Posts Tagged 'biogeochemistry'

Editorial: the changing carbonate systems in coastal, estuarine, shelf areas and marginal seas

Published 23 November 2023 Science Leave a Comment
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Editorial on the Research Topic
The changing carbonate systems in coastal, estuarine, shelf areas and marginal seas

Global atmospheric CO2 concentrations have increased from 320 ppm in the 1960s to the present-day value of 420 ppm, primarily due to anthropogenic activities. This increase influences the seawater carbonate system, impacting the marine ecosystem. There are still gaps that need to be resolved for predicting how these marine systems respond to current and future CO2 levels. Any actions to mitigate the change in pH will require adaptive management of multiple stressors across several spatial scales. Combined, these perspectives yield a more comprehensive picture of events during ocean acidification (OA).

This Research Topic brings together articles from different regions, including coastal, estuarine, and shelf areas and marginal seas, all susceptible to changing atmospheric conditions, riverine inputs, air-sea CO2 exchanges, and multiple acid-base reactions that can alter carbonate chemistry. Articles on the long-term trends of CO2 system descriptors and the interactions with calcifying organisms were also sought. The present Research Topic is primarily based on original articles devoted to carbonate systems in the marginal seas, but it is a pity that some interesting papers dealing with freshwater inflows, estuaries, and related coastal areas were not accepted.

Fransson et al. examined the effects of glacial and sea-ice meltwater on ocean acidification in the waters near the 79 North Glacier (79 NG) and the northeast Greenland shelf. The researchers investigated various ocean acidification factors and the influence of freshening, primary production, and air-sea CO2 exchange. One of the key findings was that the biological removal of CO2 through primary production played a crucial role in offsetting the negative impact of freshwater dilution on the aragonite saturation state (ΩAr), which is a measure of ocean acidification. This compensation effect was most pronounced in 2012, especially in the vicinity of the 79 NG front, where there was a significant presence of glacial meltwater and surface stratification. In 2016, a different scenario was observed, with a more homogenized water column due to sea-ice meltwater. In this case, the compensation effect of biological CO2 removal on ΩAr was weaker compared to 2012. The study also suggests that in the future, with ongoing climate and ocean chemistry changes, the increasing influence of meltwater may surpass the mitigating effects of biological CO2 removal. This could lead to unfavorable conditions for organisms that rely on calcium carbonate for their shells and skeletons. Thus, all the proposed factors need to be closely monitored as they could have significant implications for marine ecosystems and calcifying organisms in the face of ongoing environmental changes.

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The appendicularian Oikopleura dioica can enhance carbon export in a high CO2 ocean

Published 22 November 2023 Science Leave a Comment
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Gelatinous zooplankton are increasingly recognized to play a key role in the ocean’s biological carbon pump. Appendicularians, a class of pelagic tunicates, are among the most abundant gelatinous plankton in the ocean, but it is an open question how their contribution to carbon export might change in the future. Here, we conducted an experiment with large volume in situ mesocosms (~55–60 m3 and 21 m depth) to investigate how ocean acidification (OA) extreme events affect food web structure and carbon export in a natural plankton community, particularly focusing on the keystone species Oikopleura dioica, a globally abundant appendicularian. We found a profound influence of O. dioica on vertical carbon fluxes, particularly during a short but intense bloom period in the high CO2 treatment, during which carbon export was 42%–64% higher than under ambient conditions. This elevated flux was mostly driven by an almost twofold increase in O. dioica biomass under high CO2. This rapid population increase was linked to enhanced fecundity (+20%) that likely resulted from physiological benefits of low pH conditions. The resulting competitive advantage of O. dioica resulted in enhanced grazing on phytoplankton and transfer of this consumed biomass into sinking particles. Using a simple carbon flux model for O. dioica, we estimate that high CO2 doubled the carbon flux of discarded mucous houses and fecal pellets, accounting for up to 39% of total carbon export from the ecosystem during the bloom. Considering the wide geographic distribution of O. dioica, our findings suggest that appendicularians may become an increasingly important vector of carbon export with ongoing OA.

