Dubravko Justic, the Texaco Distinguished Professor in the LSU Department of Oceanography & Coastal Sciences, and Research Associate Lixia Wang recently co-authored a study suggesting that measuring the volume rather than the area of the Gulf of Mexico's dead zone, is more appropriate for monitoring its effects on marine organisms.

 

"The dead zone, a hypoxic zone, is a region of low oxygen that results from runoff of high nutrients, such as nitrogen and phosphorus, often found in fertilizer, flowing from the Mississippi River into the coastal ocean. It is the largest recurring hypoxic zone in the U.S., occurring most summers, and is located off the coast of Louisiana. This nutrient pollution, coupled with other factors, is believed to have a negative impact on fisheries because it depletes the oxygen required to support most marine life in bottom and near-bottom waters. [...]"

Source: Science Daily

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Widespread and sometimes drastic marine oxygen declines are stressing sensitive species—a trend that will continue with climate change

"Escaping predators, digestion and other animal activities—including those of humans—require oxygen. But that essential ingredient is no longer so easy for marine life to obtain, several new studies reveal.

In the past decade ocean oxygen levels have taken a dive—an alarming trend that is linked to climate change, says Andreas Oschlies, an oceanographer at the Helmholtz Center for Ocean Research Kiel in Germany, whose team tracks ocean oxygen levels worldwide. “We were surprised by the intensity of the changes we saw, how rapidly oxygen is going down in the ocean and how large the effects on marine ecosystems are,” he says. [...]"

Source: Scientific American
Author: Laura Poppick

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Abstract.

"Over the past decades, observations have confirmed decreasing oxygen levels and shoaling of oxygen minimum zones (OMZs) in the tropical oceans. Such changes impact the biogeochemical cycling of micronutrients such as Cd, but the potential consequences are only poorly constrained. Here, we present seawater Cd concentrations and isotope compositions for 12 depth profiles at coastal, nearshore and offshore stations from 4°S to 14°S in the eastern tropical South Pacific, where one of the world's strongest OMZs prevails. [...]"

Source: Earth and PLanetary Science Letters
Authors: Ruifang C. Xie et al.
DOI: 10.1016/j.epsl.2019.02.001

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Abstract.

"Hydroxylamine (NH₂OH), a short‐lived intermediate in the nitrogen cycle, is a potential precursor of nitrous oxide (N₂O) in the ocean. However, measurements of NH₂OH in the ocean are sparse. Here we present a data set of depth profiles of NH₂OH from the equatorial Atlantic Ocean and the eastern tropical South Pacific and compare it to N₂O, nitrate, and nitrite profiles under varying oxygen conditions. The presence of NH₂OH in surface waters points toward surface nitrification in the upper 100 m. [...]"

Source: Geophysical Research Letters
Authors: Frederike Korth et al.
DOI: 10.1029/2018GL080466

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Abstract.

"We quantified the removal of fixed nitrogen as N₂ production by anammox and N₂ and N₂O production by denitrification over a distance of 1900 km along the coasts of Chile and Peru, using short‐term incubations with ¹⁵N‐labeled substrates. The eastern South Pacific contains an oxygen minimum zone (OMZ) characterized by an anoxic, nitrate‐ and nitrite‐rich layer of ∼ 200‐m thickness below 30–90 m of oxic water. [...]"

Source: Limnology and Oceanography
Authors: Tage Dalsgaard et al.
DOI: 10.4319/lo.2012.57.5.1331

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Abstract.

"Data from 13 autonomous profiling BGC-Argo floats, equipped with biogeochemical and bio-optical sensors deployed between 2011 and 2016, were used to explore the potential of bio-optical methods to map deep biomass distribution in the Arabian Sea oxygen minimum zone (OMZ). Dissolved oxygen sensors revealed concentrations below 5 μmol kg⁻¹ for much of the depth range between 200 and 400 m and below 1 μmol kg⁻¹ in the centre of the OMZ, which is well below climatological values. [...]"

Source: Journal of Marine Systems
Authors: Bożena Wojtasiewicz et al.
DOI: 10.1016/j.jmarsys.2018.07.002

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Abstract.

"Previous work estimating the N₂excess above background due to denitrification has suggested that nitrate deficit-type methods may be an underestimate of fixed nitrogen (N) loss in the major oxygen deficient zones of the ocean. The N2excess approach has the advantage over nitrate deficit-type methods in that it does not depend on stoichiometric assumptions of fixed N to phosphate or oxygen utilization and avoids any uncertainly regarding the pathway of N loss. [...]"

