Posts Tagged 'chemistry'

Nitrous oxide and methane in a changing Arctic Ocean

Human activities are changing the Arctic environment at an unprecedented rate resulting in rapid warming, freshening, sea ice retreat and ocean acidification of the Arctic Ocean. Trace gases such as nitrous oxide (N2O) and methane (CH4) play important roles in both the atmospheric reactivity and radiative budget of the Arctic and thus have a high potential to influence the region’s climate. However, little is known about how these rapid physical and chemical changes will impact the emissions of major climate-relevant trace gases from the Arctic Ocean. The combined consequences of these stressors present a complex combination of environmental changes which might impact on trace gas production and their subsequent release to the Arctic atmosphere. Here we present our current understanding of nitrous oxide and methane cycling in the Arctic Ocean and its relevance for regional and global atmosphere and climate and offer our thoughts on how this might change over coming decades.

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The Gulf of St. Lawrence biogeochemical model: a modelling tool for fisheries and ocean management

The goal of this paper is to give a detailed description of the coupled physical-biogeochemical model of the Gulf of St. Lawrence that includes dissolved oxygen and carbonate system components, as well as a detailed analysis of the riverine contribution for different nitrogen and carbonate system components. A particular attention was paid to the representation of the microbial loop in order to maintain the appropriate level of the different biogeochemical components within the system over long term simulations. The skill of the model is demonstrated using in situ data, satellite data and estimated fluxes from different studies based on observational data. The model reproduces the main features of the system such as the phytoplankton bloom, hypoxic areas, pH and calcium carbonate saturation states. The model also reproduces well the estimated transport of nitrate from one region to the other. We revisited previous estimates of the riverine nutrient contribution to surface nitrate in the Lower St. Lawrence Estuary using the model. We also explain the mechanisms that lead to high ammonium concentrations, low dissolved oxygen, and undersaturated calcium carbonate conditions on the Magdalen Shallows.

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Coral micro- and macro-morphological skeletal properties in response to life-long acclimatization at CO2 vents in Papua New Guinea

This study investigates the effects of long-term exposure to OA on skeletal parameters of four tropical zooxanthellate corals naturally living at CO2 seeps and adjacent control sites from two locations (Dobu and Upa Upasina) in the Papua New Guinea underwater volcanic vent system. The seeps are characterized by seawater pH values ranging from 8.0 to about 7.7. The skeletal porosity of Galaxea fascicularisAcropora millepora, massive Porites, and Pocillopora damicornis was higher (up to ~ 40%, depending on the species) at the seep sites compared to the control sites. Pocillopora damicornis also showed a decrease of micro-density (up to ~ 7%). Thus, further investigations conducted on this species showed an increase of the volume fraction of the larger pores (up to ~ 7%), a decrease of the intraskeletal organic matrix content (up to ~ 15%), and an increase of the intraskeletal water content (up to ~ 59%) at the seep sites. The organic matrix related strain and crystallite size did not vary between seep and control sites. This multi-species study showed a common phenotypic response among different zooxanthellate corals subjected to the same environmental pressures, leading to the development of a more porous skeletal phenotype under OA.

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A replication study on coral δ11B and B/Ca and their variation in modern and fossil Porites: implications for coral calcifying fluid chemistry and seawater pH changes over the last millennium

Boron systematics offer a unique opportunity to reveal coral calcifying fluid (CF) chemistry and seawater pH (pHsw). Here we assess the intercolony differences of skeletal δ11B and B/Ca, and examine their variation in modern and fossil Porites spp. collected from the east Hainan Island in the northern South China Sea (SCS), to explore changes in coral CF chemistry and pHsw over the last millennium. This enables us to assess whether ocean acidification (OA) has disturbed the ability of corals to control their CF chemistry, and whether splicing coral δ11B-pH records can trace long-term pHsw variability. We demonstrate that coral boron systematics bear remarkable intercolony differences, with mean offset as high as 1.05‰ for δ11B and 183.1μmol/mol for B/Ca. With this in mind, we show that fossil corals exhibit no significant difference in their CF carbonate chemistry, but all have systematically higher CF pH (pHcf, by an average of 0.12 units) and almost equivalent CF dissolved inorganic carbon (DICcf) concentration, compared to modern corals. This suggests greater OA impacts on coral pHcf but less noticeable effects on DICcf. In addition, the ∼0.12 decline in pHcf translates to about 0.24 reduction in pHsw, similar to another coral-based estimate (∼0.24) from south Hainan Island, corroborating significant OA in the northern SCS since the industrial era. Nevertheless, we find that pHsw in the east Hainan Island has staged a recovery from 1980 to 2010, slowing down the OA pace, highlighting important roles of other local forcing on pHsw regulation.

