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Denitrifying community in coastal sediments performs aerobic and anaerobic respiration simultaneously

Abstract.

"Nitrogen (N) input to the coastal oceans has increased considerably because of anthropogenic activities, however, concurrent increases have not occurred in open oceans. It has been suggested that benthic denitrification in sandy coastal sediments is a sink for this N. Sandy sediments are dynamic permeable environments, where electron acceptor and donor concentrations fluctuate over short temporal and spatial scales. The response of denitrifiers to these fluctuations are largely unknown, although previous observations suggest they may denitrify under aerobic conditions. We examined the response of benthic denitrification to fluctuating oxygen concentrations, finding that denitrification not only occurred at high O2 concentrations but was stimulated by frequent switches between oxic and anoxic conditions. [...]"

Source: The ISME Journal
Authors: Hannah K Marchant et al.
DOI: 10.1038/ismej.2017.51

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Intensification and deepening of the Arabian Sea Oxygen Minimum Zone in response to increase in Indian monsoon wind intensity

Abstract.

The decline in oxygen supply to the ocean associated with global warming is expected to expand oxygen minimum zones (OMZs). This global trend can be attenuated or amplified by regional processes. In the Arabian Sea, the World’s thickest OMZ is highly vulnerable to changes in the Indian monsoon wind. Evidence from paleo records and future climate projections indicate strong variations of the Indian monsoon wind intensity over climatic timescales. Yet, the response of the OMZ to these wind changes remains poorly understood and its amplitude and timescale unexplored. Here, we investigate the impacts of perturbations in Indian monsoon wind intensity (from −50 % to +50 %) on the size and intensity of the Arabian Sea OMZ, and examine the biogeochemical and ecological implications of these changes.

Source: Biogeosciences (in Review)
Authors: Zouhair Lachkar, Marina Lévy, and Shafer Smith
DOI: 10.5194/bg-2017-146

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A Review of Protist Grazing Below the Photic Zone Emphasizing Studies of Oxygen-Depleted Water Columns and Recent Applications of In situ Approaches

Abstract.

"Little is still known of the impacts of protist grazing on bacterioplankton communities in the dark ocean. Furthermore, the accuracy of assessments of in situ microbial activities, including protist grazing, can be affected by sampling artifacts introduced during sample retrieval and downstream manipulations. Potential artifacts may be increased when working with deep-sea samples or samples from chemically unique water columns such as oxygen minimum zones (OMZs). OMZs are oxygen-depleted regions in the ocean, where oxygen concentrations can drop to <20 μM. These regions are typically located near eastern boundary upwelling systems and currently occur in waters occupying below about 8% of total ocean surface area, representing ~1% of the ocean's volume. [...]"

Source: Frontiers in Marine Science
Authors: Luis E. Medina et al.
DOI: 10.3389/fmars.2017.00105

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Upwelling and isolation in oxygen-depleted anticyclonic modewater eddies and implications for nitrate cycling

Abstract. 

"The temporal evolution of the physical and biogeochemical structure of an oxygen-depleted anticyclonic modewater eddy is investigated over a 2-month period using high-resolution glider and ship data. A weakly stratified eddy core (squared buoyancy frequency N2  ∼  0.1  ×  10−4 s−2) at shallow depth is identified with a horizontal extent of about 70 km and bounded by maxima in N2. The upper N2 maximum (3–5  ×  10−4 s−2) coincides with the mixed layer base and the lower N2 maximum (0.4  ×  10−4 s−2) is found at about 200 m depth in the eddy centre. The eddy core shows a constant slope in temperature/salinity (TS) characteristic over the 2 months, but an erosion of the core progressively narrows down the TS range. The eddy minimal oxygen concentrations decreased by about 5 µmol kg−1in 2 months, confirming earlier estimates of oxygen consumption rates in these eddies. [...]"

 

Source: Biogeosciences
Authors: Johannes Karstensen et al.
DOI: 10.5194/bg-14-2167-2017

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Influence of dissolved oxygen on the protectiveness and morphological characteristics of calcareous deposits with galvanostatic polarization

Abstract.