Source: Global Geochemical Cycles (2012)
Authors: Bonnie X. Chang, Allan H. Devol and Steven R. Emerson
DOI: 10.1029/2011GB004207

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"Growing populations and tightening water supplies have spurred people in many places—including the Middle East, Australia, California and China—to look to the oceans and other salty waters as a source of new drinking water. But desalination plants are energy intensive and create a potentially environment-harming waste called brine (made up of concentrated salt and chemical residues), which is dumped into the ocean, injected underground or spread on land.

Despite the ecological threats, “there was no comprehensive assessment about brine—how much we produce,” says Manzoor Qadir, assistant director of the United Nations University Institute on Water, Environment and Health. So he and his colleagues calculated that figure and found it is 50 percent greater than the desalination industry’s previous rough estimate. In fact, it is enough to cover Florida with 30 centimeters of brine every year. [...]"

Source: Scientific American

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Abstract.

"We investigated anammox, denitrification and dissimilatory reduction of nitrite to ammonium (DNRA) activity in the Eastern Tropical South Pacific oxygen minimum zone (OMZ) off northern Chile, at high‐depth resolution through the oxycline into the anoxic OMZ core. This was accompanied by high‐resolution nutrient and oxygen profiles to link changes in nitrogen transformation rates to physicochemical characteristics of the water column. Denitrification was detected at most depths, but anammox was the most active N2‐producing process, while DNRA was not detectable. [...]"

Source: Environmental Microbiology
Authors: Loreto De Brabandere et al.
DOI: 10.1111/1462-2920.12255

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Abstract.

"The so-called secondary nitrite maximum (SNM) is a pronounced subsurface feature in many oxygen deficient zones of the ocean. A nitrite layer of up to several hundred meters thickness indicates high microbial activity and nitrogen loss from the system. To study the effects of primary and export production on the SNM, we have developed a one-dimensional ecosystem model for oxygen deficient zones. Our model couples the marine nitrogen and oxygen cycles with physical water column processes, includes euphotic, aphotic, aerobic and anaerobic processes and thereby dynamically describes source and sink processes for nitrite. [...]"

Source: Ecological Modelling
Authors: Aike Beckmann and Inga Hense
DOI: 10.1016/j.ecolmodel.2017.05.014

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Abstract.

"Oligotrophic and productive areas of the ocean differ in plankton community composition and biomass transfer efficiency. Here, we describe the plankton community along a latitudinal transect in the tropical and subtropical Atlantic Ocean. Prochlorococcus dominated the autotrophic community at the surface and mixed layer of oligotrophic stations, replaced by phototrophic picoeukaryotes and Synechococcus in productive waters. [...]"

Source: Scientific Reports
Authors: Laia Armengol et al.
DOI: 10.1038/s41598-019-38507-9

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Abstract.

"Benthic foraminifera populate a diverse range of marine habitats. Their ability to use alternative electron acceptors—nitrate (NO3−) or oxygen (O2)—makes them important mediators of benthic nitrogen cycling. Nevertheless, the metabolic scaling of the two alternative respiration pathways and the environmental determinants of foraminiferal denitrification rates are yet unknown. We measured denitrification and O2 respiration rates for 10 benthic foraminifer species sampled in the Peruvian oxygen minimum zone (OMZ). [...]"

Source: PNAS
Authors: Nicolaas Glock et al.
DOI: 10.1073/pnas.1813887116

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Abstract.

"The eastern Mediterranean Sea sedimentary record is characterised by intervals of organic rich sediment (sapropels), indicating periods of severe anoxia triggered by astronomical forcing. It has been hypothesized that nitrogen fixation was crucial in injecting the Mediterranean Sea with bioavailable nitrogen (N) during sapropel events. However, the evolution of the N biogeochemical cycle of sapropels is poorly understood. For example, the role of the complementary removal reaction, anaerobic ammonium oxidation (anammox), has not been investigated because the traditional lipid biomarkers for anammox, ladderane fatty acids, are not stable over long periods in the sedimentary record. [...]

Source: Biogeosciences
Authors: Darci Rush et al.
DOI: 10.5194/bg-2019-27

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Abstract.

