Posts Tagged 'chemistry'

Organic alkalinity in shallow habitats of San Francisco Estuary

Estuaries are prone to increasing acidification due to growing population and urbanization in addition to global climate change. Acidification is largely studied through measuring or calculating carbonate chemistry parameters (dissolved inorganic carbon, pCO2, pH, total alkalinity), therefore a robust understanding of site carbonate chemistry is key to properly assessing habitat vulnerability to instances of acidification. A challenge in doing ocean acidification (OA) work in estuaries is that in contrast to offshore marine settings, carbonate chemistry in estuaries is more dynamic, varying both spatially and temporally. Carbonate chemistry in estuaries is also more compositionally complex, because of relatively high levels of organic alkalinity (AORG). AORG is normally deemed negligible in marine settings, but it is higher in nearshore environments due to dissolved organic matter inputs from sources such as intertidal salt marshes and terrestrial runoff. Challenges associated with quantifying AORG, and the inherent molecular complexity of AORG, have resulted in very little data in existence and a lot remains unknown pertaining to its prevalence in estuaries. To address this knowledge gap, we conducted a first-order investigation of AORG within shallow habitats of the San Francisco Estuary (SFE) to document how AORG varies spatially and temporally with pH and total alkalinity (TA). In four distinct sites (deep main channel, shallow eelgrass embayment, mudflat, and tidal creek) AORG ranged from non-detectable to 189 µmol/kg, which are comparable to AORG values reported for similar sites in the United States North and Southeast. Calculating pCO2 and saturation states of aragonite and calcite by assuming that AORG is absent resulted in an overestimation of these values as AORG (contributing to TA) ranges ~10%. Our findings show that AORG should be taken into consideration to make accurate carbonate chemistry calculations in estuarine settings.

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Coupling of carbon and oxygen in the Pearl River plume in summer: upwelling, hypoxia, reoxygenation and enhanced acidification


Acidification and hypoxia are universal environmental issues in coastal seas, especially in large river estuaries such as the Pearl River estuary. In July and August of 2015, two legs of a field survey were conducted in the Pearl River plume. Leg 1 was sampled during the influence of upwelling favorable winds, while Leg 2 was during downwelling favorable winds. During both legs, instead of the typically observed dissolved inorganic carbon (DIC) consumption and dissolved oxygen (DO) over-saturation, upwelling-induced high DIC (>2000 μmol kg-1), low pH (7.7-7.8) and low DO (140-150 μmol kg-1) values were observed in surface waters at the estuary mouth and the area off Hong Kong. In the bottom waters, hypoxia, acidification (pH 7.6-7.8) and DIC accumulation (DIC addition of ∼ 100-180 μmol kg-1) were observed. Hypoxia was less severe during Leg 2 compared to Leg 1. The stoichiometry of oxygen depletion to DIC addition was 0.89 for bottom water, suggesting remineralization was dominated by marine sourced organic matter. However, a comparison of data from the two legs showed that the stoichiometry of oxygen consumption to DIC accumulation was significantly higher during Leg 2 (0.73±0.03 for Leg 1 vs. 0.80±0.05 for Leg 2), although N/P ratios were the same (13.54±1.93 for Leg 1 vs. 13.51±2.04 for Leg 2). This phenomenon was attributed mainly to enhanced ventilation (re-oxygenation) under the influence of the downwelling favorable winds during Leg 2. Although ventilation relieves some hypoxia, it might enhance acidification in bottom waters after a short-term ventilation event. The enhanced acidification after short-term ventilation is worthy of further study considering that most hypoxia and acidification are found in shallow coastal seas.