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Microbial associates of an endemic Mediterranean seagrass enhance the access of the host and the surrounding seawater to inorganic nitrogen under ocean acidification

Published 21 November 2023 Science Leave a Comment
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Seagrasses are important primary producers in oceans worldwide. They live in shallow coastal waters that are experiencing carbon dioxide enrichment and ocean acidification. Posidonia oceanica, an endemic seagrass species that dominates the Mediterranean Sea, achieves high abundances in seawater with relatively low concentrations of dissolved inorganic nitrogen. Here we tested whether microbial metabolisms associated with P. oceanica and surrounding seawater enhance seagrass access to nitrogen. Using stable isotope enrichments of intact seagrass with amino acids, we showed that ammonification by free-living and seagrass-associated microbes produce ammonium that is likely used by seagrass and surrounding particulate organic matter. Metagenomic analysis of the epiphytic biofilm on the blades and rhizomes support the ubiquity of microbial ammonification genes in this system. Further, we leveraged the presence of natural carbon dioxide vents and show that the presence of P. oceanica enhanced the uptake of nitrogen by water column particulate organic matter, increasing carbon fixation by a factor of 8.6–17.4 with the greatest effect at CO2 vent sites. However, microbial ammonification was reduced at lower pH, suggesting that future ocean climate change will compromise this microbial process. Thus, the seagrass holobiont enhances water column productivity, even in the context of ocean acidification.

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Anthropogenic CO2, air-sea CO2 fluxes and acidification in the Southern Ocean: results from a time-series analysis at station OISO-KERFIX (51°S-68°E)

Published 20 November 2023 Science Closed
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The temporal variation of the carbonate system, air-sea CO2 fluxes and pH is analyzed in the Southern Indian Ocean, south of the Polar Front, based on in-situ data obtained from 1985 to 2021 at a fixed station (50°40’S–68°25’E) and results from a neural network model that reconstructs the fugacity of CO2 (fCO2) and fluxes at monthly scale. Anthropogenic CO2 (Cant) was estimated in the water column and detected down to the bottom (1600 m) in 1985 resulting in an aragonite saturation horizon at 600 m that migrated up to 400 m in 2021 due to the accumulation of Cant. In subsurface, the trend of Cant is estimated at +0.53 (±0.01) µmol.kg-1.yr-1 with a detectable increase in recent years. At the surface during austral winter the oceanic fCO2 increased at a rate close or slightly lower than in the atmosphere. To the contrary, in summer, we observed contrasting fCO2 and dissolved inorganic carbon (CT) trends depending on the decade and emphasizing the role of biological drivers on air-sea CO2 fluxes and pH inter-annual variability. The region moved from an annual source of 0.8 molC.m-2.yr-1 in 1985 to a sink of -0.5 molC.m-2.yr-1 in 2020. In 1985–2020, the annual pH trend in surface of -0.0165 (± 0.0040).decade-1 was mainly controlled by anthropogenic CO2 but the trend was modulated by natural processes. Using historical data from November 1962 we estimated the long-term trend for fCO2, CT and pH confirming that the progressive acidification was driven by atmospheric CO2 increase. In 59 years this leads to a diminution of 11 % for both aragonite and calcite saturation state. As atmospheric CO2 will desperately continue rising in the future, the pH and carbonate saturation state will decrease at a faster rate than observed in recent years. A projection of future CT concentrations for a high emission scenario (SSP5-8.5) indicates that the surface pH in 2100 would decrease to 7.32 in winter. This is up to -0.86 lower than pre-industrial pH and -0.71 lower than pH observed in 2020. The aragonite under-saturation in surface waters would be reached as soon as 2050 (scenario SSP5-8.5) and 20 years later for a stabilization scenario (SSP2-4.5) with potential impacts on phytoplankton species and higher trophic levels in the rich ecosystems of the Kerguelen Island area.

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Atlantic-origin water extension into the Pacific Arctic induced an anomalous biogeochemical event

Published 15 November 2023 Science Closed
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The Arctic Ocean is facing dramatic environmental and ecosystem changes. In this context, an international multiship survey project was undertaken in 2020 to obtain current baseline data. During the survey, unusually low dissolved oxygen and acidified water were found in a high-seas fishable area of the western (Pacific-side) Arctic Ocean. Herein, we show that the Beaufort Gyre shrinks to the east of an ocean ridge and forms a front between the water within the gyre and the water from the eastern (Atlantic-side) Arctic. That phenomenon triggers a frontal northward flow along the ocean ridge. This flow likely transports the low oxygen and acidified water toward the high-seas fishable area; similar biogeochemical properties had previously been observed only on the shelf-slope north of the East Siberian Sea.

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Fig. 1: Schematic of the Arctic Ocean circulation and the study area with hydrographic stations.

a, b Maps of the Arctic Ocean and the study area. In a, yellow, blue, and red arrows represent flows from the shelf-slope at the north of the East Siberian Sea (ESS), and from the Pacific and Atlantic oceans in 2017–2020. Ocean circulation and water masses are abbreviated as follows: Beaufort Gyre (BG), Transpolar Drift (TPD), Pacific Water (PW), Lower Halocline Water (LHW), and Atlantic Water (AW). Geographical locations are abbreviated as follows: Canada Basin (CB), Chukchi Plateau (CP), Mendeleyev Ridge (MR), Makarov Basin (MB), and Lomonosov Ridge (LR). In b red, green, and blue dots denote the hydrographic stations conducted by the Research Vessel (R/V) Araon (Korea), R/V Mirai (Japan), and Canadian Coast Guard Ship Louis S. St-Laurent (Canada), under the 2020 Synoptic Arctic Survey project. Black dots indicate other hydrographic stations between 2002 and 2019 listed in Supplementary Table 1.

