A Review of Protist Grazing Below the Photic Zone Emphasizing Studies of Oxygen-Depleted Water Columns and Recent Applications of In situ Approaches
"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.
Upwelling and isolation in oxygen-depleted anticyclonic modewater eddies and implications for nitrate cycling
"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 (T∕S) characteristic over the 2 months, but an erosion of the core progressively narrows down the T∕S 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. [...]"
Authors: Johannes Karstensen et al.
Influence of dissolved oxygen on the protectiveness and morphological characteristics of calcareous deposits with galvanostatic polarization
"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
Upper Ocean O2 trends: 1958-2015
"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.
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. "
Temperature Decouples Ammonium and Nitrite Oxidation in Coastal Waters
"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
New Species of Ciliates (Genus: Strombidium sp.) from hypoxic waters of the Bay of Bengal, Northern Indian Ocean
"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
Deoxygenation of the Baltic Sea during the last century
"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.
Decadal dynamics and predictability of oxygen and subsurface tracers in the California Current System
"The oxygen of the source waters that feed the upwelling in the California Current System show prominent multi-decadal fluctuations that are not significantly correlated with the dominant modes of Pacific climate variability. By combining observations and ocean reanalysis products between 1950-2010, we show that decadal changes in oxygen are linked to subsurface salinity variability and primarily controlled by ocean circulation dynamics. We find that subsurface anomalies in the core of the North Pacific Current propagate the oxygen signal downstream into the coastal upwelling system following the path of the mean gyre circulation with a timescale of 10-years. [...]"
Source: Geophysical Reasearch Letters
Authors: Mercedes Pozo Buil, Emanuele Di Lorenzo
Aerobic and anaerobic ammonium oxidizers in the Cariaco Basin: distributions of major taxa and nitrogen species across the redoxcline
"Depth distributions of cells and functional gene copies from anaerobic ammonium-oxidizing (anammox) bacteria, aerobic ammonium-oxidizing bacteria (AOB) and archaea (AOA) in the Cariaco Basin, Venezuela were obtained using FISH and q-PCR assays. These distributions were compared to concentrations of dissolved ammonium (NH4+), nitrite (NO2-), nitrate (NO3-), hydrogen sulfide (H2S) and oxygen (O2) along the redoxcline during 3 cruises. Cell counts of anammox bacteria and copies of their nitrite reductase gene (Scalindua-nirS) were consistently observed in 2 distinct layers: the suboxic zone (≤1.1 × 106 cells l-1) and the upper euxinic zone (≤4.7 × 106 cells l-1). [...]"
Source: Aquatic Microbial Ecology 79
Authors: Sara Cernadas-Martín et al.
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