Continue reading ‘A replication study on coral δ11B and B/Ca and their variation in modern and fossil Porites: implications for coral calcifying fluid chemistry and seawater pH changes over the last millennium’

Spatial variability of summer hydrography in the central Arabian Gulf

Highlights

  • Physicochemical parameters were measured in 3 transects covering the EEZ of Qatar.
  • Spatial variability of physicochemical parameters has been analyzed.
  • Vertical variations at the deeper stations have been linked with the water masses.
  • Physicochemical parameters have been inter-correlated and clustered.

Abstract

The Arabian Gulf is a very significant ocean body, which hosts more than 55% of the oil reserves of the world and produces about 30% of the total production, and thus, it is likely to face high risk and adverse problems by the intensified environmental stressors and severe climatic changes. Therefore, understanding the hydrography of the Gulf is very essential to identify various marine environmental issues and subsequently, developing marine protection and management plans. In this study, hydrography data collected at 11 stations along 3 linear transects in the early summer of 2016 were analyzed. The physicochemical parameters exhibited apparent variations along each transect, both laterally and vertically, connected to stratification, formation of different water masses and excessive heating. The temperature and salinity decreased laterally from nearshore to offshore, while layered density structures were identified in the offshore regions. The pH, dissolved oxygen (DO) and chlorophyll fluorescence (Fo) exhibited distinct horizontal and vertical variations. The observed pH is within the normal ranges, indicating that seawater acidification may not be a threat. The highest DO (6.13–8.37 mg/l) was observed in a layer of 24-36 m water depth in the deeper regions of the central transect.

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Abundances and morphotypes of the coccolithophore Emiliania huxleyi in southern Patagonia compared to neighbouring oceans and Northern Hemisphere fjords

Coccolithophores are potentially affected by ongoing ocean acidification, where rising CO2 lowers seawater pH and calcite saturation state (Ωcal). Southern Patagonian fjords and channels provide natural laboratories for studying these issues due to high variability in physical and chemical conditions. We surveyed coccolithophore assemblages in Patagonian fjords during late spring 2015 and early spring 2017. Surface Ωcal exhibited large variations driven mostly by freshwater inputs. High-Ωcal conditions (max. 3.6) occurred in the Archipelago Madre de Dios. Ωcal ranged from 2.0–2.6 in the western Strait of Magellan and 1.5–2.2 in the inner channel and was subsaturating (0.5) in Skyring Sound. Emiliania huxleyi was the only coccolithophore widely distributed in Patagonian fjords (> 96 % of total coccolithophores), only disappearing in the Skyring Sound, a semi-closed mesohaline system. Correspondence analysis associated higher E. huxleyi biomasses with lower diatom biomasses. The highest E. huxleyi abundances in Patagonia were in the lower range of those reported in Norwegian fjords. Predominant morphotypes were distinct from those previously documented in nearby oceans but similar to those of Norwegian fjords. Moderately calcified forms of E. huxleyi A morphotype were uniformly distributed throughout Patagonia fjords. The exceptional R/hyper-calcified coccoliths, associated with low Ωcal values in Chilean and Peruvian coastal upwellings, were a minor component associated with high Ωcal levels in Patagonia. Outlying mean index (OMI) niche analysis suggested that pH and Ωcal conditions explained most variation in the realized niches of E. huxleyi morphotypes. The moderately calcified A morphotype exhibited the widest niche breadth (generalist), while the R/hyper-calcified morphotype exhibited a more restricted realized niche (specialist). Nevertheless, when considering an expanded sampling domain, including nearby southeast Pacific coastal and offshore waters, even the R/hyper-calcified morphotype exhibited a higher niche breadth than other closely phylogenetically related coccolithophore species. The occurrence of E. huxleyi in naturally low pH–Ωcal environments indicates that its ecological response is plastic and capable of adaptation.