Key Points

  • Coastal upwelling induced acidification accompanied by oxygen consumption in surface water was observed for the first time in the Pearl River estuary
  • Different stoichiometry of oxygen to carbon under influence of upwelling and down-welling favorable winds were observed
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Variability and drivers of carbonate chemistry at shellfish aquaculture sites in the Salish Sea, British Columbia

Ocean acidification reduces seawater pH and calcium carbonate saturation states (Ω), which can have detrimental effects on calcifying organisms such as shellfish. Nearshore areas, where shellfish aquaculture typically operates, have limited data available to characterize variability in key ocean acidification parameters pH and Ω, as samples are costly to analyse and difficult to collect. This study collected samples from four nearshore locations at shellfish aquaculture sites on the Canadian Pacific coast from 2015–2018 and analysed them for dissolved inorganic carbon (DIC) and total alkalinity (TA), enabling the calculation of pH and Ω for all seasons. The study evaluated the diel and seasonal variability in carbonate chemistry conditions at each location and estimated the contribution of drivers to seasonal and diel changes in pH and Ω. Nearshore locations experience a greater range of variability and seasonal and daily changes in pH and Ω than open waters. Biological uptake of DIC by phytoplankton is the major driver of seasonal and diel changes in pH and Ω at our nearshore sites. The study found that freshwater is not a key driver of diel variability, despite large changes over the day in some locations. Shellfish mortality events coincide with highly favourable pH and Ω conditions during summer and are most likely linked to high surface temperatures and disease rather than ocean acidification. To reduce shellfish mortality, shellfish could be hung lower in the water column (5–20 m) to avoid high temperatures and disease, while still experiencing favourable pH and Ω conditions for shellfish.

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Future physical and biogeochemical ocean conditions under climate change along the British Columbia continental margin

Climate change impacts coastal ecosystems through large scale changes in temperature, stratification, circulation and ocean acidification. Here, the potential response of the British Columbia continental margin to climate change is investigated using a regional ocean circulation-biogeochemical model to downscale climate change projections from the Canadian regional and global climate models (CanRCM4/CanESM2) under two Intergovernmental Panel on Climate Change emission scenarios (RCP 4.5 and RCP 8.5). Projections of future physical and biogeochemical conditions for the 2041–2070 period are compared to the recent past (1981–2010). We found an overall annual average warming of >1.6°C in sea surface temperature, increase in stratification in the upper layer, and decrease in surface pH of as much as 0.21. Increasing stratification and changing winds have a limited impact on nitrate availability, phytoplankton biomass and primary production, whilst ocean warming increases primary production by up to 30% in most of the model domain. Increased atmospheric CO2 contributes to acidification over the model domain with a decrease in pH and aragonite saturation (Ωarag) at all depths resulting in an increase of 20 to 32% of the volume of Ωarag ≤1 in the upper 100 m of the continental shelf depending on the climate scenario. Our projected results, therefore, show that future climate change may alter the amount of food available for higher trophic levels and the habitat of benthic species, since bottom waters on the shelf will be undersaturated with respect to aragonite for 2–3 months in mid-summer. Both climate change scenarios results in a similar pattern of changes but projected changes were stronger and more extensive under RCP 8.5 showing the benefit of mitigation efforts in reducing the effect of climate change on marine ecosystem stressors.

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Can an eelgrass dominated bay ameliorate acidification in an urban estuary?

