Archive for pacific

The Equatorial Undercurrent influences the fate of the Oxygen Minimum Zone in the Pacific

Posted by mmaheigan 
· Tuesday, November 12th, 2019 

While the ocean as a whole is losing oxygen due to warming, oxygen minimum zones (OMZs) are maintained by a delicate balance of biological and physical processes; it is unclear how each one of them is going to evolve in the future. Changes to OMZs could affect the global uptake of carbon, the generation of greenhouse gases, and interactions among marine life. Current generation coarse-resolution (~1°) climate models compromise the ability to simulate low-oxygen waters and their response to climate change in the future because they fail to reproduce a major ocean current, the Equatorial Undercurrent (EUC). These shortcomings lead to an overly tilted upper oxygen minimum zone (OMZ) (Figure 1), thus exaggerating sensitivity to circulation changes and overwhelming other key processes like diffusion and biology. The EUC also plays a vital role in feeding the eastern Pacific upwelling region, connecting it to global climate variability.

Figure: Top: The boundary of the Oxygen Minimum Zone (OMZ) along the Equator is unrealistically tilted for current generation (coarse resolution) climate models, and improves with increased horizontal resolution. The tilt is due to a bad representation of the Equatorial Undercurrent in the coarse model, also seen in other coarse models. The exaggerated tilt of the OMZ boundary at the Equator leads to increased inter-annual variability of the depth of the upper OMZ boundary, via changes in the zonal flow (left). This phenomenon is found in most CMIP5 models (right) and could be responsible for the current inability to predict the change in OMZ extent for the next century.

A recent high‐resolution climate model study in Geophysical Research Letters significantly improved the representation of both the EUC and OMZ, suggesting that the EUC is a key player in OMZ variability. This study emphasizes the importance of improving transport processes in global circulation models to better simulate oxygen distribution and predict future OMZ extent. The results of this study imply that the fundamental dynamics maintaining this key ocean current could be categorically misrepresented in the current generation of climate models, potentially influencing the ability to predict future climate variability and trends.

 

Authors:
Julius J.M. Busecke (Princeton University)
Laure Resplandy (Princeton University)
John P. Dunne (NOAA/GFDL)

Biogeochemical controls of surface ocean phosphate

Posted by mmaheigan 
· Tuesday, November 12th, 2019 

Phosphorus availability is important for phytoplankton growth and more broadly ocean biogeochemical cycles. However, phosphate concentration is often below the analytical detection limit of the standard auto-analyzer technique. Thus, we know little about geographic phosphate variation across most low latitude regions. To address this issue, a global collaboration of scientists conducted a study published in Science Advances on combined phosphate measurements using high-sensitivity methods that yielded a detailed map of surface phosphate (Figure 1).

Figure 1: Fine-scale global variation of surface phosphate. Surface phosphate measured using high-sensitivity techniques revealed previously unrecognized low latitude differences in phosphate drawdown.

The study’s new globally expansive phosphate data set revealed previously unrecognized low-phosphate areas, including large regions of the Pacific Ocean—really low phosphate in the western North Pacific and to a lesser extent in the South Pacific. Although atmospheric iron input and nitrogen fixation are commonly described as regulators of surface phosphate, this study shows that shifts in the elemental stoichiometry (N:P:Fe) of the vertical nutrient supply play an additional role. Previous studies and climate models have suggested that the availability of phosphate is a first-order driver of ocean biogeochemical changes. Interestingly, this study suggests that marine ecosystems are more resilient to phosphate stress than previously thought. These findings underscore the importance of accurately quantifying nutrients at low concentrations for understanding the regulation of ocean ecosystem processes and biogeochemistry now and under future climate conditions.

And the data are of course available in BCO-DMO!