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Temporal and spatial variabilities of chemical and physical parameters on the Heron Island coral reef platform

Globally, coral reefs are threatened by ocean warming and acidification. The degree to which acidification will impact reefs is dependent on the local hydrodynamics, benthic community composition, and biogeochemical processes, all of which vary on different temporal and spatial scales. Characterizing the natural spatiotemporal variability of seawater carbonate chemistry across different reefs is critical for elucidating future impacts on coral reefs. To date, most studies have focused on select habitats, whereas fewer studies have focused on reef scale variability. Here, we investigate the temporal and spatial seawater physicochemical variability across the entire Heron Island coral reef platform, Great Barrier Reef, Australia, for a limited duration of six days. Autonomous sensor measurements at three sites across the platform were complemented by reef-wide boat surveys and discrete sampling of seawater carbonate chemistry during the morning and evening. Variability in both temporal and spatial physicochemical properties were predominantly driven by solar irradiance (and its effect on biological activity) and the semidiurnal tidal cycles but were influenced by the local geomorphology resulting in isolation of the platform during low tide and rapid flooding during rising tides. As a result, seawater from previous tidal cycles was sometimes trapped in different parts of the reef leading to unexpected biogeochemical trends in space and time. This study illustrates the differences and limitations of data obtained from high-frequency measurements in a few locations compared to low-frequency measurements at high spatial resolution and coverage, showing the need for a combined approach to develop predictive capability of seawater physicochemical properties on coral reefs.

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Toward unified pH of saline solutions

Fluctuations of pH in coastal systems are generally surveyed through potentiometric pH measurements. A new concept of a unified pH scale was introduced with the great advantage of enabling comparability of absolute values, pHabs, pertaining to any medium. Using water as an anchor solvent, yielding pHH2Oabs, enables referencing the pHabs values to the conventional aqueous pH scale. The current work aims at contributing to implement pHH2Oabs to saline solutions. To this purpose, differential potentiometric measurements, with a salt bridge of ionic liquid [N2225][NTf2], were carried out aiming at overcoming problems related to residual liquid junction potentials that affect the quality of such measurements. The ability to measure pHH2Oabs with acceptable uncertainty was evaluated using Tris-Tris·HCl standard buffer solutions prepared in a background matrix close to the characteristics of estuarine systems (salinity of 20) as well as with NaCl solutions with ionic strength between 0.005 and 0.8 mol kg−1. The present study shows that for high ionic strength solutions, such as seawater, challenges remain when addressing the assessment and quantification of ocean acidification in relation to climate change. Improvements are envisaged from the eventual selection of a more adequate ionic liquid.

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Seasonality and life history complexity determine vulnerability of Dungeness crab to multiple climate stressors

Abstract

Scaling climate change impacts from individual responses to population-level vulnerability is a pressing challenge for scientists and society. We assessed vulnerability of the most valuable fished species in the Northwest U.S.—Dungeness crab—to climate stressors using a novel combination of ocean, population, and larval transport models with stage-specific consequences of ocean acidification, hypoxia, and warming. Integration across pelagic and benthic life stages revealed increased population-level vulnerability to each stressor by 2100 under RCP 8.5. Under future conditions, chronic vulnerability to low pH emerged year-round for all life stages, whereas vulnerability to low oxygen continued to be acute, developing seasonally and impacting adults, which are critical to population growth. Our results demonstrate how ontogenetic habitat shifts and seasonal ocean conditions interactively impact population-level vulnerability. Because most valuable U.S. fisheries rely on species with complex life cycles in seasonal seas, chronic and acute perspectives are necessary to assess population-level vulnerability to climate change.

Plain Language Summary

The release of carbon dioxide (CO2) into the atmosphere by human activities is altering ocean conditions including pH, oxygen, and temperature. One way to understand how these changing conditions will affect ecologically, economically, and culturally important marine species is to scale individual responses from laboratory experiments to population-level impacts. In this study, we assessed the vulnerability of Dungeness crab, one of the most valuable fisheries in the NW USA, to stressful conditions based on the predicted habitat exposure and response of each life stage (eggs, larvae, juveniles, and adults). The degree of vulnerability was determined by the seasonality of the ocean conditions in combination with the crab’s complex life cycle. This approach revealed that Dungeness crab life stages and populations will be more vulnerable to low pH, low oxygen, and high temperature in the future (year 2100) under an aggressive CO2 emissions scenario. Based on these results, we recommend that fishery managers incorporate changing conditions into their decision-making to protect vulnerable life stages in areas prone to stressful conditions (e.g., adult crabs in hypoxic areas). Our approach can be adapted for many other economically and ecologically important marine species in order to inform conservation and management strategies.

