Mmol O2 Research Articles

Drivers of Ecosystem Metabolism in Two Managed Shallow Lakes with Different Salinity and Trophic Conditions: The Sauce Grande and La Salada Lakes (Argentina)

Understanding the drivers and how they affect ecosystem metabolism is essential for developing effective management policy and plans. In this study, net ecosystem production (NEP), ecosystem respiration (R), and gross primary production (GPP) rates were estimated in relation to physicochemical, hydrological, and meteorological variables in La Salada (LS) and Sauce Grande (SG), two shallow lakes located in an important agricultural region with water management. LS is a mesosaline, mesotrophic-eutrophic lake, whereas SG is a hyposaline and eutrophic lake. GPP and R showed daily and seasonal variations, with R exceeding GPP during most of the study period in both lakes. Net heterotrophic conditions prevailed during the study period (NEP LS: −1.1 mmol O2 m−2 day−1 and NEP SG: −1.25 mmol O2 m−2 day−1). From data analysis, the temperature, wind speed, and lake volume are the main drivers of ecosystem metabolism for both lakes. Despite the significant differences between the two lakes, the NEP values were similar. The different hydrological characteristics (endorheic vs. flushing lake) were crucial in explaining why the two different systems presented similar ecosystem metabolic rates, emphasizing the importance of water management.

Cobalt doped ceria for abundant storage of surface active oxygen and efficient elemental mercury oxidation in coal combustion flue gas

Cobalt doped CeO2 (Co-CeO2) prepared by a single-step hydrothermal method was used for Hg0 catalytic oxidation. The catalysts were characterized by scanning electron microscope, transmission electron microscope, X-ray diffraction, Raman spectroscopy, electron paramagnetic resonance, X-ray photoelectron spectroscopy, H2-temperature programmed reduction test, and thermogravimetric measurement. The results show that the exposed planes for the flake-shaped Co-CeO2 mainly were (110) and (100) with low oxygen vacancy formation energies. Abundant oxygen vacancy defects were identified on the Co-CeO2, which further induced plentiful chemisorbed oxygen on the surface. The oxygen storage capacity for the Co-CeO2 was up to 1.43 μmol O2 m−2 at 200 °C, and a large portion of this capacity was for chemisorbed oxygen. Without the introduction of gas-phase O2, Hg0 oxidation probably occurred through a Mars-Maessen mechanism, where active chemisorbed oxygen originated from oxygen vacancy defects reacts with adsorbed Hg0 to form HgO. Active chlorine species was generated from the interactions of chemisorbed oxygen with trace amount gaseous HCl. The active chlorine compensated the slight inhibitive effects of SO2 and H2O, leading to above 90% Hg0 oxidation under simulated low-rank coal burning flue gas atmosphere at a gas hourly space velocity of 160,000 h−1. The abundant active chemisorbed oxygen played important role in and guaranteed an efficient Hg0 oxidation in extremely adverse application environment. These results reveal the role of doping metals on structure-catalytic property relations for CeO2, which opened a new strategy for controlling mercury emission from coal-fired flue gas using CeO2 based catalysts by tuning the morphology and exposed planes.

Hazards of decreasing marine oxygen: the near-term and millennial-scale benefits of meeting the Paris climate targets

Abstract. Ocean deoxygenation is recognized as key ecosystem stressor of the future ocean and associated climate-related ocean risks are relevant for current policy decisions. In particular, benefits of reaching the ambitious 1.5 ∘C warming target mentioned by the Paris Agreement compared to higher temperature targets are of high interest. Here, we model oceanic oxygen, warming and their compound hazard in terms of metabolic conditions on multi-millennial timescales for a range of equilibrium temperature targets. Scenarios where radiative forcing is stabilized by 2300 are used in ensemble simulations with the Bern3D Earth System Model of Intermediate Complexity. Transiently, the global mean ocean oxygen concentration decreases by a few percent under low forcing and by 40 % under high forcing. Deoxygenation peaks about a thousand years after stabilization of radiative forcing and new steady-state conditions are established after AD 8000 in our model. Hypoxic waters expand over the next millennium and recovery is slow and remains incomplete under high forcing. Largest transient decreases in oxygen are projected for the deep sea. Distinct and near-linear relationships between the equilibrium temperature response and marine O2 loss emerge. These point to the effectiveness of the Paris climate target in reducing marine hazards and risks. Mitigation measures are projected to reduce peak decreases in oceanic oxygen inventory by 4.4 % ∘C−1 of avoided equilibrium warming. In the upper ocean, the decline of a metabolic index, quantified by the ratio of O2 supply to an organism's O2 demand, is reduced by 6.2 % ∘C−1 of avoided equilibrium warming. Definitions of peak hypoxia demonstrate strong sensitivity to additional warming. Volumes of water with less than 50 mmol O2 m−3, for instance, increase between 36 % and 76 % ∘C−1 of equilibrium temperature response. Our results show that millennial-scale responses should be considered in assessments of ocean deoxygenation and associated climate-related ocean risks. Peak hazards occur long after stabilization of radiative forcing and new steady-state conditions establish after AD 8000.

