Ocean oxygen levels

[Juan C. García]

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

https://www.scientificamerican.com/article/the-ocean-is-running-out-of-breath-scientists-warn/?utm_source=newsletter&utm_medium=email&utm_campaign=weekly-review&utm_content=link&utm_term=2019-02-27_featured-this-week&spMailingID=58597866&spUserID=Mzg1NDE5MjQyNDEyS0&spJobID=1583431364&spReportId=MTU4MzQzMTM2NAS2

[vox_mundi]

In Ancient Oceans that Resembled Our Own, Oxygen Loss Triggered Mass Extinction 
https://phys.org/news/2019-03-ancient-oceans-resembled-oxygen-loss.html

Roughly 430 million years ago, during the Earth's Silurian Period, global oceans were experiencing changes that would seem eerily familiar today. Melting polar ice sheets meant sea levels were steadily rising, and ocean oxygen was falling fast around the world.

At around the same time, a global die-off known among scientists as the Ireviken extinction event devastated scores of ancient species. Eighty percent of conodonts, which resembled small eels, were wiped out, along with half of all trilobites, which scuttled along the seafloor like their distant, modern-day relative the horseshoe crab.

Now, for the first time, a Florida State University team of researchers has uncovered conclusive evidence linking the period's sea level rise and ocean oxygen depletion to the widespread decimation of marine species. Their work highlights a dramatic story about the urgent threat posed by reduced oxygen conditions to the rich tapestry of ocean life.

... The experiments revealed significant global oxygen depletion contemporaneous with the Ireviken event. Compounded with the rising sea level, which brought deoxygenated waters into shallower and more habitable areas, the reduced oxygen conditions were more than enough to play a central role in the mass extinction. This was the first direct evidence of a credible link between expansive oxygen loss and the Ireviken extinction event.

Only about 8 percent or less of the global oceans experienced significantly reducing conditions with very little to no oxygen and high levels of toxic sulfide, suggesting that these conditions didn't need to advance to whole-ocean scale to have an outsized, destructive effect. 

Seth A. Young et al. Geochemical evidence for expansion of marine euxinia during an early Silurian (Llandovery–Wenlock boundary) mass extinction, Earth and Planetary Science Letters (2019)

[Sam]

Add to this the follow on effects of hydrogen sulfide emission from the oceans and lowering atmospheric oxygen levels.

Some speculate that these were major factors in the extinctions on land.

These effects are apparently short lived in geologic terms (thousands of years). That is no consolation to those killed by them.

[kassy]

Article
Open Access
Published: 16 November 2020
Potential virus-mediated nitrogen cycling in oxygen-depleted oceanic waters

Abstract
Viruses play an important role in the ecology and biogeochemistry of marine ecosystems. Beyond mortality and gene transfer, viruses can reprogram microbial metabolism during infection by expressing auxiliary metabolic genes (AMGs) involved in photosynthesis, central carbon metabolism, and nutrient cycling. While previous studies have focused on AMG diversity in the sunlit and dark ocean, less is known about the role of viruses in shaping metabolic networks along redox gradients associated with marine oxygen minimum zones (OMZs). Here, we analyzed relatively quantitative viral metagenomic datasets that profiled the oxygen gradient across Eastern Tropical South Pacific (ETSP) OMZ waters, assessing whether OMZ viruses might impact nitrogen (N) cycling via AMGs. Identified viral genomes encoded six N-cycle AMGs associated with denitrification, nitrification, assimilatory nitrate reduction, and nitrite transport. The majority of these AMGs (80%) were identified in T4-like Myoviridae phages, predicted to infect Cyanobacteria and Proteobacteria, or in unclassified archaeal viruses predicted to infect Thaumarchaeota. Four AMGs were exclusive to anoxic waters and had distributions that paralleled homologous microbial genes. Together, these findings suggest viruses modulate N-cycling processes within the ETSP OMZ and may contribute to nitrogen loss throughout the global oceans thus providing a baseline for their inclusion in the ecosystem and geochemical models.

...

Earth’s biogeochemical cycles are driven by microbial interaction networks, with significant contributions from the oceans [1, 2]. These networks and the distribution of metabolic pathways within them are modulated by environmental factors, grazing, and viral infections. Ocean viruses are abundant, kill ~20–40% of microbial cells per day,

...