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Possible future scenarios for two major Arctic Gateways connecting Subarctic and Arctic marine systems: I. climate and physical–chemical oceanography

We review recent trends and projected future physical and chemical changes under climate change in transition zones between Arctic and Subarctic regions with a focus on the two major inflow gateways to the Arctic, one in the Pacific (i.e. Bering Sea, Bering Strait, and the Chukchi Sea) and the other in the Atlantic (i.e. Fram Strait and the Barents Sea). Sea-ice coverage in the gateways has been disappearing during the last few decades. Projected higher air and sea temperatures in these gateways in the future will further reduce sea ice, and cause its later formation and earlier retreat. An intensification of the hydrological cycle will result in less snow, more rain, and increased river runoff. Ocean temperatures are projected to increase, leading to higher heat fluxes through the gateways. Increased upwelling at the Arctic continental shelf is expected as sea ice retreats. The pH of the water will decline as more atmospheric CO2 is absorbed. Long-term surface nutrient levels in the gateways will likely decrease due to increased stratification and reduced vertical mixing. Some effects of these environmental changes on humans in Arctic coastal communities are also presented.

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Co‑occurrence of aquatic heatwaves with atmospheric heatwaves, low dissolved oxygen, and low pH events in estuarine ecosystems

Heatwaves are increasing in frequency, duration, and intensity in the atmosphere and marine environment with rapid changes to ecosystems occurring as a result. However, heatwaves in estuarine ecosystems have received little attention despite the effects of high temperatures on biogeochemical cycling and fisheries and the susceptibility of estuaries to heatwaves given their low volume. Likewise, estuarine heatwave co-occurrence with extremes in water quality variables such as dissolved oxygen (DO) and pH have not been considered and would represent periods of enhanced stress. This study analyzed 1440 station years of high-frequency data from the National Estuarine Research Reserve System (NERRS) to assess trends in the frequency, duration, and severity of estuarine heatwaves and their co-occurrences with atmospheric heatwaves, low DO, and low pH events between 1996 and 2019. Estuaries are warming faster than the open and coastal ocean, with an estuarine heatwave mean annual occurrence of 2 ± 2 events, ranging up to 10 events per year, and lasting up to 44 days (mean duration = 8 days). Estuarine heatwaves co-occur with an atmospheric heatwave 6–71% of the time, depending on location, with an average estuarine heatwave lag range of 0–2 days. Similarly, low DO or low pH events co-occur with an estuarine heatwave 2–45% and 0–18% of the time, respectively, with an average low DO lag of 3 ± 2 days and low pH lag of 4 ± 2 days. Triple co-occurrence of an estuarine heatwave with a low DO and low pH event was rare, ranging between 0 and 7% of all estuarine heatwaves. Amongst all the stations, there have been significant reductions in the frequency, intensity, duration, and rate of low DO event onset and decline over time. Likewise, low pH events have decreased in frequency, duration, and intensity over the study period, driven in part by reductions in all severity classifications of low pH events. This study provides the first baseline assessment of estuarine heatwave events and their co-occurrence with deleterious water quality conditions for a large set of estuaries distributed throughout the USA.

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Diel metabolism of Yellow Sea green tide algae alters bacterial community composition under in situ seawater acidification of coastal areas

Highlights

  • Metabolism of algae mat leads to a diel pH and CO2 fluctuation in affected seawater.
  • Bacterial communities in diffusive boundary layer of the algae had a diel change.
  • Flavobacteriaceae was shown increased at night but sharp decreased at daytime.
  • Harmful algal bloom might influence coastal ocean acidification.