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The estuarine environment and pH variation: natural limits and experimental observation of the acidification effect on phosphorus bioavailability (in Portuguese)

Published 14 September 2023 Science Closed
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This study shows the variation of pH in the Cananéia-Iguape Estuarine-Lagoon Complex (CIELC). Data from 3 years (2019, 2021, 2022) were obtained in 17 points presenting the following ranges: temperature (14.88-27.05 ºC), pH (7.16-8.40) and DIP (0.20-11.28 µmol L-1) along a saline gradient (0.05-32.09) under different hydrodynamics, biogeochemical processes and anthropogenic influence. The pH buffering capacity due to the presence of weak acid salts in saline water (S ≥ 30) was associated to the lowest DIP, decreasing with low salinity values, confirming the direct correlation among salinity and pH. The highest temperatures in the winter of 2021, corroborated with the abnormal climate event in that year. An in vitro experiment showed results of the interaction of PID and sediments with different textures, with and without the presence of the benthic microbiota under a considerable decreasing of the pH (acidification) in relation to the natural condition of this environment. The P sediment flux characterized Iguape sector as a P sink with or without biota, Ararapira sector as a P source with biota and Cananéia, as P source without biota. The salt water buffered the pH and sediment buffered DIP both associated to the biogeochemical and hydrodynamic processes contribute to the homeostasis in the system.

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Estuarine acidification under a changing climate

Published 11 August 2023 Science Closed
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The increase of anthropogenic carbon dioxide (CO2) has decreased seawater pH and carbonate mineral saturation state, a process known as ocean acidification (OA), which threatens the health of organisms and ecosystems. In estuaries and coastal hypoxic waters, anthropogenic CO2-induced acidification is enhanced by intense respiration and weak acid–base buffer capacity. Here I provide a succinct review of our state of knowledge of drivers for and biogeochemical impacts on estuarine acidification. I will review how river–ocean mixing, air–water gas exchange, biological production–respiration, anaerobic respiration, calcium carbonate (CaCO3) dissolution, and benthic inputs influence aquatic acid–base properties in estuarine waters. I will emphasize the spatial and temporal dynamics of partial pressure of CO2 (pCO2), pH, and calcium carbonate mineral saturation states (Ω), with examples from the Chesapeake Bay, the Mississippi River plume and hypoxic zone, and other estuaries to illustrate how natural and anthropogenic processes may lead to estuarine and coastal acidification.

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Model development to assess carbon fluxes during shell formation in blue mussels

Published 28 July 2023 Newsletters and reports , Science Closed
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In order to quantify the amount of carbonate, precipitated as calcium-carbonate in the shells of blue mussel (Mytilus edulis) in a temperate climate, an existing Dynamic Energy Budget (DEB) model for the blue mussel was adapted by separating shell growth from soft tissue growth. Hereby, two parameters were added to the original DEB-model, a calcification cost [J/mgCaCO3] and an energy allocation fraction [-], which resulted in the energy allocated for structural growth being divided between shell and meat growth. As values for these new parameters were lacking, they were calibrated by fitting the model to field data. Calibration results showed that an Energy allocation fraction of 0.5 and a calcification cost of 0.9 J/mgCaCO3, resulted in the best fit when fitted on 2017 and 2018 field data separately. These values however, show the best fit for data obtained within the first couple of years of the shellfish life, and do not take later years into account. Also it could be discussed that some parameters vary throughout the lifespan of the species. The results were compared to a regular DEB model, where the shell output was calculated through a simple allometric relationship. It is sometimes assumed that the carbon storage in shell material as calcium carbonate could be regarded as a form of carbon sequestration, with a positive impact on the atmospheric CO2 concentrations. However, studies on the physical-chemical processes related to shell formation have shown that from an oceanographic perspective, shell formation should be regarded as a source of atmospheric CO2 rather than a sink. The removal of carbonates, through the biocalcification process, reduces the buffer capacity (alkalinity) of the water to store CO2. As a result CO2 is released from the water to the atmosphere when shell material is formed. The actual amount of CO2 that escapes from the water to the atmosphere as a result of biocalcification depends strongly on local water characteristics. In this study, the effect of calcification by mussels on the CO2 flux to the atmosphere is studied using an adapted DEB model where energy costs of calcification are modelled explicitly. The model was subsequently run under two future climate scenarios, (RCP 4.5 and RCP 8.3) with elevated temperature and decreased pH, and the total released CO2 as a result of shell formation was calculated with the SeaCarb model. This showed growth of mussels, under future climate conditions to be slower, and with that the cumulative shell mass and carbonate precipitated to CaCO3 to decrease. Yet the amount of CO2 released, due to biocalcification, increased. This is due to the fact that the amount of CO2 released/gr of CaCO3 precipitated will be higher, as a result of the decreased buffering capacity of seawater under future climatic environmental conditions.

