Assessment of seasonal variations in surface water quality of Laguna Lake Sta...
FertittaThesisPoster
1. Assessing the impacts of nutrient load reduction scenarios
on the extent and severity of hypoxia
in the northern Gulf of Mexico
David Fertitta, Dubravko Justi´c, Lixia Wang, R. Eugene Turner
Department of Oceanography and Coastal Sciences, Louisiana State University
INTRODUCTION
The northern Gulf of Mexico has one of the largest coastal hypoxic zones (also known as
’dead zones’) in the world (up to 22,000 km2; 8,495 square miles). Hypoxia (< 2 mg O2
per liter) is an environmental phenomenon where the concentration of dissolved oxygen
in the water column decreases to a level that can no longer support the life of aquatic
organisms, including commercially important species of fish and shrimp. It is, therefore,
considered to be one of the most important environmental problems in the US. In 2001,
the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force set a goal to reduce
the 5-year running average of the Gulf’s hypoxic zone to less than 5,000 km2. This ac-
tion plan envisioned that a 30% reduction in the Mississippi River nitrogen load would
be needed to reach this management goal. However, recent modeling studies based on
statistical models suggest that higher nutrient reductions may be needed. We have con-
ducted simulations using a complex three-dimensional computer model developed in Dr.
Justi´c’s laboratory at LSU to further examine how
different nutrient reduction strategies affect the
areal extent and severity of hypoxia in the northern
Gulf of Mexico. The effects of reductions in Mis-
sissippi River nutrient load have been assessed in
conjunction with different climate scenarios to bet-
ter understand the probable future conditions and
to provide a better estimate of the reduction of ni-
trogen needed in the Mississippi River Watershed
to meet the Task Force Goal.
METHODS
Two models of different complexity were used to assess reductions of nitrate loading in
the Mississippi River Watershed: an empirically derived statistical model [6] and a pre-
dictive coupled hydrodynamic-water quality model, FVCOM-LaTex [2][7]. Using these two
models, I examined the effects of two different nutrient reduction scenarios: S1 = 25%
reduction in riverine nitrate load relative to 2002, and, S2 = 50% reduction in riverine ni-
trate load relative to 2002. These load reductions were chosen because they bracket the
Task Force nutrient reduction goal of 30% as well a more severe nutrient reduction target
of 45% proposed by Scavia et al. (2003). The year 2002 was used as a baseline year
for model comparisons because it had the highest recorded summertime hypoxic zone
(22,000 km2, 8,495 square miles [1]). Two additional model scenarios that assumed a
20% increase in riverine nitrate loading (S3, statistical model only) and concurrent 20%
increase in riverine nitrate loading and a 4oC increase in temperature (S4, FVCOM-LaTex
model only) were used to explore the potential effects of future climate change [4].
Statistical Model
A statistical model uses May Mississippi River nitrate (NO3+NO2) loading to predict the
extent of hypoxic zone on the Louisiana-Texas shelf during the hypoxia shelfwide cruise,
typically carried out during the last week in July [6]. Data for May nitrate loading (157,000
metric tons in 2002) were obtained from the USGS [9]. A logarithmic regression of
y = 15114 ln(x) 158433 provided the best fit for use in predicting the summertime extent
of the hypoxic area as a function of the May Mississippi River nitrate load.
FVCOM-LaTex Model
The FVCOM-LaTex model is a coupled hydrodynamic-water quality model. It is based on
the Finite Volume Coastal Ocean Model (FVCOM), a three-dimensional, primitive equation
ocean circulation model employing an unstructured grid and a finite volume discretization
method. The water quality model is a modified version of the Water Analysis Simulation
Program (WASP) that includes novel formulations for the dynamics of various biogeo-
chemical parameters. The FVCOM-LaTex model is also able to take into account factors
beyond just nutrient reductions or increases and can be used to assess the potential im-
pacts of future climate change on hypoxia.
RESULTS
Statistical Model
The results of the statistical model are summarized in Table 1.