Abstract

Ocean acidification in coastal seawaters is a complex process, with coastal pH being affected by numerous factors including watershed and biological processes that also support metabolically diverse bacterial communities. The world’s largest macroalgal blooms have occurred consecutively in the Yellow Sea over the last 13 years. In particular, algal mats formed by Yellow Sea green tides (YSGT) significantly influence coastal environments. Herein, we hypothesized that 1) inorganic carbonate chemistry in coastal areas is altered by diel metabolism of these giant algal mats and that 2) bacterial community composition in diffusive boundary layers might be altered along diel cycles due to algal mat metabolism. In situ studies indicated that algal mat metabolism led to changes in diel pH and CO2 in affected seawaters. Such metabolic activities could intensify diel pH fluctuations in algal mat diffusive boundary layers, as noted by pH fluctuations of 0.22 ± 0.01 units, and pCO2 fluctuations of 214.62 ± 29.37 μatm per day. In contrast, pH fluctuations of 0.11 ± 0.02 units and pCO2 fluctuations of 79.02 ± 42.70 μatm were noted in unaffected areas. Furthermore, the bacterial community composition associated with diffusive algal boundary layers, including those of ambient bacteria and epiphytic bacteria, exhibited diel changes, while endophytic bacterial communities were relatively stable. Flavobacteriaceae were particularly highly abundant taxa in the ambient and epiphytic bacterial communities and exhibited increased abundances at night but sharp decreases in abundances during daytime. Flavobacteriaceae are heterotrophic taxa that could contribute to coastal area acidification at night due to the transformation of organic carbon to inorganic carbon. These results provide new insights to understand the variability in coastal ocean acidification via harmful algal blooms while providing a framework for evaluating the effects of YSGT on costal carbon cycling.

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Study of the salinity and pH dilution pattern of discharged brine of the Konarak desalination plant into the a case study

This research aims to study the salinity and pH dilution pattern of discharged brine of the Konarak desalination plant into the Chabahar bay, their relation on coastal environment, and type of its brine discharge. Due to the shallow water depth of the coast and type of brine discharge, evaluating the salinity and pH was done with a sampling of surface seawater. The type of brine disposal is a direct surface discharge of negatively buoyant flow in the coastal environment of Chabahar bay. The brine discharge mechanism is a shore-attached surface jet, which is most likely influenced by the cross-flow deflection, dynamic shoreline interaction, and more minor by bottom attachment factors. The laboratory simulations using actual brine and seawater and either satellite pictures support the finding dilution pattern. The zone of initial dilution is under 50 m which, in the long run, can affect the quality of water of intake seawater pool of the plant.

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pH variability in catchment flows to estuaries – a South African perspective

Highlights

  • Catchment geology was found to be the dominant driver of pH in river inflow.
  • Catchment vegetation/anthropogenic pressures act as modifiers to ambient pH.
  • Trends in inter-annual variability were linked to anthropogenic pressures.
  • Relationship between pH and flow was present in systems with rainfall seasonality.
  • Highlights the potential effects of upstream catchment practices on downstream.

Abstract

River inflow plays an integral role on the water quality characteristics of estuaries, including pH. This study aimed to investigate pH variability in river inflow to South African estuaries and how these might be influenced by catchment characteristics. Specifically, three hypotheses were tested: 1) catchment geology is a dominant influencing factor of pH in river inflow, 2) catchment vegetation and anthropogenic pressures, e.g., urban and agriculture, act as modifiers of geology-driven ambient pH, and 3) seasonality in river flow rates can alter pH levels (e.g., pH decreasing during periods of high flow). First, drivers of pH variability were explored in relation to electrical conductivity, total alkalinity, and catchment geology type, as well as vegetation and key anthropogenic pressures. Thereafter, temporal variability was evaluated considering both inter-annual and seasonal variability also including variability in flow rates. Values of pH displayed an exponential relationship with electrical conductivity and total alkalinity, and as hypothesised pH variability was primarily influenced by catchment geology. Results also indicated that catchment vegetation (e.g., peatlands and fynbos) and/or anthropogenic pressures (e.g., urban and agriculture) act as modifiers causing pH to deviate from geology-driven ambient equilibria, especially in Table Mountain Group sandstone-dominated systems. Only limited trends in inter-annual variability were observed, mostly linked to increases in pH with increases in anthropogenic pressures. Further, a significant inverse relationship between pH and river flows was present, mostly in systems showing marked seasonality in rainfall. This study adds to our understanding of the variability of pH in river inflows to estuaries, and highlights some of the key influencing factors. Indeed, results suggest that anthropogenic pressures in catchments potentially are leading to alkalinisation of river inflow, contrary to the potential effect of ocean acidification. Finally, this study highlights the ripple effects of upstream catchment practices on downstream coastal ecosystems such as estuaries, emphasising the need for integrated catchment-to-coast management.