Oxygen Saturation Surrounding Deep Water Formation Events in the Labrador Sea From Argo‐O2 Data

AbstractDeep water formation supplies oxygen‐rich water to the deep sea, spreading throughout the ocean by means of the global thermohaline circulation. Models suggest that dissolved gases in newly formed deep water do not come to equilibrium with the atmosphere. However, direct measurements during wintertime convection are scarce, and the controls over the extent of these disequilibria are poorly quantified. Here we show that, when convection reached deeper than 800 m, oxygen in the Labrador Sea was consistently undersaturated at −6.1% to −7.6% at the end of convection. Deeper convection resulted in greater undersaturation, while convection ending later in the year resulted in values closer to equilibrium, from which we produce a predictive relationship. We use dissolved oxygen data from six profiling Argo floats in the Labrador Sea between 2003 and 2016, allowing direct observations of wintertime convection. Three of the six optode oxygen sensors displayed substantial average in situ drift of −3.03 μmol O2 kg−1 yr−1 (−0.94% O2 yr−1), which we corrected to stable deepwater oxygen values from repeat ship surveys. Observations of low oxygen intrusions during restratification and a simple mixing calculation demonstrate that lateral processes act to lower the oxygen inventory of the central Labrador Sea. This suggests that the Labrador Sea is a net sink for atmospheric oxygen, but uncertainties in parameterizing gas exchange limit our ability to quantify the net uptake. Our results constrain the oxygen concentration of newly formed Labrador Sea Water and allow more precise estimates of oxygen utilization and nutrient regeneration in this water mass.

Benthic Carbon Mineralization in Hadal Trenches: Insights From In Situ Determination of Benthic Oxygen Consumption

AbstractHadal trenches have been proposed as depocenters of organic material and hot spots for organic matter mineralization. In this study, we for the first time quantified the total benthic O2 uptake in hadal trenches using in situ chamber incubations. Three trenches in the tropical Pacific were targeted and exhibited relatively high diagenetic activity given the great water depths, that is, the Mariana Trench (2.0 × 102 μmol O2 m−2 d−1, 10,853 m), the Mussau Trench (2.7 ± 0.1 × 102 μmol O2 m−2 d−1, 7,011 m), and the New Britain Trench (6.0 ± 0.1 × 102 μmol O2 m−2 d−1, 8,225 m). Combined with the analyses of total organic carbon and δ13C of total organic carbon in the sediments and previously published in situ O2 microprofiles from hadal settings, we suggest that hadal benthic carbon mineralization partly is governed by the surface production and also is linked to the distance from land. Therefore, we highlight that terrestrial organic matter can be of importance in sustaining benthic communities in some hadal settings.

Modelling biogeochemical processes in sediments from the north-western Adriatic Sea: response to enhanced particulate organic carbon fluxes

Abstract. This work presents the result of a study carried out in the north-western Adriatic Sea, by combining two different types of biogeochemical models with field sampling efforts. A longline mussel farm was taken as a local source of perturbation to the natural particulate organic carbon (POC) downward flux. This flux was first quantified by means of a pelagic model of POC deposition coupled to sediment trap data, and its effects on sediment bioirrigation capacity and organic matter (OM) degradation pathways were investigated constraining an early diagenesis model by using original data collected in sediment porewater. The measurements were performed at stations located inside and outside the area affected by mussel farm deposition. Model-predicted POC fluxes showed marked spatial and temporal variability, which was mostly associated with the dynamics of the farming cycle. Sediment trap data at the two sampled stations (inside and outside of the mussel farm) showed average POC background flux of 20.0–24.2 mmol C m−2 d−1. The difference of organic carbon (OC) fluxes between the two stations was in agreement with model results, ranging between 3.3 and 14.2 mmol C m−2 d−1, and was primarily associated with mussel physiological conditions. Although restricted, these changes in POC fluxes induced visible effects on sediment biogeochemistry. Observed oxygen microprofiles presented a 50 % decrease in oxygen penetration depth (from 2.3 to 1.4 mm), accompanied by an increase in the O2 influx at the station below the mussel farm (19–31 versus 10–12 mmol O2 m−2 d−1) characterised by higher POC flux. Dissolved inorganic carbon (DIC) and NH4+ concentrations showed similar behaviour, with a more evident effect of bioirrigation underneath the farm. This was confirmed through constraining the early diagenesis model, of which calibration leads to an estimation of enhanced and shallower bioirrigation underneath the farm: bioirrigation rates of 40 yr−1 and irrigation depth of 15 cm were estimated inside the shellfish deposition footprint versus 20 yr−1 and 20 cm outside. These findings were confirmed by independent data on macrofauna composition collected at the study site. Early diagenesis model results indicated a larger organic matter mineralisation below the mussel farm (11.1 versus 18.7 mmol m−2 d−1), characterised by similar proportions between oxic and anoxic degradation rates at the two stations, with an increase in the absolute values of oxygen consumed by OM degradation and reduced substances re-oxidation underneath the mussel farm.