 

Authors:
Adam C. Martiny (University of California, Irvine)
Michael W. Lomas (Bigelow Laboratory for Ocean Sciences)
Weiwei Fu (University of California, Irvine)
Philip W. Boyd (University of Tasmania)
Yuh-ling L. Chen (National Sun Yat-sen University)
Gregory A. Cutter (Old Dominion University)
Michael J. Ellwood (Australian National University)
Ken Furuya (The University of Tokyo)
Fuminori Hashihama (Tokyo University of Marine Science and Technology)
Jota Kanda (Tokyo University of Marine Science and Technology)
David M. Karl (University of Hawaii)
Taketoshi Kodama (Japan Fisheries Research and Education Agency)
Qian P. Li (Chinese Academy of Sciences)
Jian Ma (Xiamen University)
Thierry Moutin (Université de Toulon)
E. Malcolm S. Woodward (Plymouth Marine Laboratory)
J. Keith Moore (University of California, Irvine)

The arsenic respiratory cycle in pelagic waters of Oxygen Deficient Zones

Posted by mmaheigan 
· Wednesday, October 30th, 2019 

Oxygen Deficient Zones (ODZs) are naturally occurring functionally anoxic regions of the open ocean which can act as proxies for early Earth’s anoxic ocean. Without free oxygen, microorganisms in these regions use alternative electron acceptors to oxidize organic material. These functionally anoxic regions are also hotspots for chemoautotrophic pathways. Some microorganisms can use arsenic based compounds to oxidize organic material, and others can couple nitrate reduction with arsenic oxidation supporting autotrophic carbon fixation thus linking arsenic respiration with carbon and nitrogen cycling. While arsenic concentrations in modern oceans are relatively low, the Precambrian ocean likely had periods of high arsenic concentrations. Integrating over time and space of anoxic waters, arsenic-based metabolisms may have had significant implications for the biogeochemical cycling of not only arsenic, but also carbon and nitrogen.

Figure 1: Arsenotrophic genes identified in the Eastern Tropical North Pacific Oxygen Deficient Zone. (A) Genomic complement for dissimilatory arsenate reduction assembled from metagenomes which likely supports respiration of organic matter. (B) Genomic complement for putative chemoautotrophic arsenite oxidation pathway assembled from metagenomes which may couple with nitrate reduction to support organic matter production. (C) Relative abundance of genes associated with arsenite oxidase (aioA), dissimilatory arsenate reduction (arrA), and forward dissimilatory sulfite reductase (dsrA) associated with sulfur reduction; abundance shown as a relative contribution to the total microbial community as estimated by abundance of RNA polymerase genes (rpoB). The genes arrA and forward-dsrA are more abundant in the particulate fraction, whereas aioA is more abundant in the free-living fraction. (D) Relative abundance of genes in the microbial community for the more abundant genes aioA-like and reverse form of dsrA associated with sulfur oxidation. aioA-like genes are relatively more abundant within the particulate fraction, with no strong partitioning between fractions identified for the reverse-dsrA genes. Arsenical reduction and chemoautotrophic arsenical oxidation are likely performed by different microbial groups within the ODZ communities.

Recent work in PNAS identified gene sequences for a complete arsenic respiratory cycle from Eastern Tropical North Pacific (ETNP) ODZ metagenomes. The authors identified arsenotrophic genes for dissimilatory arsenate reduction from one group of microorganisms and genes for a putative chemoautotrophic arsenite oxidation pathway from another group within the ETNP ODZ microbial community. Analysis of genomic sequences from a free-living sample and from particulate-associated sample indicate niche differentiation of these pathways—arsenate reduction genes enriched within the particulate fraction and arenite oxidation enriched in the free-living water column. In addition to the presence of these genes in metagenomes, the authors identified the active expression of these arsenotrophic genes in publicly available metatranscriptomes from the ETNP and Eastern Tropical South Pacific ODZs. Theyalso found an abundance of sequences in the ETNP ODZ for the gene aioA-like, which is a closely related enzyme to arsenite oxidase (aioA), but with an unconfirmed function. The identification of these actively expressed genes in modern ODZs enables further investigation of these cycles that were likely important in early oceans. These findings also highlight that there are still yet to be discovered respiratory pathways in ODZs. Arsenotrophy, in conjunction with other niche respiratory pathways – both known and as yet undiscovered – likely sum to a considerable contribution of energy flow and elemental cycling through these anoxic systems.

Authors:
Jaclyn Saunders (University of Washington; present affiliation Woods Hole Oceanographic Institution)
Clara Fuchsman (University of Washington; present affiliation Horn Point Laboratory)
Cedar McKay & Gabrielle Rocap (University of Washington)

 

See related University of Washington press-release

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