Scenarios N-Loading (metric tons) Hypoxic Area (km2)
Actual 2002 157,000 22,000
S1 (-25% N) 117,750 18,043
S2 (-50% N) 78,500 11,915
S3 (+20% N) 188,400 25,147
FVCOM-LaTex Model
Seasonal changes in the area of hypoxia
The simulated areas of hypoxia in the northern Gulf of Mexico obtained by the FVCOM-
LaTex model for scenarios S1 and S2 show trends comparable to those obtained by
the statistical model. The corresponding decreases in the area of hypoxia were ob-
served throughout the course of hypoxic zone development with each nitrate reduction
(25% and 50%) compared to the standard simulated value of summer 2002. How-
ever, the future climate change scenario associated with a 20% increase in riverine
nutrient loading and a 4oC increase in temperature shows a markedly larger area of
hypoxia on the Louisiana-Texas shelf (47,450 km2) compared to the prediction of the
statistical model for the same 20% increase in riverine nitrate loading (25,147 km2).
Changes in the spatial patterns of hypoxia
The simulated and observed spatial distributions of the hypoxic zone for July 21-26, 2002
are similar in extent across the Louisiana-Texas shelf and show a good agreement be-
tween the observed data and the model results. The two nutrient reduction scenarios
both indicate a reduction in the extent of the area of hypoxic waters. In contrast, the
scenario simulating a 20% increase in nutrients and a 4oC increase in global temper-
ature greatly increases the spatial distribution of hypoxia in the northern Gulf of Mex-
ico so that hypoxic bottom waters cover much of the Louisiana-Texas continental shelf.
( a ) Observed (top) vs simulated
hypoxic area (bottom) for 2002.
( b ) Simulated hypoxic areas for scenarios
S1 (top), S2 (center), and S4 (bottom)
CONCLUSIONS
Given the current state of hypoxia in the northern Gulf of Mexico, bringing the five-year run-
ning average area of hypoxia to 5,000 km2 remains a challenging task. Both the statistical
model and the FVCOM-LaTex model indicated that when using the 2002 as the reference
year, reductions of 25% and 50% in riverine nitrate loading will still result in hypoxic areas
substantially larger than the 5,000 km2 management goal. The statistical model was able
to predict what would happen if riverine nitrogen levels were to increase by 20% compared
to the loading in 2002, but it was crucial to employ the use of the FVCOM La-Tex model
to better understand future possible conditions of hypoxia in the northern Gulf of Mexico.
Due to the large difference in projected hypoxic area, it can be concluded that the effects
of climate change could potentially have a significant impact on the size and spatial extent
of hypoxia in the northern Gulf of Mexico and could interfere with nutrient management
efforts in the Mississippi River watershed. The results of this study suggest that even
a 50% reduction in the Mississippi River nitrate load will not be sufficient to reduce the
5-year average of hypoxic area to less than 5,000 km2 and that hypoxia in the northern
Gulf of Mexico may be exacerbated as a result of climate change in spite of reductions in
the Mississippi River nitrate load. Thus, it appears that, for a foreseeable future, a large
hypoxic zone will continue to persist on the Louisiana-Texas shelf unless further nutrient
reduction strategies are implemented.
REFERENCES AND ACKNOWLEDGMENTS
References
[1 ] Gulf Hypoxia. http://www.gulfhypoxia.net/
[2 ] D. Justi´c, L. Wang. 2013. Continental Shelf Research 72: 163-179.
[3 ] D. Justi´c, N.N. Rabalais, R.E. Turner, 1996. Limnology and Oceanography 41 (5): 9921003.
[4 ] D. Justi´c, N.N. Rabalais, R.E. Turner. 2003. Journal of Marine Systems 42: 115-126.
[5 ] Task Force (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force). 2001. U.S. Environmental Protection Agency; Washington, DC.
[6 ] R.E. Turner, N.N. Rabalais, D. Justi´c. 2012. Marine Pollution Bulletin 64: 318-323.
[7 ] L. Wang, D. Justi´c. 2009. Continental Shelf Research 29:1464-1476.
Acknowledgments
We would like to acknowledge the support of the following institutions and individuals:
1. The Tiger Athletic Foundation and the LSU Honors College for their scholarship support during the de-
velopment of this project.