Mussel farming impact on pelagic production and respiration rates in a coastal upwelling embayment (Ría de Vigo, NW Spain)

This paper provides the first diagnosis of the impact of mussel farming on the primary production (PP) and the metabolic balance in a coastal upwelling region (Ría de Vigo). Measurements of size-fractionated PP and microbial plankton metabolism were performed outside (reference station; ReS) and inside the farming area (raft station; RaS). At ReS, integrated PP was higher during upwelling (1.05 ± 0.45 g C m−2 d−1) with microphytoplankton dominating carbon fixation (74 ± 14%). The significance of nanophytoplankton and picophytoplankton increased during winter linked to lower PP (0.24 ± 0.03 g C m−2 d−1). Water column at ReS was always autotrophic with net community production (NCP) ranging from 186 ± 67 mmol O2 m−2 d−1 during upwelling to 43 ± 22 mmol O2 m−2 d−1 in winter. At RaS, there was a decrease in PP attributable not only to mussel consumption but also to the lower irradiance under mussel rafts. Concomitant decrease in NCP was also observed (by 56%), yet remained autotrophic, supporting the view that under current conditions food does not limit mussel growth in the Ría, thus securing the carrying capacity of the system in terms of production.

Refined Estimates of Net Community Production in the Subarctic Northeast Pacific Derived From ΔO2/Ar Measurements With N2O‐Based Corrections for Vertical Mixing

AbstractWe present the first field application of a N2O‐based approach to correct for vertical mixing in the estimation of net community production (NCP) from mixed layer O2 measurements. Using new ship‐based observations of N2O and biological oxygen saturation anomalies (ΔO2/Ar) from the Subarctic Northeast Pacific, we provide refined mixed layer NCP estimates across contrasting hydrographic regimes and a comprehensive assessment of the methodological considerations and limitations of the approach. Increased vertical mixing coefficients at the base of the mixed layer, derived using N2O measurements, corresponded with periods of heightened wind speed and coastal upwelling. Corrections were most significant in coastal regions where the vertical supply of low‐O2 water can otherwise falsely imply net heterotrophy from negative ΔO2/Ar measurements. After correcting for the mixing flux, all coastal stations showed autotrophic signatures, with maximum NCP exceeding 100 mmol O2 m−2 d−1 in the spring and summer. Vertical fluxes were lower in off‐shelf waters but often contributed more than 50% to corrected NCP. At some oceanic stations, however, the cooccurrence of N2O minima and O2 maxima resulted in biased (overestimated) N2O corrections. Evaluating vertical fluxes in these regions remains a challenge for ship‐based studies. Nonetheless, our refined NCP estimates show better coherence with surface chlorophyll, temperature, and mixed layer depth than uncorrected values. Potential mixed layer N2O production introduces some uncertainty in the approach, but errors are likely to be small. Ultimately, this work provides rationale for the adoption of the N2O correction to refine NCP estimates, particularly in coastal waters.

Quantification of light-induced miniSOG superoxide production using the selective marker, 2-hydroxyethidium.

Genetically-encoded photosensitizers produce reactive oxygen species (ROS) in response to light. Transgenic expression of fusion proteins can target the photosensitizers to specific cell regions and permit the spatial and temporal control of ROS production. These ROS-generating proteins (RGPs) are widely used for cell ablation, mutagenesis and chromophore-assisted light inactivation of target proteins. However, the species produced by RGPs are unclear due to indirect measures with confounding interpretations. Recently, the RGP mini "Singlet Oxygen Generator" (miniSOG) was engineered from Arabidopsis thaliana phototropin 2. While miniSOG produces singlet oxygen (1O2), the contribution of superoxide (O2•-) to miniSOG-generated ROS remains unclear. We measured the light-dependent O2•- production of purified miniSOG using HPLC separation of dihydroethidium (DHE) oxidation products. We demonstrate that DHE is insensitive to 1O2 and establish that DHE is a suitable indicator to measure O2•- production in a system that produces both 1O2 and O2•-. We report that miniSOG produces both 1O2 and O2•-, as can its free chromophore, flavin mononucleotide. miniSOG produced O2•- at a rate of ~4.0µmol O2•-/min/µmol photosensitizer for an excitation fluence rate of 5.9mW/mm2 at 470 ± 20nm, and the rate remained consistent across fluences (light doses). Overall, the contribution of O2•- to miniSOG phenotypes should be considered.