Eelgrass (Zostera marina) beds are known to provide habitat, including nursery habitat, to many ecologically and commercially important species such as Dungeness crab (Metacarcinus magister), Pacific herring (Clupea pallasii), and Olympia oyster (Ostrea lurida). Species that are vulnerable to acidification. Recent research suggests that eelgrass dominated habitats can buffer the effects of acidification through their ability to influence pH and total alkalinity (TA), and thereby the saturation states (𝛀𝛀) of calcite (𝛀𝛀calcite) and aragonite (𝛀𝛀arag). The San Francisco Estuary (SFE) was used as a case study site due to the naturally occurring extensive eelgrass beds within a highly urbanized estuary, which experiences ocean and coastal acidification (OA and CoA), as well as regularly occurring upwelling events. To understand the role of eelgrass community metabolism in altering the pH, TA, and 𝛀𝛀 in the SFE, I compared two major SFE habitats (a deep, mainstream estuarine channel and an eelgrass dominated embayment) using a combination of long-term water quality data and discrete field sampling to examine how they differed in: 1) seasonal and diurnal trends in pH, dissolved oxygen (DO), salinity, and water temperature and 2) tidal trends in pH, TA, and 𝛀𝛀. To better understand changes in TA at the estuary level, the relationship between salinity and TA of the SFE was examined using current and historical data. At a seasonal scale, median pH ranged between 7.8-8.0 at both sites, but rose to as high as pH = 9.0 in the eelgrass dominated habitat during the spring and winter. Strong Pearson correlation between pH and DO (% saturation) in the eelgrass dominated habitat (R2 = 0.51-0.74) suggested community metabolism was an important factor affecting pH at this site compared to the deep, mainstream estuarine channel (R2 = 0.095-0.28), where there was a weak correlation. At a diurnal scale, average pH ranged between 7.89-8.05 in the eelgrass dominated habitat and between 7.82-7.84 in the deep, mainstream estuarine channel. At a tidal scale, pH, TA, 𝛀𝛀calcite, and 𝛀𝛀arag were all greater in the eelgrass dominated habitat compared to the deep, mainstream estuarine channel. Overall, the eelgrass dominated habitat exhibited the potential to provide refugia from acidification in an urban estuary. On an estuarine level, the current relationship between salinity and TA in the SFE showed an increase in TA by more than 150 μmol kg-1 at the freshwater end member since the 1980s. The exact mechanism for this increase in TA at the watershed scale is unknown but could be attributed to a lack of filtration of collected water samples, leaching of alkaline metals into the sample bottles over time, or a profound increase in the bicarbonate flux from significant land use changes from an increase in agricultural watersheds, and thus, an increase in agricultural discharge.

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Upwelling intensity and source water properties drive high interannual variability of corrosive events in the California Current

Ocean acidification is progressing rapidly in the California Current System (CCS), a region already susceptible to reduced aragonite saturation state due to seasonal coastal upwelling. Results from a high-resolution (~ 3 km), coupled physical-biogeochemical model highlight that the intensity, duration, and severity of undersaturation events exhibit high interannual variability along the central CCS shelfbreak. Variability in dissolved inorganic carbon (DIC) along the bottom of the 100-m isobath explains 70–90% of event severity variance over the range of latitudes where most severe conditions occur. An empirical orthogonal function (EOF) analysis further reveals that interannual event variability is explained by a combination coastal upwelling intensity and DIC content in upwelled source waters. Simulated regional DIC exhibits low frequency temporal variability resembling that of the Pacific Decadal Oscillation, and is explained by changes to water mass composition in the CCS. While regional DIC concentrations and upwelling intensity individually explain 9 and 43% of year-to-year variability in undersaturation event severity, their combined influence accounts for 66% of the variance. The mechanistic description of exposure to undersaturated conditions presented here provides important context for monitoring the progression of ocean acidification in the CCS and identifies conditions leading to increased vulnerability for ecologically and commercially important species.

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Dynamics of the seawater carbonate system in the East Siberian Sea: the diversity of driving forces

The East Siberian Sea (ESS) is a large and the shallowest part of the Arctic Ocean. It is characterized by high biogeochemical activity, but the seawater carbonate system remains understudied, especially during the late autumn season. Data from the research vessel (RV) “Professor Multanovsky” cruise were used to assess the dynamics of the seawater carbonate system, air–sea CO2 fluxes, and the calcium carbonate corrosive waters in the two biogeochemical provinces of the ESS shortly before freeze-up. The ESS waters were mainly a sink for atmospheric CO2 due to the limited dispersion of river waters, autumn water cooling, and phytoplankton blooms in its eastern autotrophic province. The mean value of the CO2 air–sea flux was 11.2 mmol m−2 day−1. The rate of CO2 uptake in the eastern ESS was an order of magnitude larger than that in the western ESS. The specific waters and ice cover dynamics determined intensive photosynthesis processes identified on the eastern shelf and in the northern deep oligotrophic waters. A part of the surface and most of the bottom ESS waters were corrosive with respect to calcium carbonate, with the lowest saturation state of aragonite (0.22) in the bottom layer of the eastern ESS. The eastern ESS was the main source of these waters into the deep basin. The observed export of corrosive shelf waters to the deep sea can have a potential impact on the ocean water ecosystem in the case of mixing with layers inhabited by calcifying organisms.