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Contrasting controls of acidification metrics across environmental gradients in the North Pacific and the adjunct Arctic Ocean: insight from a transregional study

Abstract

The spatiotemporal variabilities and drivers of ocean acidification (OA) metrics, [H+], pH, and aragonite saturation state (Ωarag) across environmental gradients remain poorly constrained. We use a novel high-precision measurement of underway pH to investigate the hemispheric-scale distributions of OA metrics from East Asia to the Arctic Ocean. While temperature and its induced air-sea gas exchange fundamentally control the OA metrics distributions, we show that biological activity exerts the most prominent but different modifications on pH and Ωarag patterns. Strong photosynthesis counteracts the temperature-driven pH pattern but reinforces that of Ωarag. Ice melt-induced dilution in the Arctic Ocean additionally strengthens the Ωarag-temperature relationship but insignificantly affects [H+] and pH. This study provides the first coherent assessment of comprehensive processes on OA metrics across large spatial regions, and highlights the potential of sea-ice melt in changing Ωarag distribution, which should be included by Earth system models projecting future climate change.

Plain Language Summary

The ocean uptake of anthropogenic carbon dioxide (CO2) is causing increase in hydrogen ion concentration ([H+]) and reductions in pH and carbonate mineral aragonite saturation state (Ωarag), together of which are commonly referred to as ocean acidification (OA). The coupled behavior of these affected OA metrics responding to physical and biogeochemical processes across environmental gradients has barely been examined in a comparative manner. To address this issue, we conduct a survey measuring high-precision underway pH from East Asia to the Arctic Ocean. We find that, besides the temperature effects which ultimately control the distributions of OA metrics, biological activity induces the strongest interruptions. Photosynthesis weakens the temperature-driven pH pattern but reinforces that of Ωarag. In addition, ice melt-induced dilution in the polar region strengthens Ωarag-temperature relationship but makes less difference to [H+] and pH. These findings are important as they are based on the first large-scale direct measurements of underway pH, and have implications for future studies on ocean acidification in the context of climate change.

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Heterogeneity around CO2 vents obscures the effects of ocean acidification on shallow reef communities

Studies that use CO2 vents as natural laboratories to investigate the impacts of ocean acidification (OA) typically employ control-impact designs or local-scale gradients in pH or pCO2, where impacted sites are compared to reference sites. While these strategies can accurately represent well-defined and stable vent systems in relatively homogenous environments, it may not adequately encompass the natural variability of heterogeneous coastal environments where many CO2 vents exist. Here, we assess the influence of spatial heterogeneity on the perceived impacts of OA at a vent system well established in the OA literature. Specifically, we use a multi-scale approach to investigate and map the spatial variability in seawater pH and benthic communities surrounding vents at Whakaari-White Island, New Zealand to better understand the scale and complexity of ecological impacts of an acidified environment. We found a network of vents embedded in complex topography throughout the study area, and spatially variable pH and pCO2 levels. The distribution of habitats (i.e. macroalgal forests and turfing algae) was most strongly related to substratum type and sea urchin densities, rather than pH. Epifaunal communities within turfing algae differed with sampling distance from vents, but this pattern was driven by higher abundances of a number of taxa immediately adjacent to vents, where pH and temperature gradients are steep and white bacterial mats are prevalent. Our results contrast with previous studies at White Island that have used a control-impact design and suggested significant impacts of elevated CO2 on benthic communities. Instead, we demonstrate a highly heterogeneous reef where it is difficult to separate effects of reduced pH from spatial variation in reef communities. We urge that future research carefully considers and quantifies the biological and physical complexity of venting environments, and suggest that in dynamic systems, such as White Island, the use of control-impact designs can oversimplify and potentially overestimate the future impacts of OA.