Nanoscale heterogeneity and low-temperature redox property of CeO2-ZrO2-La2O3-Y2O3 quaternary solid solution

In this paper, co-precipitation and surfactant-assisted co-precipitation were employed to synthesize CeO2-ZrO2-La2O3-Y2O3 quaternary solid solutions (CZ and CZ-N12), respectively. The redox properties, textural properties, structural properties, surface chemistry as well as the nano-structural features of the materials were investigated by hydrogen-temperature programmed reduction (H2-TPR), oxygen storage capacity complete (OSCC), powder X-ray diffraction (XRD), N2 adsorption-desorption, Raman, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) characterizations. Surfactant assisted co-precipitation shows great advantages than co-precipitation, which is presented as: CZ-N12 exhibits a low reduction temperature of 400 °C and a high OSC value of 424 μmol O2 g−1, which are about 149 °C lower and 165 μmol O2 g−1 higher than those of CZ, respectively. CZ-N12 also possesses better thermal stability than CZ, after aging at 1000 °C for 4 h, CZ-N12-a owns a high surface area of 38 m2 g−1 (18 m2 g−1 for CZ-a). In addition, XPS studies disclose that CZ-N12 exhibits Ce-enriched surface, high concentration of Ce3+ and higher amount of Oβ compared with CZ. TEM observations confirm the existence of compositional nanoscale domains in CZ-N12, which may be caused by the combustion of the surfactant. These results demonstrate that the greatly enhanced low-temperature redox activity of CZ-N12 is strongly related to the compositional and structural heterogeneity in nanoscale.

Metabolic activity throughout early development of dusky kob Argyrosomus japonicus (Sciaenidae)

The physiology of fishes in the early stages of development remains poorly assessed despite the importance of identifying energy bottlenecks in organisms faced with changing environmental conditions. This study describes the metabolic activity of dusky kob Argyrosomus japonicus throughout its early development, from hatchling to settlement stage. Standard, routine and active metabolic rates (SMR, RMR and AMR, respectively) were assessed to determine the species’ metabolic scope and identify how metabolism changes with growth and development. Distinct metabolic changes occurred in association with developmental changes during the early life stages, with flexion-stage larvae showing significantly reduced metabolic scope (approx. 0.30 µmol O2 ind.–1 h–1), representing an energy bottleneck. Based on these findings, it is likely that larvae of A. japonicus are most susceptible to environmental perturbations during flexion. The variability of metabolic rates during the diel cycle was also assessed and revealed that the early-stage larvae showed no preference for daylight, although settlement-stage juveniles were more active during daylight hours (RMR = 12.78 µmol O2 ind.–1 h–1) than at night (RMR = 5.87 µmol O2 ind.–1 h–1). These results suggest that metabolic measurements of the SMR of A. japonicus larvae can be taken at any time of the diel cycle until the settlement phase, when readings should take place at night.

Plankton community respiration and bacterial metabolism in a North Atlantic Shelf Sea during spring bloom development (April 2015)

Spring phytoplankton blooms are important events in Shelf Sea pelagic systems as the increase in carbon production results in increased food availability for higher trophic levels and the export of carbon to deeper waters and the sea-floor. It is usually accepted that the increase in phytoplankton abundance and production is followed by an increase in plankton respiration. However, this expectation is derived from field studies with a low temporal sampling resolution (5–15 days). In this study we have measured the time course of plankton abundance, gross primary production, plankton community respiration, respiration of the plankton size classes (>0.8 µm and 0.2–0.8 µm) and bacterial production at ≤5 day intervals during April 2015 in order to examine the phasing of plankton autotrophic and heterotrophic processes. Euphotic depth-integrated plankton community respiration increased five-fold (from 22 ± 4 mmol O2 m−2 d−1 on 4th April to 119 ± 4 mmol O2 m−2 d−1 on 15th April) at the same time as gross primary production also increased five-fold, (from 114 ± 5 to 613 ± 28 mmol C m−2 d−1). Bacterial production began to increase during the development of the bloom, but did not reach its maximum until 5 days after the peak in primary production and plankton respiration. The increase in plankton community respiration was driven by an increase in the respiration attributable to the >0.8 µm size fraction of the plankton community (which would include phytoplankton, microzooplankton and particle attached bacteria). Euphotic depth-integrated respiration of the 0.2–0.8 µm size fraction (predominantly free living bacteria) decreased and then remained relatively constant (16 ± 3 – 11 ± 1 mmol O2 m−2 d−1) between the first day of sampling (4th April) and the days following the peak in chlorophyll-a (20th and 25th April). Recent locally synthesized organic carbon was more than sufficient to fulfil the bacterial carbon requirement in the euphotic zone during this productive period. Changes in bacterial growth efficiencies (BGE, the ratio of bacterial production to bacterial carbon demand) were driven by changes in bacterial production rates increasing from <30 ± 14% on 4th April to 51 ± 11% on 25th of April. This study therefore shows a concurrent rather than a phased increase in primary production and community respiration attributable to cells >0.8 µm during the development of the spring bloom, followed 5 days later by a peak in bacterial production. In addition, the size fractionated respiration rates and high growth efficiencies suggest that free living bacteria are not the major producers of CO2 before, during and a few days after this shelf sea spring phytoplankton bloom.