In summary the conclusions of the project were:

  • Biocalcification (shell formation) of marine organisms, such as bivalves, cannot be regarded as a process resulting in negative CO2 emission to the atmosphere;
  • The actual amount of CO2 that, due to biocalcification, is released from the water to the atmosphere depends on the physicochemical characteristics of the water, which are influenced by (future) climate conditions;
  • Our first model calculations suggest that at future climate conditions mussel’s grow rate will be somewhat reduced. While the amount of CO2 that due to biocalcification, escapes to the atmosphere during its life-time will slightly increase. Making the ratio of g CO2 release/g CaCO3 precipitated slightly higher;
  • Our model calculations should be considered an exercise rather than a definite prediction of how mussels will respond to future climate scenarios. Additional information/experimentation is strongly needed to validate the model settings, and to test the validity of the above mentioned outcome of the model.
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Canada’s marine carbon sink: an early career perspective on the state of research and existing knowledge gaps

Published 26 July 2023 Science Closed
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Improving our understanding of how the ocean absorbs carbon dioxide is critical to climate change mitigation efforts. We, a group of early career ocean professionals working in Canada, summarize current research and identify steps forward to improve our understanding of the marine carbon sink in Canadian national and offshore waters. We have compiled an extensive collection of reported surface ocean air–sea carbon dioxide exchange values within each of Canada’s three adjacent ocean basins. We review the current understanding of air–sea carbon fluxes and identify major challenges limiting our understanding in the Pacific, the Arctic, and the Atlantic Ocean. We focus on ways of reducing uncertainty to inform Canada’s carbon stocktake, establish baselines for marine carbon dioxide removal projects, and support efforts to mitigate and adapt to ocean acidification. Future directions recommended by this group include investing in maturing and building capacity in the use of marine carbon sensors, improving ocean biogeochemical models fit-for-purpose in regional and ocean carbon dioxide removal applications, creating transparent and robust monitoring, verification, and reporting protocols for marine carbon dioxide removal, tailoring community-specific approaches to co-generate knowledge with First Nations, and advancing training opportunities for early career ocean professionals in marine carbon science and technology.

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Evaluating the drivers of air-sea CO2 exchange and ocean acidification in coastal waters around New Zealand