2. Dr. John Westra, Department of Agricultural Economics and Agribusiness, LSU.
LSU Discover Day, 2015, Baton Rouge, LA
![Assessing the impacts of nutrient load reduction scenarios
on the extent and severity of hypoxia
in the northern Gulf of Mexico
David Fertitta, Dubravko Justi´c, Lixia Wang, R. Eugene Turner
Department of Oceanography and Coastal Sciences, Louisiana State University
INTRODUCTION
The northern Gulf of Mexico has one of the largest coastal hypoxic zones (also known as
’dead zones’) in the world (up to 22,000 km2; 8,495 square miles). Hypoxia (< 2 mg O2
per liter) is an environmental phenomenon where the concentration of dissolved oxygen
in the water column decreases to a level that can no longer support the life of aquatic
organisms, including commercially important species of fish and shrimp. It is, therefore,
considered to be one of the most important environmental problems in the US. In 2001,
the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force set a goal to reduce
the 5-year running average of the Gulf’s hypoxic zone to less than 5,000 km2. This ac-
tion plan envisioned that a 30% reduction in the Mississippi River nitrogen load would
be needed to reach this management goal. However, recent modeling studies based on
statistical models suggest that higher nutrient reductions may be needed. We have con-
ducted simulations using a complex three-dimensional computer model developed in Dr.
Justi´c’s laboratory at LSU to further examine how
different nutrient reduction strategies affect the
areal extent and severity of hypoxia in the northern
Gulf of Mexico. The effects of reductions in Mis-
sissippi River nutrient load have been assessed in
conjunction with different climate scenarios to bet-
ter understand the probable future conditions and
to provide a better estimate of the reduction of ni-
trogen needed in the Mississippi River Watershed
to meet the Task Force Goal.
METHODS
Two models of different complexity were used to assess reductions of nitrate loading in
the Mississippi River Watershed: an empirically derived statistical model [6] and a pre-
dictive coupled hydrodynamic-water quality model, FVCOM-LaTex [2][7]. Using these two
models, I examined the effects of two different nutrient reduction scenarios: S1 = 25%
reduction in riverine nitrate load relative to 2002, and, S2 = 50% reduction in riverine ni-
trate load relative to 2002. These load reductions were chosen because they bracket the
Task Force nutrient reduction goal of 30% as well a more severe nutrient reduction target
of 45% proposed by Scavia et al. (2003). The year 2002 was used as a baseline year
for model comparisons because it had the highest recorded summertime hypoxic zone
(22,000 km2, 8,495 square miles [1]). Two additional model scenarios that assumed a
20% increase in riverine nitrate loading (S3, statistical model only) and concurrent 20%
increase in riverine nitrate loading and a 4oC increase in temperature (S4, FVCOM-LaTex
model only) were used to explore the potential effects of future climate change [4].
Statistical Model
A statistical model uses May Mississippi River nitrate (NO3+NO2) loading to predict the
extent of hypoxic zone on the Louisiana-Texas shelf during the hypoxia shelfwide cruise,
typically carried out during the last week in July [6]. Data for May nitrate loading (157,000
metric tons in 2002) were obtained from the USGS [9]. A logarithmic regression of
y = 15114 ln(x) 158433 provided the best fit for use in predicting the summertime extent
of the hypoxic area as a function of the May Mississippi River nitrate load.
FVCOM-LaTex Model
The FVCOM-LaTex model is a coupled hydrodynamic-water quality model. It is based on
the Finite Volume Coastal Ocean Model (FVCOM), a three-dimensional, primitive equation
ocean circulation model employing an unstructured grid and a finite volume discretization
method. The water quality model is a modified version of the Water Analysis Simulation
Program (WASP) that includes novel formulations for the dynamics of various biogeo-
chemical parameters. The FVCOM-LaTex model is also able to take into account factors
beyond just nutrient reductions or increases and can be used to assess the potential im-
pacts of future climate change on hypoxia.
RESULTS
Statistical Model
The results of the statistical model are summarized in Table 1.