Plankton Respiration, Production, and Trophic State in Tropical Coastal and Shelf Waters Adjacent to Northern Australia

In a changing ocean, tropical waters can be instructive as to the potential effects of climate induced changes on marine ecosystem structure and function. We describe the relationships between planktonic community respiration (CR), net community production (NCP), gross primary production (GPP) and environmental variables in 14 regions and three ecosystem types (coastal, coral reef and open sea) from Australia, Papua New Guinea and Indonesia. The data are compiled from separate studies conducted between 2002 and 2014 with the goal of better parameterizing the metabolic balance in tropical marine waters. Overall, these regions were strongly autotrophic (average GPP:CR ratio: 2.14 ± 0.98), though our dataset of 783 paired measurements did include some oceanic stations where heterotrophy (GPP:CR < 1) was predominant, and some coastal stations that were intermittently heterotrophic. Our statistical analysis suggested that temperature was the most important determinant of CR in coral reef and ocean ecosystems but less so in coastal ecosystems, where chlorophyll concentration was more important. In contrast, chlorophyll and sampling depth were more important in regulating GPP than temperature. The relationships between temperatures and metabolic rates showed that these were ecosystem-dependent, with coastal ecosystems showing less response to temperature than coral reef and open sea sites. The threshold of GPP to achieve metabolic balance fell in a range between 0.715 mmol O2 m-3 d-1 in the Coral Sea to 10.052 mmol O2 m-3 d-1 in mangrove waterways of Hinchinbrook Channel. These data allow regions in and around northern Australia to be ranked in terms of trophic state, ranging from the oligotrophic Scott Reef (GPP:CR = 0.84 ± 0.08) to productive surface waters of the Kimberley coast (GPP:CR = 5.21 ± 0.62). The measurement of pelagic metabolism shows potential as a quantitative tool to monitor the trophic state of coastal waters.

Vertical segregation among pathways mediating nitrogen loss (N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; and N&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt;O production) across the oxygen gradient in a coastal upwelling ecosystem

Abstract. The upwelling system off central Chile (36.5° S) is seasonally subjected to oxygen (O2)-deficient waters, with a strong vertical gradient in O2 (from oxic to anoxic conditions) that spans a few metres (30–50 m interval) over the shelf. This condition inhibits and/or stimulates processes involved in nitrogen (N) removal (e.g. anammox, denitrification, and nitrification). During austral spring (September 2013) and summer (January 2014), the main pathways involved in N loss and its speciation, in the form of N2 and/or N2O, were studied using 15N-tracer incubations, inhibitor assays, and the natural abundance of nitrate isotopes along with hydrographic information. Incubations were developed using water retrieved from the oxycline (25 m depth) and bottom waters (85 m depth) over the continental shelf off Concepción, Chile. Results of 15N-labelled incubations revealed higher N removal activity during the austral summer, with denitrification as the dominant N2-producing pathway, which occurred together with anammox at all times. Interestingly, in both spring and summer maximum potential N removal rates were observed in the oxycline, where a greater availability of oxygen was observed (maximum O2 fluctuation between 270 and 40 µmol L−1) relative to the hypoxic bottom waters ( &lt; 20 µmol O2 L−1). Different pathways were responsible for N2O produced in the oxycline and bottom waters, with ammonium oxidation and dissimilatory nitrite reduction, respectively, as the main source processes. Ammonium produced by dissimilatory nitrite reduction to ammonium (DNiRA) could sustain both anammox and nitrification rates, including the ammonium utilized for N2O production. The temporal and vertical variability of δ15N-NO3− confirms that multiple N-cycling processes are modulating the isotopic nitrate composition over the shelf off central Chile during spring and summer. N removal processes in this coastal system appear to be related to the availability and distribution of oxygen and particles, which are a source of organic matter and the fuel for the production of other electron donors (i.e. ammonium) and acceptors (i.e. nitrate and nitrite) after its remineralization. These results highlight the links between several pathways involved in N loss. They also establish that different mechanisms supported by alternative N substrates are responsible for substantial accumulation of N2O, which are frequently observed as hotspots in the oxycline and bottom waters. Considering the extreme variation in oxygen observed in several coastal upwelling systems, these findings could help to understand the ecological and biogeochemical implications due to global warming where intensification and/or expansion of the oceanic OMZs is projected.