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Response of CO2 sink and biogeochemistry to sea-ice loss in the Western Arctic Ocean

The oceanic uptake of atmospheric CO2 is of global importance as it affects the pace of climate change. The Arctic Ocean acts as a carbon sink for atmospheric CO2, benefiting from high solubility of CO2 in cold seawater and high summer biological production. It has been known that amplified warming and accelerated sea ice loss in the Arctic Ocean since 1980s have profoundly altered the Arctic Ocean environment and related biogeochemical processes. However, less is known about how oceanic CO2 uptake and biological production changes in different biogeochemical provinces in respond to warming and sea ice loss and how fast are these changes. Based on results from two cruises conducted in the western Arctic Ocean in 2016 and 2018, we examined seasonal and regional variabilities in metabolic status and the coupling of biological production and oceanic CO2 uptake, which provided a mechanistic view of the summer evolution of net community production and CO2 flux in the various stages of ice-melt and nutrient status. By compiling historical datasets of underway measurements of sea surface partial pressure of CO2 (pCO2), we found that despite the western Arctic Ocean as a whole continuing to act as an oceanic carbon sink, regional carbon flux dynamics differ greatly; the Chukchi Sea continues to absorb CO2 at pace with the atmospheric CO2 increase, whereas Beaufort Sea and Canada Basin become a weakened or diminishing CO2 sink as the sea surface CO2 increased at more than twice the rate of CO2 in the atmosphere. In addition to examination of the long-term trend of sea surface CO2, we further assessed seasonal and interannual variations in CO2 uptake between 1994 and 2019. Two complementary approaches (observation-based and model-based) were conducted. Our results suggest that CO2 uptake in the Chukchi Sea significantly increased at a rate of 1.4 ±0.4 Tg C decade-1, which was primarily due to a longer ice-free period with a larger open area and increased primary production and partially due to enhanced wind. However, no significant change in CO2 uptake was found in the Canada Basin and Beaufort Sea. Our model results further revealed that the greatly decreased sea ice extent in summer indeed promoted CO2 uptake and resulted in a weak increased CO2 sink by 0.6±0.3 Tg C decade-1 in the Canada Basin, but this increasing sink was counteracted by a rapidly decreasing air-sea CO2 gradient.

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Air-sea gas fluxes and remineralization from a novel combination of pH and O2 sensors on a glider

Accurate, low-power sensors are needed to characterize biogeochemical variability on underwater glider missions. However, the needs for high accuracy and low power consumption can be difficult to achieve together. To overcome this difficulty, we integrated a novel sensor combination into a Seaglider, comprising a spectrophotometric lab-on-a-chip (LoC) pH sensor and a potentiometric pH sensor, in addition to the standard oxygen (O2) optode. The stable, but less frequent (every 10 min) LoC data were used to calibrate the high-resolution (1 s) potentiometric sensor measurements. The glider was deployed for a 10-day pilot mission in August 2019. This represented the first such deployment of either type of pH sensor on a glider. The LoC pH had a mean offset of +0.005±0.008 with respect to pH calculated from total dissolved inorganic carbon content, c(DIC), and total alkalinity, AT, in co-located water samples. The potentiometric sensor required a thermal-lag correction to resolve the pH variations in the steep thermocline between surface and bottom mixed layers, in addition to scale calibration. Using the glider pH data and a regional parameterization of AT as a function of salinity, we derived the dissolved CO2 content and glider c(DIC). Glider surface CO2 and O2 contents were used to derive air-sea fluxes, Φ(CO2) and Φ(O2). Φ(CO2) was mostly directed into the ocean with a median of −0.4 mmol m–2 d–1. In contrast, Φ(O2) was always out of the ocean with a median of +40 mmol m–2 d–1. Bottom water apparent oxygen utilization (AOU) was (35±1) μmol kg–1, whereas apparent carbon production (ACP) was (11±1) μmol kg–1, with mostly insignificant differences along the deployment transect. This deployment shows the potential of using pH sensors on autonomous observing platforms such as Seagliders to quantify the interactions between biogeochemical processes and the marine carbonate system at high spatiotemporal resolution.