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Long-term trends of pH and inorganic carbon in the Eastern North Atlantic: the ESTOC site

Using 25 years of data from the North-East Atlantic Ocean at the ESTOC site, we confirm the surface ocean is actively absorbing carbon emissions caused by human activities and undergoing ocean acidification. The carbon dioxide is also increasing in the subsurface and deepest waters. Seawater salinity normalized inorganic carbon (NCT), fugacity of CO2 (fCO2) and anthropogenic CO2 increase at a rate of 1.17 ± 0.07 µmol kg -1 , 2.1 ± 0.1 µatm yr -1 and 1.06 ± 0.11 μmol kg −1 yr −1 , respectively, while the ocean pHT fixed to the average temperature of 21ºC, declines at a rate of 0.002 ± 0.0001 pH yr -1 in the first 100 m. These rates are 20% higher than values determined for the period 1995-2010. Over the 25 years, the average surface fCO2 increased by 52.5 µatm while the pHT declined by 0.051 pH units (~13% increase in acidity), like the observed seasonal signal. After 2020, seawater conditions are outside the range of surface fCO2 and pHT seasonal amplitude observed in the 1990s. It was also predicted by the year 2040, fCO2 seawater data will be smaller than atmospheric one and the area will be acting as a sink the full year around.Parameterizations of AT, CT, pHT and fCO2 using observations of water temperature, salinity and dissolved oxygen were determined for the ESTOC site with standard error of estimation of 6.5 µmol kg -1 , 6.8 µmol kg -1 , 0.010 pH and 9.6 µatm, respectively, and were applied to the North-East Atlantic Ocean. The observations and the parameterizations showed that the trends of the carbonate variables along the water column in the eastern subtropical ESTOC region are dominated by anthropogenically induced changes, observed in the whole water profile.

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Estuarine acidification under a changing climate

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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Seasonal production dynamics of high latitude seaweeds in a changing ocean: implications for bottom-up effects on temperate coastal food webs

As the oceans absorb excess heat and CO2 from the atmosphere, marine primary producers face significant changes to their abiotic environments and their biotic interactions with other species. Understanding the bottom-up consequences of these effects on marine food webs is essential to informing adaptive management plans that can sustain ecosystem and cultural services. In response to this need, this dissertation provides an in-depth consideration of the effects of global change on foundational macroalgal (seaweed) species in a poorly studied, yet highly productive region of our world’s oceans. To explore how seaweeds within seasonally dynamic giant kelp forest ecosystems will respond to ocean warming and acidification, I employ a variety of methods: year-round environmental monitoring using an in situ sensor array, monthly subtidal community surveys, and a series of manipulative experiments. I find that a complementary phenology of macroalgal production currently characterizes these communities, providing complex habitat and a nutritionally diverse energy supply to support higher trophic levels throughout the year. I also find that future ocean warming and acidification will lead to substantial shifts in the phenology, quantity and quality of macroalgal production in these systems. My results suggest that the giant kelp Macrocystis pyrifera may be relatively resilient to the effects of global change in future winter and summer seasons at high latitudes. In contrast, the calcifying coralline algae Bossiella orbigniana and Crusticorallina spp. and the understory kelps Hedophyllum nigripes and Neoagarum fimbriatum will experience a suite of negative impacts, especially in future winter conditions. The resulting indirect effects on macroalgal-supported coastal food webs will be profound, with projected reductions in habitat and seasonal food supply on rocky reefs. Coming at a time of heightened interest in seaweed production potential at high latitudes, this dissertation provides a comprehensive evaluation of the future of these foundational organisms in a changing environment.

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Development of optical fibre pH sensors for marine microenvironments

The oceans absorb approximately one-third of the CO2 emission into the atmosphere causing a decline in seawater pH, a process known as ocean acidification (OA). This decline in pH reflects changes to the seawater carbonate system and is expected to have an impact on the marine environment and ecosystems. pH changes along coastal ecosystems are highly variable and knowledge of these marine environments can be used to enhance our understanding of OA impacts on marine organisms.