Seasonal oxygen dynamics in a warm temperate estuary: effects of hydrologic variability on measurements of primary production, respiration, and net metabolism.

Seasonal responses in estuarine metabolism (primary production, respiration, and net metabolism) were examined using two complementary approaches. Total ecosystem metabolism rates were calculated from dissolved oxygen time series using Odum's open water method. Water column rates were calculated from oxygen-based bottle experiments. The study was conducted over a spring-summer season in the Pensacola Bay estuary at a shallow seagrass-dominated site and a deeper bare-bottomed site. Water column integrated gross production rates more than doubled (58.7 to 130.9 mmol O2 m-2 d-1) from spring to summer, coinciding with a sharp increase in water column chlorophyll-a, and a decrease in surface salinity. As expected, ecosystem gross production rates were consistently higher than water column rates, but showed a different spring-summer pattern, decreasing at the shoal site from 197 to 168 mmol O2 m-2 d-1 and sharply increasing at the channel site from 93.4 to 197.4 mmol O2 m-2 d-1. The consistency among approaches was evaluated by calculating residual metabolism rates (ecosystem - water column). At the shoal site, residual gross production rates decreased from spring to summer from 176.8 to 99.1 mmol O2 m-2 d-1, but were generally consistent with expectations for seagrass environments, indicating that the open water method captured both water column and benthic processes. However, at the channel site, where benthic production was strongly light-limited, residual gross production varied from 15.7 mmol O2 m-2 d-1 in spring to 86.7 mmol O2 m-2 d-1 in summer. The summer rates were much higher than could be realistically attributed to benthic processes, and likely reflected a violation of the open water method due to water column stratification. While the use of sensors for estimating complex ecosystem processes holds promise for coastal monitoring programs, careful attention to the sampling design, and to the underlying assumptions of the methods, is critical for correctly interpreting the results. This study demonstrated how using a combination of approaches yielded a fuller understanding of the ecosystem response to hydrologic and seasonal variability.

A multi‐year time‐series of N2O dynamics in a seasonally anoxic fjord: Saanich Inlet, British Columbia

AbstractWe present an 8‐yr time‐series of monthly water column N2O measurements from Saanich Inlet, a seasonally anoxic fjord in southern British Columbia. We document seasonal and inter‐annual variability in N2O concentrations driven by physical and biological forcing, and examine the relationship between N2O and O2 concentrations across the redoxcline in this system. Near‐surface N2O concentrations were typically supersaturated, and increased with depth to a maximum near the oxic‐anoxic transition. Genes associated with both the nitrification and denitrification pathways were widely distributed throughout the water column, suggesting the potential for simultaneous N2O production from nitrification and incomplete denitrification. Maximum N2O concentrations in Saanich Inlet were similar to other anoxic basins, but lower than those previously documented in coastal upwelling and open ocean oxygen minimum zones. N2O and O2 were inversely correlated throughout the water column, except below 10 μmol O2 L−1 where net N2O consumption appears to occur. In surface waters, maximum N2O concentrations and sea–air fluxes occurred during late summer/early fall, when O2 levels in the oxycline were low. Annual deep basin renewals often persisted over multiple months, supplying O2, , and N2O to the deep basin, with N2O subsequently declining to undetectable levels (&lt; 0.4 nmol N2O L−1) by late spring. An unusually weak renewal was observed during 2009, possibly linked to reduced upwelling off the coast of British Columbia during El Niño. Our results can be used as a baseline to identify the effect of longer‐term changes in Saanich Inlet, such as deoxygenation and its effect on N2O concentrations.

Water Oxidation Reaction on Fe(III)-Cr(III) Mixed Oxide Under Electrochemical and Photochemical Condition