"The influence of dissolved oxygen on calcareous deposits formed under galvanostatic polarization mode was studied. When the dissolved oxygen concentration was less than 7 mg L−1, the cathodic protection potential showed a plateau at the initial polarization, and then quickly shifted negatively. While the dissolved oxygen was more than 9 mg L−1, the potential shifted negatively in a linear form. After 168 h of polarization, the final protection potential shifted negatively with the decreasing dissolved oxygen concentration.  The deposition progress was monitored by electrochemical impedance spectroscopy, and only one single loop was found in Nyquist diagram, indicating deposits of ineffective protectiveness precipitation under the experimental conditions. [...]"

 

Source: Journal of Ocean University of China
Authors: Chengjie Li, Min Du, Rongjie Gao
DOI: 10.1007/s11802-017-2933-4
 

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Upper Ocean O2 trends: 1958-2015

Abstract.

"Historic observations of dissolved oxygen (O2) in the ocean are analyzed to quantify multi-decadal trends and variability from 1958 to 2015. Additional quality control is applied and the resultant oxygen anomaly field is used to quantify upper ocean O2 trends at global and hemispheric scales. A widespread negative O2 trend is beginning to emerge from the envelope of interannual variability. Ocean reanalysis data is used to evaluate relationships with changes in ocean heat content (OHC) and oxygen solubility (O2,sat). Global O2 decline is evident after the 1980s, accompanied by an increase in global OHC. [...]"

 

Source: Geophysical Reasearch Letters
Authors: Takamitsu Ito et al.
DOI: 10.1002/2017gl073613

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Rising water temperatures endanger health of coastal ecosystems, study finds

"Increasing water temperatures are responsible for the accumulation of a chemical called nitrite in marine environments throughout the world, a symptom of broader changes in normal ocean biochemical pathways that could ultimately disrupt ocean food webs, according to new research from the University of Georgia. " 

Source: phys.org

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Temperature Decouples Ammonium and Nitrite Oxidation in Coastal Waters

Abstract.

"Nitrification is a two-step process linking the reduced and oxidized sides of the nitrogen cycle. These steps are typically tightly coupled with the primary intermediate, nitrite, rarely accumulating in coastal environments. Nitrite concentrations can exceed 10 μM during summer in estuarine waters adjacent to Sapelo Island, Georgia, U.S.A. Similar peaks at other locations have been attributed to decoupling of the two steps of nitrification by hypoxia; however, the waters around Sapelo Island are aerobic and well-mixed.  Experiments examining the response to temperature shifts of a nitrifying assemblage composed of the same organisms found in the field indicate that ammonia- and nitrite-oxidation become uncoupled between 20 and 30 °C, leading to nitrite accumulation. [...]"

Source: Environmental Science & Technology 
Authors: Sylvia C. Schaefer, James T. Hollibaugh
DOI: 10.1021/acs.est.6b03483

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New Species of Ciliates (Genus: Strombidium sp.) from hypoxic waters of the Bay of Bengal, Northern Indian Ocean

Abstract.

"Present study describes a new species of Strombidium (oligotrich ciliates) found in the cold sub-surface (125m below surface) oxic-hypoxic boundary of the Bay of Bengal. We name it as Strombidium mansai and describe its morphology." 

[...] "The  biological productivity of Bay of Bengal is governed by mesoscale eddies and tropical cyclones that trigger occasional higher productivity that could also add to the consumption of dissolved nutrients at sub-surface depth. These zones of hypoxic waters are found to be dominated by large number of bacterial community probavly driving unique microbial community in the Idian Ocean."

Source: Indian Journal of Geo Marine Science
Authors: Sai Elangovan, Mangesh Gauns

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Deoxygenation of the Baltic Sea during the last century

Abstract.

"Deoxygenation is a global problem in coastal and open regions of the ocean, and has led to expanding areas of oxygen minimum zones and coastal hypoxia. The recent expansion of hypoxia in coastal ecosystems has been primarily attributed to global warming and enhanced nutrient input from land and atmosphere. The largest anthropogenically induced hypoxic area in the world is the Baltic Sea, where the relative importance of physical forcing versus eutrophication is still debated. We have analyzed water column oxygen and salinity profiles to reconstruct oxygen and stratification conditions over the last 115 y and compare the influence of both climate and anthropogenic forcing on hypoxia. [...]"

Source: Procedings of the National Academy of Sciences of the United States of America (PNAS)
Authors: Jacob Carstensen et al.
DOI: 10.1073/pnas.1323156111

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