The micro-to-centimetre thick layer directly surrounding many aquatic organisms is known as the diffusion boundary layer (DBL). The DBL creates a region which reduces the exposure of calcifying species to OA conditions. The pH within the DBL is dependent on light-controlled metabolic activities and exhibits different pH behaviour to bulk seawater. The challenge of detecting in situ pH variations and attributing OA effects highlights a need for a fresh research approach and innovative analyses.

The objective of this research is to develop optical fibre pH sensors capable of continuous pH measurement, and suitable for measuring pH variation in marine microenvironments. The development, characterisation, and applications of optical fibre pH sensors are described. The pH sensing components consist of pH-sensitive indicators immobilised in an optimised sol-gel matrix, minimising indicator leaching without the need for a covalent bond. This research explores two approaches, absorbance-based and fluorescence-based pH sensors.

The absorbance-based sensor applied meta-cresol purple (mCP) as the pH-sensitive indicator. The pH sensor has a usable lifetime of 7 days and a dynamic range of pH 7.4 to 9.7. This self-referencing pH sensor was utilised for real-time pH measurements within the DBL of the seaweed Ulva sp., and successfully used to monitor metabolic activity for 100 hours, achieving a precision of 0.02 pH units. This sensor conformed to the GOA-ON Weather quality guideline and demonstrated its capability to identify short-term variation in biological and environmental studies.

The fluorescence pH sensor utilises a time-domain dual-lifetime referencing scheme (t-DLR). The fluorescence pH sensing materials required the synthesis of pH-sensitive iminocoumarin and the encapsulation of pH-inert reference Ru(dpp)3 in polyacrylonitrile (PAN). The pH is determined from the ratio of the combined excitation intensity to the emission intensity of the reference indicator. This approach allows the signal to be referenced internally, independent of fluorescence dye concentration and variations in excitation light intensity. The t-DLR instrumentation used commercial electronic and optical components, integrated with custom-made electronic circuits. The pH sensor has a dynamic range of pH 7.8 to 9.3 and a precision of 0.02 pH units. The pH sensor was insensitive to changes in salinity and had negligible dye leaching and minimal photobleaching.

This work accomplished the development of mCP-based and dual-layer t-DLR fluorescent-based optical fibre pH sensors. This highlights the versatility of optical fibre pH sensors and the potential for a wider range of applications.

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Variable exposure to multiple climate stressors across the California marine protected area network and policy implications

The efficacy of marine protected areas (MPAs) may be reduced when climate change disrupts the ecosystems and human communities around which they are designed. The effects of ocean warming on MPA functioning have received attention but less is known about how multiple climatic stressors may influence MPAs efficacy. Using a novel dataset incorporating 8.8 million oceanographic observations, we assess exposure to potentially stressful temperatures, dissolved oxygen concentrations, and pH levels across the California MPA network. This dataset covers more than two-thirds of California’s 124 MPAs and multiple biogeographic domains. However, spatial-temporal and methodological patchiness constrains the extent to which systematic evaluation of exposure is possible across the network. Across a set of nine well-monitored MPAs, the most frequently observed combination of stressful conditions was hypoxic conditions (<140 umol/kg) co-occurring with low pH (<7.75). Conversely, MPAs exposed most frequently to anomalously warm conditions were less likely to experience hypoxia and low pH, although exposure to hypoxia varied throughout the 2014–2016 marine heatwaves. Finally, we found that the spatial patterns of exposure to hypoxia and low pH across the MPA network remained stable across years. This multiple stressor analysis both confirms and challenges prior hypotheses regarding MPA efficacy under global environmental change.

