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Quantifying the impacts of stratification and nutrient loading on hypoxia in the northern Gulf of Mexico.

Obenour DR, Michalak AM, Zhou Y, Scavia D - Environ. Sci. Technol. (2012)

Bottom Line: This study provides a novel solution to this problem by determining the effect of stratification based on its spatial relationship with bottom-water dissolved oxygen (BWDO) concentration using a geostatistical regression.Ten years (1998-2007) of midsummer Gulf of Mexico BWDO measurements are modeled using stratification metrics along with trends based on spatial coordinates and bathymetry, which together explain 27-61% of the spatial variability in BWDO for individual years.Overall, 82% of the year-to-year variability in mean BWDO is explained.

View Article: PubMed Central - PubMed

Affiliation: School of Natural Resources & Environment, University of Michigan, Ann Arbor, Michigan 48109-1041, United States. obenour@umich.edu

ABSTRACT
Stratification and nutrient loading are two primary factors leading to hypoxia in coastal systems. However, where these factors are temporally correlated, it can be difficult to isolate and quantify their individual impacts. This study provides a novel solution to this problem by determining the effect of stratification based on its spatial relationship with bottom-water dissolved oxygen (BWDO) concentration using a geostatistical regression. Ten years (1998-2007) of midsummer Gulf of Mexico BWDO measurements are modeled using stratification metrics along with trends based on spatial coordinates and bathymetry, which together explain 27-61% of the spatial variability in BWDO for individual years. Because stratification effects explain only a portion of the year-to-year variability in mean BWDO; the remaining variability is explained by other factors, with May nitrate plus nitrite river concentration the most important. Overall, 82% of the year-to-year variability in mean BWDO is explained. The results suggest that while both stratification and nutrients play important roles in determining the annual extent of midsummer hypoxia, reducing nutrient inputs alone will substantially reduce the average extent.

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Related in: MedlinePlus

Map of shelf bathymetry and monitoringsites used in study.
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fig1: Map of shelf bathymetry and monitoringsites used in study.

Mentions: We use data frommidsummer hypoxia monitoring cruises conducted between 1998 and 2007along the Louisiana-Texas shelf. These cruises are performed by theLouisiana Universities Marine Consortium (LUMCON), and the data wereretrieved from the National Ocean Data Center.17 To help ensure a consistent spatial envelope for this study,only locations sampled during at least 9 of the 10 years were includedin the analysis (Figure 1), resulting in 61–64monitoring locations for each year. Sampling locations were geo-referencedusing the UTM Zone15 projection, and water depths were determinedfrom a 3-arc-second digital elevation model (DEM) obtained from NOAA.18 We use the dissolved oxygen, salinity, and temperatureprofile data collected at these locations. Typically, data were collectedby two different instruments at each monitoring site: a Hydrolab anda Sea-Bird profiler.19 Based on a comparisonwith the DEM, the Hydrolab and Sea-Bird typically reached to withinzero and one meters of the sea floor, respectively, with minor variability.For this study, the BWDO values were taken from whichever instrumentreached the greatest recorded depth (typically the Hydrolab). To determinesalinity and temperature profiles, Sea-Bird data, which have bettervertical resolution, were chosen preferentially over Hydrolab data;and when the Hydrolab reached a greater depth, we appended these additionalmeasurements to the Sea-Bird profile.


Quantifying the impacts of stratification and nutrient loading on hypoxia in the northern Gulf of Mexico.

Obenour DR, Michalak AM, Zhou Y, Scavia D - Environ. Sci. Technol. (2012)

Map of shelf bathymetry and monitoringsites used in study.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3823025&req=5

fig1: Map of shelf bathymetry and monitoringsites used in study.
Mentions: We use data frommidsummer hypoxia monitoring cruises conducted between 1998 and 2007along the Louisiana-Texas shelf. These cruises are performed by theLouisiana Universities Marine Consortium (LUMCON), and the data wereretrieved from the National Ocean Data Center.17 To help ensure a consistent spatial envelope for this study,only locations sampled during at least 9 of the 10 years were includedin the analysis (Figure 1), resulting in 61–64monitoring locations for each year. Sampling locations were geo-referencedusing the UTM Zone15 projection, and water depths were determinedfrom a 3-arc-second digital elevation model (DEM) obtained from NOAA.18 We use the dissolved oxygen, salinity, and temperatureprofile data collected at these locations. Typically, data were collectedby two different instruments at each monitoring site: a Hydrolab anda Sea-Bird profiler.19 Based on a comparisonwith the DEM, the Hydrolab and Sea-Bird typically reached to withinzero and one meters of the sea floor, respectively, with minor variability.For this study, the BWDO values were taken from whichever instrumentreached the greatest recorded depth (typically the Hydrolab). To determinesalinity and temperature profiles, Sea-Bird data, which have bettervertical resolution, were chosen preferentially over Hydrolab data;and when the Hydrolab reached a greater depth, we appended these additionalmeasurements to the Sea-Bird profile.

Bottom Line: This study provides a novel solution to this problem by determining the effect of stratification based on its spatial relationship with bottom-water dissolved oxygen (BWDO) concentration using a geostatistical regression.Ten years (1998-2007) of midsummer Gulf of Mexico BWDO measurements are modeled using stratification metrics along with trends based on spatial coordinates and bathymetry, which together explain 27-61% of the spatial variability in BWDO for individual years.Overall, 82% of the year-to-year variability in mean BWDO is explained.

View Article: PubMed Central - PubMed

Affiliation: School of Natural Resources & Environment, University of Michigan, Ann Arbor, Michigan 48109-1041, United States. obenour@umich.edu

ABSTRACT
Stratification and nutrient loading are two primary factors leading to hypoxia in coastal systems. However, where these factors are temporally correlated, it can be difficult to isolate and quantify their individual impacts. This study provides a novel solution to this problem by determining the effect of stratification based on its spatial relationship with bottom-water dissolved oxygen (BWDO) concentration using a geostatistical regression. Ten years (1998-2007) of midsummer Gulf of Mexico BWDO measurements are modeled using stratification metrics along with trends based on spatial coordinates and bathymetry, which together explain 27-61% of the spatial variability in BWDO for individual years. Because stratification effects explain only a portion of the year-to-year variability in mean BWDO; the remaining variability is explained by other factors, with May nitrate plus nitrite river concentration the most important. Overall, 82% of the year-to-year variability in mean BWDO is explained. The results suggest that while both stratification and nutrients play important roles in determining the annual extent of midsummer hypoxia, reducing nutrient inputs alone will substantially reduce the average extent.

Show MeSH
Related in: MedlinePlus