News

Flooding Makes Big 'Dead Zone' Off Louisiana Coast Likely

"The year's widespread flooding has made it likely that a big, oxygen-starved "dead zone" off Louisiana's coast will form this summer, the head of the National Centers for Coastal Ocean Science said Thursday. Preliminary computer model runs "indicate a large to very large year," for the area where there's too little oxygen to support marine life, Steven Thur told the Mississippi River/Gulf of Mexico Hypoxia Task Force during a meeting livestreamed from Baton Rouge. [...]"

Source: The New York Times

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Extent of the annual Gulf of Mexico hypoxic zone influences microbial community structure

Abstract.

"Rich geochemical datasets generated over the past 30 years have provided fine-scale resolution on the northern Gulf of Mexico (nGOM) coastal hypoxic (≤ 2 mg of O2 L-1) zone. In contrast, little is known about microbial community structure and activity in the hypoxic zone despite the implication that microbial respiration is responsible for forming low dissolved oxygen (DO) conditions. [...]"

Source: PLoS ONE
Authors: Lauren Gillies Campbell et al.
DOI: 10.1371/journal.pone.0209055

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Yield stability analysis reveals sources of large-scale nitrogen loss from the US Midwest

Abstract.

"Loss of reactive nitrogen (N) from agricultural fields in the U.S. Midwest is a principal cause of the persistent hypoxic zone in the Gulf of Mexico. We used eight years of high resolution satellite imagery, field boundaries, crop data layers, and yield stability classes to estimate the proportion of N fertilizer removed in harvest (NUE) versus left as surplus N in 8 million corn (Zea mays) fields at subfield resolutions of 30 × 30 m (0.09 ha) across 30 million ha of 10 Midwest states. [...]"

Source: Scientific Reports
Authors: Bruno Basso et al.
DOI: 10.1038/s41598-019-42271-1

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'Dead zone' volume more important than area to fish, fisheries

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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Hypoxic volume is more responsive than hypoxic area to nutrient load reductions in the northern Gulf of Mexico – and it matters to fish and fisheries

Abstract.

"While impacts of low oxygen on marine organisms have been reviewed from physiological and ecological perspectives, relating broad population- and ecosystem-level effects to the areal extent of hypoxia (dissolved oxygen concentration below 64 µM, or 2 mg/l) has proven difficult. We suggest that hypoxic volume is a more appropriate metric compared to hypoxic area because volume better integrates the effects of hypoxia on ecological processes relevant to many marine taxa. [...]"

Source: IOP Science
Authors: Donald Scavia et al.
DOI: 10.1088/1748-9326/aaf938

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A Novel Approach for Measuring the Gulf of Mexico Dead Zone: Toward Better Temporal and Spatial Estimates for Management Applications

Abstract.

"Nearly every summer, a large hypoxic zone forms in the northern Gulf of Mexico. Research on the causes and consequences of hypoxia requires reliable estimates of hypoxic extent, which can vary at submonthly time scales due to hydro-meteorological variability. Here, we use an innovative space-time geostatistical model and data collected by multiple research organizations to estimate bottom-water dissolved oxygen (BWDO) concentrations and hypoxic area across summers from 1985 to 2016. [...]"

Source: Environmental Science and Technology 
Authors: V. Rohith Reddy Matli et al.
DOI: 10.1021/acs.est.8b03474

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What could cause the Mississippi Bight to become hypoxic?

"Coastal regions with low dissolved oxygen (known as hypoxia) can lead to poor water quality and harm regional fisheries. These areas of low dissolved oxygen are expanding and expected to continue growing in coming years due to human impacts on the environment.

A recent article published in Continental Shelf Research explores aspects of the environmental conditions that can potentially lead to hypoxia in the Mississippi Bight region of the northern Gulf of Mexico. This area extends from Apalachicola in Florida to the Mississippi River Delta. [...]"

Source:  EurekAlert!

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Tracking sea surface salinity and dissolved oxygen on a river-influenced, seasonally stratified shelf, Mississippi Bight, northern Gulf of Mexico

Abstract.

"River discharge, and its resulting region of freshwater influence (ROFI) in the coastal ocean, has a critical influence on physical and biogeochemical processes in seasonally stratified shelf ecosystems. Multi-year (2010–2016) observations of satellite-derived sea surface salinity (SSS) and in situ water column hydrographic data during summer 2016 were used to investigate physical aspects of the ROFI east of the Mississippi River Delta to better assess regional susceptibility to hypoxia in the summer months. [...]"

Source: Continental Shelf Research
Authors: Brian Dzwonkowski et al.
DOI: 10.1016/j.csr.2018.09.009

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Drivers of oxygen consumption in the northern Gulf of Mexico hypoxic waters – A stable carbon isotope perspective

Abstract.

"We examined the stable carbon isotopic composition of remineralized organic carbon (δ13COCx) in the northern Gulf of Mexico (nGoM) using incubations (sediment and water) and a three end‐member mixing model. δ13COCx in incubating sediments was ‐18.1±1.3‰, and δ13COCx in incubating near‐surface and near‐bottom waters varied with salinity, ranging from ‐30.4‰ to ‐16.2‰ from brackish water to full strength Gulf water. The average δ13COCx was ‐18.6 ±1.8‰ at salinity >23. A three end‐member mixing model based on a multi‐year dataset collected in previous summer hypoxia cruises (2011, 2012, 2014, 2015 and 2016) suggested that δ13COCx in near‐bottom waters across the nGoM (5‐50 m) was ‐18.1±0.6‰. [...]" 

Source: Geophysical Reasearch Letters
Authors: Hongjie Wang et al.
DOI: 10.1029/2018GL078571

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Gulf of Mexico 'dead zone' forecasted to exceed the size of Connecticut

"Scientists have predicted the dead zone, or area with little to no oxygen in the northern Gulf of Mexico, will become larger than the state of Connecticut by the end of July. The dead zone will cover about 6,620 square miles of the bottom of the continental shelf off Louisiana and Texas. While there are more than 500 dead zones around the world, the northern Gulf of Mexico dead zone is the second largest human-caused coastal hypoxic area in the world."

Source: Phys.org

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