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An empirical projection of ocean acidification in southwestern Japan over the 21st century

Most of Japan’s coral reefs are distributed in the Ryukyu Islands, in the southwestern part of Japan. Since they support biodiversity in the tropical and subtropical seas and are vulnerable to ocean acidification as well as ocean heat waves and pollution, projecting acidification over multidecadal or longer periods of time is a topic. Currently, the majority of long-term acidification projections are based on Earth System Models (ESMs), and the validation of these projections relies on intercomparisons among ESMs. This study evaluated the multi-decadal trends in total dissolved inorganic carbon (DIC) around the Ryukyu Islands over the past 25 years from 1995 to 2019. Multiple linear regression using temperature, salinity and time parameters as explanatory variables was applied to evaluate the salinity-normalized dissolved inorganic carbon (nDIC) concentrations. The coefficient of time (+1.15 ± 0.03 μmol kg−1 yr−1) was insignificantly different from the growth rate of nDIC that was calculated from the growth rate of atmospheric CO2 concentrations during the same period. Assuming that nDIC in this region will continue to increase at a rate that is consistent with the expected growth rate of atmospheric CO2 concentrations, we projected future trends of pH and aragonite saturation state (ΩA) under scenarios RCP4.5 and RCP8.5. The empirical projection of acidification by the end of the 21st century was generally consistent with projections based on ESMs. At present, global corals are generally distributed in waters with ΩA > 3.0. According to the empirical projection under the RCP8.5 scenario, ΩA around Okinawa Island would fall below 3.0 in winter in the 2030s and throughout the year in the 2060s.

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Differences in carbonate chemistry up-regulation of long-lived reef-building corals

With climate projections questioning the future survival of stony corals and their dominance as tropical reef builders, it is critical to understand the adaptive capacity of corals to ongoing climate change. Biological mediation of the carbonate chemistry of the coral calcifying fluid is a fundamental component for assessing the response of corals to global threats. The Tara Pacific expedition (2016–2018) provided an opportunity to investigate calcification patterns in extant corals throughout the Pacific Ocean. Cores from colonies of the massive Porites and Diploastrea genera were collected from different environments to assess calcification parameters of long-lived reef-building corals. At the basin scale of the Pacific Ocean, we show that both genera systematically up-regulate their calcifying fluid pH and dissolved inorganic carbon to achieve efficient skeletal precipitation. However, while Porites corals increase the aragonite saturation state of the calcifying fluid (Ωcf) at higher temperatures to enhance their calcification capacity, Diploastrea show a steady homeostatic Ωcf across the Pacific temperature gradient. Thus, the extent to which Diploastrea responds to ocean warming and/or acidification is unclear, and it deserves further attention whether this is beneficial or detrimental to future survival of this coral genus.

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

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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Laboratory experiments in ocean alkalinity enhancement research

Recent concern about the consequences of continuing increases in atmospheric CO2 as a key heat-trapping agent (USGCRP, 2017; IPCC, 2021) have prompted ocean experts to come together to discuss how to provide science-based solutions. Ocean alkalinity enhancement (OAE) is being considered not only as a ocean carbon dioxide removal (CDR) approach, but also as a potential way to mitigate ocean acidification. Over the last two decades, inter-laboratory comparisons have proven valuable in evaluating the reliability of methodologies associated with sampling and analysis of carbonate chemistry parameters, which have been routinely used in ocean acidification research (Bockmon and Dickson, 2015). Given the complexity of processes and mechanisms related to ecosystem responses to OAE, consolidating protocols to ensure compatibility across studies is fundamental for synthesis and upscaling analysis. This chapter provides an overview of best practice in OAE laboratory experimentation and facilitates awareness of the importance of applying standardized methods to promote data re- use, inter-lab comparisons, and transparency. This chapter provides the reader with the tools to (1) identify the criteria to achieve the best laboratory practice and experimental design; (2) provide guidance on the selection of response variables for various purposes (physiological, biogeochemical, ecological, evolutionary) for inter-lab comparisons; (3) offer recommendation for a minimum set of variables that should be sampled and propose additional variables critical for different types of synthesis and upscaling; and (4) identify protocols for standardized measurements of response variables.

