm experiment was conducted in the coastal upwelling area off Callao (Peru) to investigate the impacts of ongoing ocean deoxygenation on biogeochemical processes, coinciding with a rare coastal El Niño event. Here we report on the temporal dynamics of carbonate chemistry in the mesocosms and surrounding Pacific waters over a continuous period of 50 d with high-temporal-resolution observations (every second day). The mesocosm experiment simulated an upwelling event in the mesocosms by addition of nitrogen (N)-deficient and CO2-enriched OMZ water. Surface water in the mesocosms was acidified by the OMZ water addition, with pHT lowered by 0.1–0.2 and pCO2 elevated to above 900 µatm. Thereafter, surface pCO2 quickly dropped to near or below the atmospheric level (405.22 µatm in 2017; Dlugokencky and Tans, 2021; NOAA/Global Monitoring Laboratory (GML)) mainly due to enhanced phytoplankton production with rapid CO2 consumption. Further observations revealed that the dominance of the dinoflagellate Akashiwo sanguinea and contamination of bird excrements played important roles in the dynamics of carbonate chemistry in the mesocosms. Compared to the simulated upwelling, natural upwelling events in the surrounding Pacific waters occurred more frequently with sea-to-air CO2 fluxes of 4.2–14.0 mmol C m−2 d−1. The positive CO2 fluxes indicated our site was a local CO2 source during our study, which may have been impacted by the coastal El Niño. However, our observations of dissolved inorganic carbon (DIC) drawdown in the mesocosms suggest that CO2 fluxes to the atmosphere can be largely dampened by biological processes. Overall, our study characterized carbonate chemistry in nearshore Pacific waters that are rarely sampled in such a temporal resolution and hence provided unique insights into the CO2 dynamics during a rare coastal El Niño event.

Continue reading ‘Temporal dynamics of surface ocean carbonate chemistry in response to natural and simulated upwelling events during the 2017 coastal El Niño near Callao, Peru’

Elevated pCO2 changes community structure and function by affecting phytoplankton group-specific mortality

Highlights

  • Persistent CO2 differences influenced the natural communities in mesocosm encloser.
  • CO2 enrichment supported higher Chl-a concentrations.
  • Elevated pCO2 decreased the mortality of the total phytoplankton and dominant small-sized diatoms.
  • Elevation of CO2 enhance small-sized diatom dominance by reducing mortality in coastal ecosystems.

Abstract

The rise of atmospheric pCO2 has created a number of problems for marine ecosystem. In this study, we initially quantified the effects of elevated pCO2 on the group-specific mortality of phytoplankton in a natural community based on the results of mesocosm experiments. Diatoms dominated the phytoplankton community, and the concentration of chlorophyll a was significantly higher in the high-pCO2 treatment than the low-pCO2 treatment. Phytoplankton mortality (percentage of dead cells) decreased during the exponential growth phase. Although the mortality of dinoflagellates did not differ significantly between the two pCO2 treatments, that of diatoms was lower in the high-pCO2 treatment. Small diatoms dominated the diatom community. Although the mortality of large diatoms did not differ significantly between the two treatments, that of small diatoms was lower in the high-pCO2 treatment. These results suggested that elevated pCO2 might enhance dominance by small diatoms and thereby change the community structure of coastal ecosystems.

Continue reading ‘Elevated pCO2 changes community structure and function by affecting phytoplankton group-specific mortality’

Ocean acidification induces distinct metabolic responses in subtropical zooplankton under oligotrophic conditions and after simulated upwelling

Highlights

  • Effects of rising atmospheric CO2 on zooplankton were studied in coastal mesocosms.
  • Mesozooplankton metabolism was more affected by elevated CO2 than microzooplankton.
  • CO2-induced effects were more marked in eutrophic than in oligotrophic conditions.
  • Elevated CO2 impacts the role of zooplankton on the carbon and nitrogen cycles.

Abstract

Ocean acidification (OA) is one of the most critical anthropogenic threats to marine ecosystems. While significant ecological responses of plankton communities to OA have been revealed mainly by small-scale laboratory approaches, the interactive effect of OA-related changes on zooplankton metabolism and their biogeochemical implications in the natural environment still remains less well understood. Here, we explore the responses of zooplankton respiration and ammonium excretion, two key processes in the nutrient cycling, to high pCO2 levels in a 9-week in situ mesocosm experiment conducted during the autumn oligotrophic season in the subtropical northeast Atlantic. By simulating an upwelling event halfway through the study, we further evaluated the combined effects of OA and nutrient availability on the physiology of micro-and mesozooplankton. OA conditions generally resulted in a reduction in the biomass-specific metabolic and enzymatic rates, particularly in the mesozooplankton community. The situation reversed after the nutrient-rich deep-water addition, which initially promoted a diatom bloom and increased heterotrophic activities in all mesocosms. Under high pCO2 conditions (>800 μatm), however, the nutrient fertilization triggered the proliferation of the harmful alga Vicicitus globosus, with important consequences for the metabolic performance of the two zooplankton size classes. Here, the zooplankton contribution to the remineralization of organic matter and nitrogen regeneration dropped by 30% and 24%, respectively, during the oligotrophic period, and by 40% and 70% during simulated upwelling. Overall, our results indicate a potential reduction in the biogeochemical role of zooplankton under future ocean conditions, with more evident effects on the large mesozooplankton and during high productivity events.

Continue reading ‘Ocean acidification induces distinct metabolic responses in subtropical zooplankton under oligotrophic conditions and after simulated upwelling’

Performance of a potentially invasive species of ornamental seaweed Caulerpa sertularioides in acidifying and warming oceans

Caulerpa, a (sub) tropical seaweed, is a notorious taxonomic group and an invasive seaweed worldwide. Similar to several species that have been introduced to benthic habitats through aquariums, Caulerpa sertularioides has also been introduced into Korean aquariums, although it is not native to the region. Thus, it is necessary to evaluate the potential of this species for invading domestic macroalgal habitats. Therefore, an indoor mesocosm experiment was conducted to examine the ecophysiological invasion risk of non-native seaweed C. sertularioides under various climate conditions and exposure to three future climate scenarios: acidification (doubled CO2), warming (5 °C increase from ambient temperature), and greenhouse (GR: combination of acidification and warming); additionally, we compared the invasion risk between future and present climates (control: 20 °C and 470 µatm CO2). High CO2 concentrations and increased temperatures positively affected the photosynthesis and growth of C. sertularioides. Photosynthesis and growth were more synergistically increased under GR conditions than under acidification and warming. Consequently, the performance of this potentially invasive species in the native macroalgal Korean habitat will be higher in the future in coastal environments. Therefore, proper management is required to prevent the geographic expansion of C. sertularioides in the Korean coastal ocean.

Continue reading ‘Performance of a potentially invasive species of ornamental seaweed Caulerpa sertularioides in acidifying and warming oceans’

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