Published 24 July 2023 Science Closed
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Ocean absorption of anthropogenic carbon dioxide (CO2) emissions mitigates the impacts of climate change but also causes ocean acidification, putting marine organisms and ecosystems at risk. Carbon dynamics in coastal environments, driven by interactions with terrestrial processes, benthic ecosystems and unique physical oceanographic processes make these ecosystems more vulnerable to ocean acidification and compounding stressors such as eutrophication, hypoxia, and warming due to climate change. Coastal margins, including shelf seas, represent areas of high biological productivity that provide important economic and cultural marine ecosystem services and play a significant role in the global carbon cycle. However, coastal processes are typically not well represented or constrained in Earth System Models despite their significance to the global carbon budget and sensitivity to human impacts. Understanding the drivers of regional carbon cycles is necessary for predicting how a system will respond to anthropogenic perturbation. The research presented herein focuses on investigating the drivers of seasonal carbon cycle, air-sea CO2 exchange and implications for ocean acidification in the coastal margins around New Zealand. Observational data collected regularly since 1998 at stations spanning subantarctic and subtropical waters, along with 4 years (2015 – 2019) of observations from a coastal ocean observing network were used to evaluate regional carbon cycles. Methods to integrate modelled and reanalysis data with observational data were developed to leverage sparsely sampled datasets to better understand the processes that control the seasonal carbon cycles and drivers of long-term variability. Nearshore coastal environments exhibited the largest seasonal to interannual variability in pH compared to shelf seas, consistent with the influence of terrestrial processes, freshwater fluxes, and dominance of benthic ecosystems on carbonate chemistry in these systems. Overall, subtropical shelf waters in the northern North Island are a stronger sink for atmospheric CO2 (4.66 mol C m-2 y-1) than subantarctic waters off the South Island (0.84 mol C m-2 y-1). CO2 fluxes are driven by air-sea gradients that are controlled by seasonal thermodynamics, biological production, and physical transport. Subtropical sites exhibit a dominance of seasonal temperature variability compared to subantarctic sites. Circulation was found to play a large role the seasonal carbon cycles around New Zealand. Advective fluxes export dissolved inorganic carbon (DIC) from the northeastern shelf of the North Island while they add DIC along the southeastern shelf break of the South Island. Decadal variability in advection along the southeastern shelf is correlated with the El Nino Southern Oscillation and Southern Annual Mode, which has reduced advection of DIC into this region, so maintaining the regional sink strength for atmospheric CO2. These changes in ocean circulation and warming due to climate change have also reduced solubility of CO2 during 2009-2018 by 2%. Simulations using the Regional Ocean Modelling System (ROMS) were used to improve understanding of how terrestrial interactions affect seasonal mixed layer dynamics and carbonate chemistry in coastal and shelf waters off the southeast the South Island. Terrestrial freshwater fluxes were shown to have a large impact on the seasonal salinity and heat budgets which are dominated by advection and turbulent mixing along the Subtropical Front. A coupled biogeochemical ROMS was developed for a domain along the northeastern shelf of the North Island which included the Firth of Thames and Hauraki Gulf. A hydrological model was used to estimate terrestrial fluxes of freshwater, nutrients, organic matter, dissolved oxygen, and heat, which enabled sensitivity analysis of coastal carbonate chemistry and air-sea gas exchange to terrestrial inputs and deconvolution of the seasonal carbon budget. Inner Firth primary production was driven nearly entirely by terrestrial nitrate loading but sensitivity to loading diminished along the land-shelf spatial gradient. Terrestrial organic matter had limited impact on seasonal air-sea exchange and carbon export. The total carbon exported from the Hauraki Gulf was estimated to be ~8 Tg C y-1 with the model, but this may represent an overestimate due to the simplicity of the biological model used. Although this model showed high skill in reproducing seasonal phytoplankton biomass, it did not reproduce hypoxic conditions observed seasonally due to inadequately represented benthic processes. This modelling framework was successful in informing drivers of the seasonal air-sea CO2 exchange across this land-ocean gradient. These studies indicate vulnerability of New Zealand’s coastal ecosystems to climate change and anthropogenic stressors. The results show the relative importance of ocean circulation, biological processes, changes in ocean heat and salinity, and land-ocean interactions in modulating carbon cycling over seasonal to decadal time scales. Methods developed within this research show how model and observational data can be combined to investigate climate change questions important for sustainable resource management.

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Carbonate pump feedbacks on alkalinity and the carbon cycle in the 21st century and beyond

Published 19 July 2023 Science Closed
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Ocean acidification is likely to impact all stages of the ocean carbonate pump, i.e. the production, export, dissolution and burial of biogenic CaCO3. However, the associated feedbacks on anthropogenic carbon uptake and ocean acidification have received little attention. It has previously been shown that Earth system model (ESM) carbonate pump parameterizations can affect and drive biases in the representation of ocean alkalinity, which is critical to the uptake of atmospheric carbon and provides buffering capacity towards associated acidification. In the sixth phase of the Coupled Model Intercomparison Project (CMIP6), we show divergent responses of CaCO3 export at 100 m this century, with anomalies by 2100 ranging from -74 % to +23 % under a high-emissions scenario. The greatest export declines are projected by ESMs that consider pelagic CaCO3 production to depend on the local calcite/aragonite saturation state. Despite the potential effects of other processes on alkalinity, there is a robust negative correlation between anomalies in CaCO3 export and salinity-normalized surface alkalinity across the CMIP6 ensemble. Motivated by this relationship and the uncertainty in CaCO3 export projections across ESMs, we perform idealized simulations with an ocean biogeochemical model and confirm a limited impact of carbonate pump anomalies on twenty-first century ocean carbon uptake and acidification. However between 2100 and 2300, we highlight a potentially abrupt shift in the dissolution of CaCO3 from deep to subsurface waters when the global scale mean calcite saturation state reaches about 1.23 at 500 m (likely when atmospheric CO2 reaches 900 to 1100 ppm). During this shift, upper ocean acidification due to anthropogenic carbon uptake induces deep ocean acidification driven by a substantial reduction in CaCO3 deep dissolution following its decreased export at depth. Although the effect of a diminished carbonate pump on global ocean carbon uptake and surface ocean acidification remains limited until 2300, it can have a large impact on regional air-sea carbon fluxes, particularly in the Southern Ocean.