Introduction Water splitting into H2 and O2is attracted as one of useful strategies to produce hydrogen. This reaction is separated to two half redox reactions. 2H+ + 2e– → H2(0 V vs. RHE) 2H2O + 4h+ → O2 + 4H+(1.23 V vs. RHE) In general, water oxidation reaction is the rate determination step in water splitting because 4 electrons are involved in the reaction. Various catalysts aiming to enhance water oxidation have therefore been investigated. Metal oxides have been applied to such water oxidation catalysts for photochemical and electrochemical water oxidation. These metal oxide materials are also applied in the form of nanoparticles on the surface of a semiconductor, which are called cocatalyst. For example, IrO x , CoO x and RuO2are well known as not only bulk catalysts but also useful cocatalyst materials for water oxidation. Recently, our group has reported that Fe(III)-Cr(III) mixed oxide nanoparticles worked as cocatalysts for photocatalytic water oxidation, even better than pristine Fe2O3. However, the detail mechanism of the promotional effect caused by Cr(III) substitution has been still unclear. Nanoparticle cocatalysts loaded on a semiconductor photocatalyst worked as reaction sites for water oxidation or reduction by collecting holes/electron generated in the semiconductor. So they can be regarded as electrocatalysts for water splitting. To investigate the effect of Cr(III) substitution for Fe2O3, we focused material characteristics using bulk Fe(III) and Cr(III) mixed oxide particles as model compounds. In the present study, we examined the catalytic performance of Fe(III)-Cr(III) mixed oxide (Fe2-x Cr x O3) for water oxidation under photochemical and electrochemical condition as shown in Scheme 1, aiming to investigate the promotional effect of Cr(III) substitution for Fe2O3. Experimental Fe2-x Cr x O3 was prepared by a precipitation method. Appropriate amounts of Fe(NO3)3·9H2O and Cr(NO3)3·9H2O were dissolved in 20 mL of pure water. After addition of 1 mL NH3 aqueous solution, the precipitates were dried at 343 K for overnight. After grinding, the products were calcined at 823 K for 2 h after heating at 623 K for 2 h. Photochemical water oxidation activity was investigated under visible light irradiation (480 < λ < 500 nm) in the presence of tris(2,2’-bipyridyl)ruthenium(II) sulfate as a photosensitizer and sodium persulfate as a sacrificial electron acceptor. Electrochemical water oxidation was conducted in a phosphate buffered aqueous solution (pH 7.5) using three-electrode configuration, which consisted of a Ag/AgCl reference electrode, a Pt wire counter electrode and the Fe2–x Cr x O3/FTO working electrode. Result and discussion In photochemical water oxidation reaction, Fe2O3 showed catalytic performance for O2 evolution. Comparing to Fe2O3, the O2 evolution activity is dropped with increasing Cr ratio. Because the photochemical reaction proceeded on the surface of oxides, the property of Fe(III)-Cr(III) mixed oxide would have great influence on the O2 evolution activity. It is suggested that the catalytic activity that Fe2-x Cr x O3possessed for water oxidation is dropped by increasing the Cr ratio. Electrochemical water oxidation activity using Fe2-x Cr x O3/FTO electrode is investigated by controlled-potential electrolysis with gas detection and solution analysis. Faradaic efficiency of water oxidation and oxidative dissolution of Cr(III) to Cr(IV) during the electrolysis were determined by the amount of evolved O2 and dissolved Cr(IV) ion. Using Cr2O3 electrode, O2 was not detected, and the observed anodic current originated from oxidative dissolution of Cr(III). On the other hand, Fe2O3 electrode showed nearly 100% Faradaic efficiency for water oxidation, producing evolution of 0.2 μmol O2 after 50 min of electrolysis. In the case of Fe0.7Cr1.3O3 and Fe1.6Cr0.4O3, the amount of evolved O2 was improved about 5 times relative to Fe2O3, although Cr dissolution occurred to some extent (10–30% efficiencies). These results indicate that the activity of electrochemical water oxidation is improved by Cr substitution, although Cr dissolution occurred at the same time. Electrochemical impedance spectroscopy indicated that the promotional effect achieved by the Cr substitution resulted from improved charge transfer inside Fe2–x Cr x O3. We believe that this conclusion may be applied to nanoparticle-based photocatalytic overall water splitting system, where Fe2–x Cr x O3 nanoparticles were employed as water oxidation cocatalysts. Figure 1

Biological production, export efficiency, and phytoplankton communities across 8000 km of the South Atlantic

AbstractIn situ oxygen tracers (triple oxygen isotope and oxygen/argon ratios) were used to evaluate meridional trends in surface biological production and export efficiency across ~8000 km of the tropical and subtropical South Atlantic in March–May 2013. We used observations of picophytoplankton, nanophytoplankton, and microphytoplankton to evaluate community structure and diversity and assessed the relationships of these characteristics with production, export efficiency, and particulate organic carbon (POC) fluxes. Rates of productivity were relatively uniform along most of the transect with net community production (NCP) between 0 and 10 mmol O2 m−2 d−1, gross primary production (GPP) between 40 and 100 mmol O2 m−2 d−1, and NCP/GPP, a measure of export efficiency, ranging from 0.1 to 0.2 (0.05–0.1 in carbon units). However, notable exceptions to this basin‐scale homogeneity included two locations with highly enhanced NCP and export efficiency compared to surrounding regions. Export of POC and particulate nitrogen, derived from sediment traps, correlated with GPP across the transect, over which the surface community was dominated numerically by picophytoplankton. NCP, however, did not correlate with POC flux; the mean difference between NCP and POC flux was similar to published estimates of dissolved organic carbon export from the surface ocean. The interrelated rates of production presented in this work contribute to the understanding, building on the framework of better‐studied ocean basins, of how carbon is biologically transported between the atmosphere and the deep ocean.