Scenarios N-Loading (metric tons) Hypoxic Area (km2)
Actual 2002 157,000 22,000
S1 (-25% N) 117,750 18,043
S2 (-50% N) 78,500 11,915
S3 (+20% N) 188,400 25,147
FVCOM-LaTex Model
Seasonal changes in the area of hypoxia
The simulated areas of hypoxia in the northern Gulf of Mexico obtained by the FVCOM-
LaTex model for scenarios S1 and S2 show trends comparable to those obtained by
the statistical model. The corresponding decreases in the area of hypoxia were ob-
served throughout the course of hypoxic zone development with each nitrate reduction
(25% and 50%) compared to the standard simulated value of summer 2002. How-
ever, the future climate change scenario associated with a 20% increase in riverine
nutrient loading and a 4oC increase in temperature shows a markedly larger area of
hypoxia on the Louisiana-Texas shelf (47,450 km2) compared to the prediction of the
statistical model for the same 20% increase in riverine nitrate loading (25,147 km2).
Changes in the spatial patterns of hypoxia
The simulated and observed spatial distributions of the hypoxic zone for July 21-26, 2002
are similar in extent across the Louisiana-Texas shelf and show a good agreement be-
tween the observed data and the model results. The two nutrient reduction scenarios
both indicate a reduction in the extent of the area of hypoxic waters. In contrast, the
scenario simulating a 20% increase in nutrients and a 4oC increase in global temper-
ature greatly increases the spatial distribution of hypoxia in the northern Gulf of Mex-
ico so that hypoxic bottom waters cover much of the Louisiana-Texas continental shelf.
( a ) Observed (top) vs simulated
hypoxic area (bottom) for 2002.
( b ) Simulated hypoxic areas for scenarios
S1 (top), S2 (center), and S4 (bottom)
CONCLUSIONS
Given the current state of hypoxia in the northern Gulf of Mexico, bringing the five-year run-
ning average area of hypoxia to 5,000 km2 remains a challenging task. Both the statistical
model and the FVCOM-LaTex model indicated that when using the 2002 as the reference
year, reductions of 25% and 50% in riverine nitrate loading will still result in hypoxic areas
substantially larger than the 5,000 km2 management goal. The statistical model was able
to predict what would happen if riverine nitrogen levels were to increase by 20% compared
to the loading in 2002, but it was crucial to employ the use of the FVCOM La-Tex model
to better understand future possible conditions of hypoxia in the northern Gulf of Mexico.
Due to the large difference in projected hypoxic area, it can be concluded that the effects
of climate change could potentially have a significant impact on the size and spatial extent
of hypoxia in the northern Gulf of Mexico and could interfere with nutrient management
efforts in the Mississippi River watershed. The results of this study suggest that even
a 50% reduction in the Mississippi River nitrate load will not be sufficient to reduce the
5-year average of hypoxic area to less than 5,000 km2 and that hypoxia in the northern
Gulf of Mexico may be exacerbated as a result of climate change in spite of reductions in
the Mississippi River nitrate load. Thus, it appears that, for a foreseeable future, a large
hypoxic zone will continue to persist on the Louisiana-Texas shelf unless further nutrient
reduction strategies are implemented.
REFERENCES AND ACKNOWLEDGMENTS
References
[1 ] Gulf Hypoxia. http://www.gulfhypoxia.net/
[2 ] D. Justi´c, L. Wang. 2013. Continental Shelf Research 72: 163-179.
[3 ] D. Justi´c, N.N. Rabalais, R.E. Turner, 1996. Limnology and Oceanography 41 (5): 9921003.
[4 ] D. Justi´c, N.N. Rabalais, R.E. Turner. 2003. Journal of Marine Systems 42: 115-126.
[5 ] Task Force (Mississippi River/Gulf of Mexico Watershed Nutrient Task Force). 2001. U.S. Environmental Protection Agency; Washington, DC.
[6 ] R.E. Turner, N.N. Rabalais, D. Justi´c. 2012. Marine Pollution Bulletin 64: 318-323.
[7 ] L. Wang, D. Justi´c. 2009. Continental Shelf Research 29:1464-1476.
Acknowledgments
We would like to acknowledge the support of the following institutions and individuals:
1. The Tiger Athletic Foundation and the LSU Honors College for their scholarship support during the de-
velopment of this project.
2. Dr. John Westra, Department of Agricultural Economics and Agribusiness, LSU.
LSU Discover Day, 2015, Baton Rouge, LA](https://image.slidesharecdn.com/31c1f356-7e28-4de4-b709-08e82a6b1758-150420193556-conversion-gate02/85/FertittaThesisPoster-1-320.jpg)