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Spatial and temporal variations in sea surface pCO2 and air-sea flux of CO2 in the Bering Sea revealed by satellite-based data during 2003–2019

Published 18 July 2023 Science Closed
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The understanding of long-time-series variations in air-sea CO2 flux in the Bering Sea is critical, as it is the passage area from the North Pacific Ocean water to the Arctic. Here, a data-driven remote sensing retrieval method is constructed based on a large amount of underway partial pressure of CO2 (pCO2) data in the Bering Sea. After several experiments, a Gaussian process regression model with input parameters of sea surface temperature, sea surface height, mixed-layer depth, chlorophyll a concentration, dry air mole fractions of CO2, and bathymetry was selected. After validation with independent data, the root mean square error of pCO2 was< 24 μatm (R2 = 0.94) with satisfactory performance. Then, we reconstructed the sea surface pCO2 in the Bering Sea from 2003 to 2019 and estimated the corresponding air-sea CO2 fluxes. Significant seasonal variations were identified, with higher sea surface pCO2 in winter/spring than in summer/autumn in both the basin and shelf area. Semiquantitative analysis reveals that the Bering Sea is a non-temperature-dominated area with a mean temperature effect on pCO2 of 12.7 μatm and a mean non-temperature effect of −51.8 μatm. From 2003 to 2019, atmospheric pCO2 increased at a rate of 2.1 μatm yr−1, while sea surface pCO2 in the basin increased rapidly (2.8 μatm yr−1); thus, the CO2 emissions from the basin increased. However, the carbon sink in the continental shelf still continuously increased. The whole Bering Sea exhibited an increasing carbon sink with the area integral of air-sea CO2 fluxes increasing from 6 to 19 TgC over 17 years. Meanwhile, the seasonal amplitudes in pCO2 in the shelf area also increased, approaching 14 μatm per decade. The reaction of the continuously added CO2 in continental seawater reduced the ocean CO2 system capacity. This is the first study to present long-time-series satellite data with high resolution in the Bering Sea, which is beneficial for studying the changes in ocean ecosystems and carbon sink capacity.

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Feeding in mixoplankton enhances phototrophy increasing bloom-induced pH changes with ocean acidification

Published 14 July 2023 Science Closed
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Plankton phototrophy consumes CO2, increasing seawater pH, while heterotrophy does the converse. Elevation of pH (>8.5) during coastal blooms becomes increasingly deleterious for plankton. Mixoplankton, which can be important bloom-formers, engage in both photoautotrophy and phagoheterotrophy; in theory, this activity could create a relatively stable pH environment for plankton growth. Using a systems biology modelling approach, we explored whether different mixoplankton functional groups could modulate the environmental pH compared to the extreme activities of phototrophic phytoplankton and heterotrophic zooplankton. Activities by most mixoplankton groups do not stabilize seawater pH. Through access to additional nutrient streams from internal recycling with phagotrophy, mixoplankton phototrophy is enhanced, elevating pH; this is especially so for constitutive and plastidic specialist non-constitutive mixoplankton. Mixoplankton blooms can exceed the size of phytoplankton blooms; the synergisms of mixoplankton physiology, accessing nutrition via phagotrophy as well as from inorganic sources, enhance or augment primary production rather than depressing it. Ocean acidification will thus enable larger coastal mixoplankton blooms to form before basification becomes detrimental. The dynamics of such bloom developments will depend on whether the mixoplankton are consuming heterotrophs and/or phototrophs and how the plankton community succession evolves.

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Biological modification of coastal pH depends on community composition and time

Published 14 July 2023 Science Closed
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Biological processes play important roles in determining how global changes manifest at local scales. Primary producers can absorb increased CO2 via daytime photosynthesis, modifying pH in aquatic ecosystems. Yet producers and consumers also increase CO2 via respiration. It is unclear whether biological modification of pH differs across the year, and, if so, what biotic and abiotic drivers underlie temporal differences. We addressed these questions using the intensive study of tide pool ecosystems in Alaska, USA, including quarterly surveys of 34 pools over 1 year and monthly surveys of five pools from spring to fall in a second year. We measured physical conditions, community composition, and changes in pH and dissolved oxygen during the day and night. We detected strong temporal patterns in pH dynamics. Our measurements indicate that pH modification varies spatially (between tide pools) and temporally (across months). This variation in pH dynamics mirrored changes in dissolved oxygen and was associated with community composition, including both relative abundance and diversity of benthic producers and consumers, whose role differed across the year, particularly at night. These results highlight the importance of the time of year when considering the ways that community composition influences pH conditions in aquatic ecosystems.