Influence of mixed upwelled waters on metabolic balance in a subtropical coastal ecosystem: São Sebastião Channel, southern Brazil

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 573:61-72 (2017) - DOI: https://doi.org/10.3354/meps12162 Influence of mixed upwelled waters on metabolic balance in a subtropical coastal ecosystem: São Sebastião Channel, southern Brazil Aurore Regaudie-de-Gioux1,4,*, Alexandre Castagna1, Amabile Ferreira1, Medea Abbrecht2, Elisabete S. Braga3, Aurea M. Ciotti1 1Laboratório Aquarela, Centro de Biologia Marinha (CEBIMar), Universidade de São Paulo, Rod. Manoel Hipólito do Rego Km 131.5, São Sebastião-SP, 11600-000, Brazil 2Universität Hohenheim, Schloss Hohenheim 1, 70599 Stuttgart, Germany 3Laboratório de Biogeoquímica de Nutrientes, Micronutrientes e Traços nos Oceanos, Instituto Oceanográfico, Universidade de São Paulo, Praça do Oceanográfico, 191, São Paulo-SP, 05508-900, Brazil 4Present address: Instituto Mediterranéo de Estudios Avanzados, calle Miquel Marqués 21, 07190 Esporles, Spain *Corresponding author: aurore.regaudie@imedea.uib-csic.es ABSTRACT: Planktonic gross primary production (GPP), community respiration (CR) and net community production (NCP) were evaluated for the first time in the central part of the Southern Brazilian Bight (SBB), in the São Sebastião Channel (SSC); and their response to the South Atlantic Coastal Water (SACW) intrusion was investigated. Sampling experiments took place during late spring 2015 and summer 2016 at a fixed station in the southern portion of the SSC. Although both chlorophyll a and nutrient concentrations were characteristic of oligotrophic and low productive ecosystems, all volumetric planktonic metabolism experiments showed high net and gross primary productions rates. Depth-integrated NCP and GPP values had a global mean ±SE of 31 ± 10.5 and 106.7 ± 11.5 mmol O2 m-2 d-1, respectively. The communities tended to be net heterotrophic (i.e. GPP/CR < 1) at volumetric GPP values less than 7.10 mmol O2 m-3 d-1. The intrusion of nutrient-rich and -mixed water (partly SACW) observed during the summer experiments had a strong influence on autotrophic planktonic communities, enhancing their NCP rates about 3-fold. Nutrient supply by terrestrial inputs and by upwelled SACW intrusion in the SSC enables high primary production to be sustained with a dominance of autotrophic communities. KEY WORDS: Plankton · Metabolism · Southern Brazilian Bight · SACW Full text in pdf format PreviousNextCite this article as: Regaudie-de-Gioux A, Castagna A, Ferreira A, Abbrecht M, Braga ES, Ciotti AM (2017) Influence of mixed upwelled waters on metabolic balance in a subtropical coastal ecosystem: São Sebastião Channel, southern Brazil. Mar Ecol Prog Ser 573:61-72. https://doi.org/10.3354/meps12162 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 573. Online publication date: June 21, 2017 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2017 Inter-Research.

Estimation of net ecosystem metabolism of seagrass meadows in the coastal waters of the East Sea and Black Sea using the noninvasive eddy covariance technique

We measured the community-scale metabolism of seagrass meadows in Bulgaria (Byala [BY]) and Korea (Hoopo Bay [HP]) to understand their ecosystem function in coastal waters. A noninvasive in situ eddy covariance technique was applied to estimate net O2 flux in the seagrass meadows. From the high-quality and high-resolution time series O2 data acquired over > 24 h, the O2 flux driven by turbulence was extracted at 15-min intervals. The spectrum analysis of vertical flow velocity and O2 concentration clearly showed well-developed turbulence characteristics in the inertial subrange region. The hourly averaged net O2 fluxes per day ranged from -474 to 326 mmol O2 m-2 d-1 (-19 ± 41 mmol O2 m-2 d-1) at BY and from -74 to 482 mmol O2 m-2 d-1 (31 ± 17 mmol O2 m-2 d-1) at HP. The net O2 production rapidly responded to photosynthetically available radiation (PAR) and showed a good relationship between production and irradiance (P-I curve). The hysteresis pattern of P-I relationships during daytime also suggested increasing heterotrophic respiration in the afternoon. With the flow velocity between 3.30 and 6.70 cm s-1, the community metabolism during daytime and nighttime was significantly increased by 20 times and 5 times, respectively. The local hydrodynamic characteristics may be vital to determining the efficiency of community photosynthesis. The net ecosystem metabolism at BY was estimated to be -17 mmol O2 m-2 d-1, which was assessed as heterotrophy. However, that at HP was 36 mmol O2 m-2 d-1, which suggested an autotrophic state.