"While the value of giant kelp (Macrocystis pyrifera) as a habitat-forming foundation species is well-understood, it is unclear how they impact the oxygen concentration and pH of the surrounding seawater, and further, how such a dynamic abiotic environment will affect eco-evolutionary dynamics in a context of global change. Here, we profiled the nearshore kelp forest environment in Southern California to understand changes in dissolved oxygen (DO) and pH with high spatiotemporal resolution. We then examined transgenerational effects using sea urchins (Strongylocentrotus purpuratus) as our study organism.  [...]"

Source: Frontiers in Marine Science
Authors: Umihiko Hoshijima and Gretchen Hofmann
DOI: 10.3389/fmars.2019.00062

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"Climate change is causing significant changes to phytoplankton in the world's oceans, and a new MIT study finds that over the coming decades these changes will affect the ocean's color, intensifying its blue regions and its green ones. Satellites should detect these changes in hue, providing early warning of wide-scale changes to marine ecosystems. [...]"

Source: ScienceDaily

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Abstract.

"Marine calcifiers are considered to be among the most vulnerable taxa to climate-forced environmental changes occurring on continental margins with effects hypothesized to occur on microstructural, biomechanical, and geochemical properties of carbonate structures. Natural gradients in temperature, salinity, oxygen, and pH on an upwelling margin combined with the broad depth distribution (100–1,100 m) of the pink fragile sea urchin, Strongylocentrotus (formerly Allocentrotus) fragilis, along the southern California shelf and slope provide an ideal system to evaluate potential effects of multiple climate variables on carbonate structures in situ. [...]"

Source: Nature Communications
Authors: Stephanie Dutkiewicz et al.
DOI: 10.1038/s41467-019-08457-x

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Abstract.

"Marine calcifiers are considered to be among the most vulnerable taxa to climate-forced environmental changes occurring on continental margins with effects hypothesized to occur on microstructural, biomechanical, and geochemical properties of carbonate structures. Natural gradients in temperature, salinity, oxygen, and pH on an upwelling margin combined with the broad depth distribution (100–1,100 m) of the pink fragile sea urchin, Strongylocentrotus (formerly Allocentrotus) fragilis, along the southern California shelf and slope provide an ideal system to evaluate potential effects of multiple climate variables on carbonate structures in situ. [...]"

Source: Frontiers in Marine Science
Authors: Kirk N. Sato et al.
DOI: 10.3389/fmars.2018.00258

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Abstract.

"The mechanisms controlling the variability of oxygen levels in the ocean are poorly quantified. We focus here on the impact of wind synoptic variability associated with tropical convective regions and extra‐tropical storms. Removing the wind higher frequencies of variability (2 days – 1 month) in an atmosphere reanalysis used to force an ocean model decreases wind stress by up to 20% in the tropics and 50% in the mid‐latitudes, weakening wind‐driven ocean circulation by 20%. Oxygen levels decrease by up to 10 mmol.m^‐3 in tropical oceans and 30 mmol.m^‐3 in subtropical gyres mainly due to changes in advective processes. [...]"

Source: Geophysical Research Letters
Authors: Olaf Duteil
DOI: 10.1029/2018GL081041

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Abstract.

"Enhanced ocean carbon storage during the Pleistocene ice ages lowered atmospheric CO2 concentrations by 80 to 100 ppm relative to interglacial levels. Leading hypotheses to explain this phenomenon invoke a greater efficiency of the ocean's biological pump, in which case carbon storage in the deep sea would have been accompanied by a corresponding reduction in dissolved oxygen. We exploit the sensitivity of organic matter preservation in marine sediments to bottom water oxygen concentration to constrain the level of dissolved oxygen in the deep central equatorial Pacific Ocean during the last glacial period (18,000 – 28,000 years BP) to have been within the range of 20‐50 μmol/kg, much less than modern value of ca. 168 μmol/kg. [...]"

Source: Global Biogeochemical Cycles
Authors: Robert F. Anderson et al.
DOI: 10.1029/2018GB006049

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Abstract.

"Oxygen availability is key in determining habitat suitability for marine fish. As a result of climate change, low oxygen conditions are predicted to occur more frequently and over a greater geographic extent. Studies assessing the long-term chronic effects and impacts for commercially important fish are rare. To assess the potential effects of climate-induced low oxygen on fisheries, physiological data, such as critical thresholds, derived from laboratory experiments on 5 commercial fish species were integrated with hindcast and future oxygen projections from the hydrodynamic-biogeochemical model GETM-ERSEM. [...]"

Source: Marine Ecology Progress Series (2017)
Authors: Bryony L. Townhill et al.
DOI: 10.3354/meps12291

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