Earth’s biogeochemical cycles are driven by microbial interaction networks, with significant contributions from the oceans [1, 2]. These networks and the distribution of metabolic pathways within them are modulated by environmental factors, grazing, and viral infections. Ocean viruses are abundant, kill ~20–40% of microbial cells per day,

...

In summary, understanding how viruses alter N-related biogeochemical cycling in OMZs is critical, considering the expansion of these suboxic and anoxic water masses and their effects in surface primary production, greenhouse gas emission, and fixed-nitrogen loss [32,33,34]. Our findings imply that OMZ viruses impact N cycling not only through lysis of key N-cycling microbes but also by modulating diverse N-metabolisms during infection. Such infected “virocells” [10] would be drastically altered in their metabolic capacity and biogeochemical outputs as has been shown now in several environmental model virus–host systems [10, 12, 129]. With these N-related virus AMGs now uncovered, future OMZ virus work can evaluate virocell-impacted nitrogen cycling, as well as develop primer sets for “viral” vs “cellular” versions to differentially quantify the biogeochemical impacts of viruses in OMZ N-cycling genes and transcripts. As standardized practices emerge for viral ecogenomics [130,131,132], they are enabling the development of global maps of ocean viruses [30, 49, 133] that can be integrated into multi-organism ecological studies [134]. Together these efforts to understand virus-mediated nutrient cycling in climate-critical environments, along with parallel efforts on land (e.g., thawing permafrosts [135, 136]), are now providing quantitative information needed to incorporate viruses into predictive models [137].

https://www.nature.com/articles/s41396-020-00825-6

[Juan C. García]

A High-Resolution Atlas of the Eastern Tropical Pacific Oxygen Deficient Zones

Abstract
Oxygen deficient zones (ODZs) are important biogeochemical provinces of the global oceans wherein standing dissolved oxygen concentrations decrease to nanomolar levels. Despite their confinement, these regions are disproportionally important to the ocean's role in modulating Earth's climate through the interactions between the marine nitrogen cycle and that of carbon. Moreover, the spatial domain of low oxygen regions of the ocean is predicted to change as a consequence of ocean warming, increased stratification, and changes in circulation and productivity. However, the expanse of the modern ODZs is poorly resolved due to a dearth of direct sampling compounded with errors that arise in the processing and gridding of the sparse measurements that do exist. Here, we take a novel approach to map the horizontal and vertical extent of the two major ODZs of the eastern tropical Pacific via analysis of meter-scale resolution electrode sensors from both ship casts and Argo profiles, rather than from discretized bottle measurements. The resulting three-dimensional data product is based on a compendium of nearly 15 million measurements taken across three decades and provides the precise locations of low oxygen water, elucidating the ODZs' three-dimensional structures. It can be utilized by researchers to validate models, plan cruise occupations, and as a comparison for future change. Calculations made with this high-resolution atlas also provide the volumes, layers of maximal areal extent, and other descriptive statistics for both Pacific ODZs. Finally, the atlas reveals fine-scale features of oxygenated water mass intrusions and regional differences across these anoxic zones.

Plain Language Summary
Using high resolution, meter-scale profiles of oxygen, we derive the locations and fine-scale structure of the Pacific Ocean's oxygen deficient zones. These zones are regions of the lowest oxygen concentrations in the global ocean and host anaerobic metabolisms that result in the loss of bio-available nitrogen and the emission of the potent greenhouse gas, nitrous oxide. This new approach, compiling tens of thousands of profiles and over 15 million individual measurements, is a leap forward in the representation of these climate critical regions as the method does not rely on arbitrarily defining a sensor-dependent detection limit. Instead, the vertical gradient in the measurements is used to not only identify the layers of functional anoxia but also to quantify the intrusion of oxygenated waters into these zones. The resulting gridded data set, the most reliable produced to date, is freely available for scientists and the general public to use to in their research and policy-making efforts. This comprehensive data set can moreover be used in the validation of climate models.

https://agupubs.onlinelibrary.wiley.com/doi/10.1029/2021GB007001

P.S. Image from:
https://www.msn.com/es-mx/noticias/mundo/ciencia-grandes-zonas-muertas-en-el-pac%C3%ADfico-trop%C3%ACcal-ante-la-costa-de-am%C3%A9rica/ar-AAScww6?ocid=msedgntp