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The potential role of Posidonia oceanica for mitigating acidification on coastal waters of Europe

Published 12 July 2023 Science Closed
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Ocean acidification is a major environmental concern that has significant ecological., economic, and social implications. The plantation and restoration of seagrass meadows in coastal waters, specifically Posidonia oceanica, is one possible method to combat ocean acidification and has the potential to have a significant positive impact on the marine environment and the overall state of the biosphere. As there has been a decline of Posidonia oceanica of about 30% in the Mediterranean Sea over the past three decades to about 1.2 mio ha in the Mediterranean Sea, the positive effects of the sea grass have diminished due to anthropogenic influence. Still, its importance as a carbon sink should not be underestimated. By using recent literature and different studies that have been analysed of the capacity of sea grass to mitigate the impacts of ocean acidification and the effects on the marine ecosystems, supported by several experiments that have been conducted, this thesis demonstrated the importance of Posidonia oceanica. The experiments showed that seagrass ecosystems have higher pH than ecosystems without seagrass, with a mean difference of 0.43. As the pH is interlocked with the CO2-levels and the oxygen levels, also experiments on these factors have been conducted. In general, the concentration of oxygen with P. oceanica present is 2mg/L higher than without. Equally, the CO2 concentration was lower with P. oceanica present. The Posidonia oceanica meadows present in the Mediterranean Sea are able to fixate about 13,3 mio tons of CO2, which is equal to 0,3% of Europeans CO2 emissions. About 2,8 mio tons of CO2 are sequestered by the sea grass, which is about 0,07% of European CO2 emissions. Furthermore, recent plantation efforts show the successful restoration of seagrass meadows and their overall benefits for the regional environment. Overall, this paper provides valuable insights into the potential role of seagrass meadows in mitigating ocean acidification and improving marine biosystems while providing specific numbers to support its findings.

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Physiological response of an Antarctic cryptophyte to increasing temperature, CO2, and irradiance

Published 12 July 2023 Science Closed
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The Southern Ocean, a globally important CO2 sink, is one of the most susceptible regions in the world to climate change. Phytoplankton of the coastal shelf waters around the Western Antarctic Peninsula have been experiencing rapid warming over the past decades and current ongoing climatic changes will expose them to ocean acidification and high light intensities due to increasing stratification. We conducted a multiple-stressor experiment to evaluate the response of the still poorly studied key Antarctic cryptophyte species Geminigera cryophila to warming in combination with ocean acidification and high irradiance. Based on the thermal growth response of G. cryophila, we grew the cryptophyte at suboptimal (2°C) and optimal (4°C) temperatures in combination with two light intensities (medium light: 100 μmol photons m−2 s−1 and high light [HL]: 500 μmol photons m−2 s−1) under ambient (400 μatm pCO2) and high pCO2 (1000 μatm pCO2) conditions. Our results reveal that G. cryophila was not susceptible to high pCO2, but was strongly affected by HL at 2°C, as both growth and carbon fixation were significantly reduced. In comparison, warming up to 4°C stimulated the growth of the cryptophyte and even alleviated the previously observed negative effects of HL at 2°C. When grown, however, at temperatures above 4°C, the cryptophyte already reached its maximal thermal limit at 8°C, pointing out its vulnerability toward even higher temperatures. Hence, our results clearly indicate that warming and high light and not pCO2 control the growth of G. cryophila.

Continue reading ‘Physiological response of an Antarctic cryptophyte to increasing temperature, CO2, and irradiance’

Effects of pH, eelgrass, and settlement substrate on the growth of juvenile magallana (crassostrea) gigas, a commercially important oyster species

Published 6 July 2023 Science Closed
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Worsening ocean acidification (OA), resulting from ongoing absorption of atmospheric carbon dioxide (CO2) by the oceans, threatens marine life globally. Calcifying organisms, especially their early life stages, are particularly vulnerable; this includes the economically important Pacific oyster, Magallana (Crassostrea) gigas. Uptake of dissolved CO2 through photosynthesis by seagrasses, like eelgrass (Zostera marina), may benefit calcifying organisms by increasing pH and carbonate availability. I conducted laboratory and field experiments to quantify carbonate chemistry modification by eelgrass and potential mitigation of OA impacts on growth in juvenile Pacific oysters. In the laboratory experiment, daytime net photosynthesis by eelgrass increased seawater pH, while nighttime net respiration reduced pH though to a lesser extent; both effects grew stronger as the pH of incoming seawater decreased. This is consistent with the expectation that eelgrass will benefit from increased aqueous CO2 levels and suggests that the importance of carbonate chemistry modification by eelgrass and its role as a refugium may increase as OA proceeds. Under the conditions tested, however, eelgrass effects on pH were modest and did not affect oyster growth in the lab or field. In the lab, oysters settled on shell flour grew faster than those on shell chunks, but unlike those on chunks, the growth rate of oysters on flour decreased significantly in low pH treatments. One hypothesis consistent with these results is that the boundary layer around shell chunks may have slowed oyster growth by limiting food availability but that it also reduced sensitivity to low pH via enhanced carbonate saturation.