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

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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Sea ice loss translates into major shifts in the carbonate environmental conditions in Arctic Shelf Sea

Healthy Arctic marine ecosystems are essential to the food security and sovereignty, culture and wellbeing of Indigenous Peoples in the Arctic. At the same time, Arctic marine ecosystems are highly susceptible to impacts of climate change and ocean acidification. While increasing ocean and air temperatures and melting sea ice act as direct stressors on the ecosystem, they also indirectly enhance ocean acidification, accelerating the associated changes in the inorganic carbon system. Yet, much is to be learned about the current state and variability of the inorganic carbon system in remote places. Here, we present pH and pCO2 time-series (2016–2020) from the Chukchi Ecosystem Observatory. The subsurface observatory is located in the midst of a biological hotspot with high primary productivity and a rich benthic food web that support coastal Iñupiat, whales, ice seals, walrus (Odobenus rosmarus), and Arctic cod (Boreogadus saida). Our observations suggest that near-bottom waters (33 m depth, 13 m above the seafloor) are a high carbon dioxide and low pH and aragonite saturation state environment in summer and fall, when organic material from the highly productive summer remineralizes. During this time, the aragonite saturation state can be as low as 0.4, triggering free CaCO3 dissolution. During the sea ice covered winter period, pH was < 8 and aragonite remained undersaturated under the sea ice. There are only two short seasonal periods with relatively higher pH and Ωarag, which we term ocean acidification relaxation events. In spring, high primary production from sea ice algae and phytoplankton blooms and ikaite dissolution lead to spikes in pH (pH > 8) and aragonite oversaturation. In late fall, strong wind driven mixing events that bring CO2 depleted surface water to the shelf also lead to events with elevated pH and Ωarag. Given the recent observations of high rates of ocean acidification, and sudden and dramatic shift of the physical, biogeochemical, and ecosystem conditions in the Chukchi Sea, it is possible that the observed extreme conditions at the Chukchi Ecosystem Observatory are significantly deviating from the carbonate conditions to which many species are adapted and may have negative impacts on the ecosystem.

Continue reading ‘Sea ice loss translates into major shifts in the carbonate environmental conditions in Arctic Shelf Sea’

Effects of climate change and eutrophication on photosynthesis and carbon-concentrating mechanisms: surprising diversity among reef algae

Increased anthropogenic CO2 emission since the start of the Industrial Revolution has brought a changing climate and various threats to coastal ecosystems including ocean warming, ocean acidification (OA), and sea level rise. Coral reef ecosystems are especially vulnerable to the climate change, because ocean warming and acidification decrease calcification and increase bleaching in coral. In addition to these impacts of climate change, coastal ecosystems are already experiencing local anthropogenic impacts such as chronic eutrophication and continuing arrival of new invasive species. In Hawai‘i, large-scale blooms of both native and invasive macroalgae are often observed in the region with coastal eutrophication by land-based anthropogenic nutrient input. Predicting the effects of OA (increased CO2 concentration in the ocean) on algae is not straightforward because many algae are already equipped with carbon-concentrating mechanisms (CCMs) with which algae can increase their internal CO2 concentration for photosynthesis. Further, nutrient availability especially that of the macronutrient, nitrogen (N) could alter the operation of algal CCMs because CCMs involve specific, large proteins such as ribulose-1,5-biphosphate carboxylase-oxygenase (RUBISCO) and carbonic anhydrases (CA). This study experimentally investigated how OA and eutrophication, independently and synergistically, affect photosynthesis and CCMs in common Hawaiian reef algae. Algae can quickly change their maximum photosynthetic rates and CCMs when grown under elevated CO2 and N. Further, we found a surprising diversity among reef algae in how they react to elevated CO2 and N with their CCMs. The results of this study suggest that many Hawaiian algae will thrive under future climate change conditions, and OA and eutrophication will likely work in their favor, accelerating the phase shift from coral-dominated to macroalgal-dominated reefs in unpredictably faster paces and with players that are not easily predicted.

Continue reading ‘Effects of climate change and eutrophication on photosynthesis and carbon-concentrating mechanisms: surprising diversity among reef algae’

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

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.

Continue reading ‘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’

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