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Contrasting land-uses in two small river basins impact the colored dissolved organic matter concentration and carbonate system along a river-coastal ocean continuum

Highlights

  • Contrasting land use in basins influences CDOM proportions in river-coastal continuum.
  • CDOM/fDOM proportions fluvial may influence the carbonate system of coastal.
  • River with high CDOM proportions have implications for mussel farming.
  • High CDOM/fDOM proportion be associated with corrosive conditions in river waters.

Abstract

Human activities have led to an increase in land use change, with effects on the structure and functioning of ecosystems. The impact of contrasting land uses along river basins on the concentration of colored dissolved organic matter (CDOM) reaching the coastal zone, and its relationship with the carbonate system of the adjacent coastal ocean, is poorly known. To understand the relationship between land use change, CDOM and its influence on the carbonate system, two watersheds with contrasting land uses in southern Chile were studied. The samples were collected at eight stations between river and adjacent coastal areas, during three sampling campaigns in the austral summer and spring. Chemical and biological samples were laboratory analysis according to protocols. Landsat 8 satellite images of the study area were used for identification and supervised classification using remote sensing tools. The Yaldad River basin with 82% of native forest and the Colu River basin with 38% of grassland (agriculture). Low total alkalinity (AT) and Dissolved Inorganic Carbon (DIC), but high CDOM proportions were observed in freshwater. A higher CDOM and humic-like compounds concentration was observed along the river-coastal ocean continuum in the Yaldad basin, characterized by a predominance of native forests. In contrast, nutrient concentrations, AT and DIC, were higher in the Colu area. Low CaCO3 saturation state (ΩAr < 2) and even undersaturation conditions were observed at the coastal ocean at Yaldad. A strong negative correlation between AT, DIC and ΩAr with CDOM/fDOM, suggested the influence of terrestrial material on the seawater carbon chemistry. Our results provide robust evidence that land uses in river basins can influence CDOM/fDOM proportion and its influence on the carbonate chemistry of the adjacent coastal, with potential implications for the shellfish farming activity in this region.

Continue reading ‘Contrasting land-uses in two small river basins impact the colored dissolved organic matter concentration and carbonate system along a river-coastal ocean continuum’

Modeling of biogeochemical consequences of a CO2 leak in the water column with bottom anoxia

Highlights

  • To study the biogeochemical consequences of a release of CO2 in an anoxic marine environment a FABM family set of models consisting of a transport model, biogeochemical model (including carbonate system processes block) and bubble fate (transport and dissolution) was applied.
  • The measurements performed during a controlled 2-h long CO2 release experiments show elevated levels of pCO2 and simultaneously decreased values in pH, that was used for the model validation.
  • The model analysis of consequences of a CO2discharge demonstrates that CO2 bubbles are dissolved shortly after termination of the leak, while changes in pH, pCO2 and TIC can be detected for several days after the leak, but only at a limited distance from the source (< 10 m in the examples evaluated here).

Abstract

In this paper we investigate the spatial extent and biogeochemical properties of a known CO2 plume using the pelagic transport-biogeochemical model BROM (Bottom RedOx Model). The model consists of a biogeochemical module, a 2-dimensional vertical transport module and gas bubble fate module, parameterizing bubbles rising and dissolution according to existing approaches. A controlled CO2 release experiment was carried out in the Horten Inner Harbor, Norway, in September 2018. This isolated bay is characterized by limited water mixing and anoxia in the bottom layer. CO2 was released at a water depth of 18 m either in a gas phase or dissolved in seawater at leak rates ranging from 0.1 l/min to 15.8 l/min. The chemical response to the release events relative to background variations was measured using chemical sensors mounted on two seabed templates located 4 m and 15 m from the release point, respectively, and compared to the values predicted by the model. The measurements show elevated levels of pCO2 and simultaneously decreased values in pH corresponding to the controlled release experiments. The model’s simulations were in good agreement with the baseline observations and the measured changes forced by the experimental leak. The model predicts that after a continuous leak of this magnitude in stagnant conditions of anoxic bottom water, a 2–3 weeks long restoration period occurs, after which the disturbances disappear. This work confirms that the footprint of a potential CO2 leak is localized in the vicinity of the source (tens of meters) where it can be detectable with available chemical sensors.

Continue reading ‘Modeling of biogeochemical consequences of a CO2 leak in the water column with bottom anoxia’

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