Continue reading ‘Effects of pH, eelgrass, and settlement substrate on the growth of juvenile magallana (crassostrea) gigas, a commercially important oyster species’

Methane-derived authigenic carbonates – a case for a globally relevant marine carbonate factory

Published 6 July 2023 Science Closed
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Precipitation of methane-derived authigenic carbonates (MDAC) is an integral part of marine methane production and consumption, but MDAC’s relative significance to the global marine carbon cycle is not well understood. Here we provide a synthesis and perspective to highlight MDAC from a global marine carbon biogeochemistry viewpoint. MDAC formation is a result and archive of carbon‑sulfur (C – S) coupling in the shallow sulfatic zone and carbon‑silicon (C – Si) coupling in deeper methanic sediments. MDAC constitute a carbon sequestration of 3.93 Tmol C yr−1 (range 2.34–5.8 Tmol C yr−1) in the modern ocean and are the third-largest carbon burial mechanism in marine sediments. This burial compares to 29% (11–57%) organic carbon and 10% (6–23%) skeletal carbonate carbon burial along continental margins. MDAC formation is also an important sink for benthic alkalinity and, thereby, a potential contributor to bottom water acidification. Our understanding of the impact of MDAC on global biogeochemical cycles has evolved over the past five decades from what was traditionally considered a passive carbon sequestration mechanism in a seep-oasis setting to what is now considered a dynamic carbonate factory expanding from deep sediments to bottom waters—a factory that has been operational since the Precambrian. We present a strong case for the need to improve regional scale quantification of MDAC accumulation rates and associated carbonate biogeochemical parameters, leading to their incorporation in present and paleo‑carbon budgets in the next phase of MDAC exploration.

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CO2 in the surface ocean

Published 3 July 2023 Science Closed
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The global ocean comprises a significant sink for human-emitted carbon dioxide, yet many different processes are at play, causing strong spatial and temporal variations in the distribution of the sea surface pCO2 and the resulting air-sea CO2 fluxes. While dominated by the temperature-driven solubility, physical transport and biogeochemistry, the increase in the sea surface CO2 partial pressure over the past decades is closely following the increase in atmospheric CO2, resulting in a decreasing pH and decreasing saturation states of calcite and aragonite minerals. Despite the increasing abundance of novel data interpolation tools, e.g. based on machine learning, the heterogeneous distribution of CO2 in the surface ocean requires a dense observing network to reconstruct global change.

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Combined effects of ocean deoxygenation, acidification, and phosphorus limitation on green tide macroalga, Ulva prolifera

Published 29 June 2023 Science Closed
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Highlights

  • Additive and antagonistic interactions between the three stressors were mainly observed.
  • Ocean deoxygenation, acidification, and P limitation can dysregulate the Ulva prolifera of photosynthetic efficiency.
  • Green tide macroalgal Ulva prolifera has a strong acclimation capacity to elevated CO2, low O2, and high N/P.
  • Ulva prolifera could use organic P to support its growth under low inorganic phosphorus conditions.
  • Increased CO2 levels can decrease the energy costs associated with CCM, and can support the growth of macroalgal cells.

Abstract

Ocean deoxygenation, acidification, and decreased phosphorus availability are predicted to increase in coastal ecosystems under future climate change. However, little is known regarding the combined effects of such environmental variables on the green tide macroalga Ulva prolifera. Here, we provide quantitative and mechanistic understanding of the acclimation mechanisms of U. prolifera to ocean deoxygenation, acidification, and phosphorus limitation under both laboratory and semi-natural (mesocosms) conditions. We found that there were significant interactions between these global environmental conditions on algal physiological performance. Although algal growth rate and photosynthesis reduced when the nitrogen-to‑phosphorus (N/P) ratio increased from 16:1 to 35:1 under ambient CO2 and O2 condition, they remained constant with further increasing N/P ratios of 105:1, 350:1, and 1050:1. However, the increasing alkaline phosphatase activities at high N/P ratios suggests that U. prolifera could use organic P to support its growth under phosphorus limitation. Deoxygenation had no effect on specific growth rate (SGR) but decreased photosynthesis under low N/P ratios of 16:1, 35:1, and 105:1, with reduced activities of several enzymes involved in N assimilation pathway being observed. Elevated CO2 promoted algal growth and alleviated the negative effect of deoxygenation on algal photosynthesis. The patterns of responses to high CO2 and low O2 treatments in in situ experiments were generally consistent with those observed in laboratory experiments. Our results generally found that the strong physiological acclimation capacity to elevated CO2, low O2, and high N/P could contribute to its large-scale blooming in coastal ecosystem.

Continue reading ‘Combined effects of ocean deoxygenation, acidification, and phosphorus limitation on green tide macroalga, Ulva prolifera’
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