The Need for Sustainable, Integrative Long-Term Monitoring of the Gulf of

Mexico Hypoxic Zone

Summit on Long-Term Monitoring of the Gulf of Mexico Hypoxic ZoneJanuary 30-31, 2007

Alan Lewitus

• Boesch & Rabalais begin monitoring (1985)

• NOAA’s Coastal Ocean Program study documented issue (NECOP 1990-96; supplemental research 1997-1999) – evidence for increasing hypoxic zone over time

History of Monitoring

Areal Extent of Gulf of Mexico Hypoxic Zone: 1985-1999

Are

a (k

m )2

Rabalais et al.

30.0

Latit

ude

29.5

29.0

28.5

-93.0 -92.0 -91.0 -90.0 -89.0

Atchafalaya R.

Mississippi R.Louisiana

Gulf of Mexico

Bottom Water Hypoxia, July 23-29, 1997

• Hypoxia has increased since the 1950’s

• River N load is main driver of hypoxia

• NO3 load is > 3X that of 1950’s:

90% of nitrate inputs from non-point sources;74% of nitrate load is from agricultural non-point sources.

CENR Conclusions

85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 00 01 02 03 04 05

Record = 22,000 km2 in 2002

Since mid 1990s, the 5-yr running average size of hypoxic zone has hovered around 15,000 km2

Rabalais et al.

Extension of Monitoring

Gulf of Mexico

LOUISIANA

AtchafalayaR

.

Mississippi

R.

C61988

1993100 km

N

94o 93o 92o 91o 90o30o

28o

29o

Gulf of Mexico

LOUISIANA

AtchafalayaR

.

Mississippi

R.

C61988

1993100 km

N

94o 93o 92o 91o 90o30o

28o

29o

Justić et al. (2002)

Areal Extent of Hypoxic Zone – Coastal Goal Metric

Areal extent of the hypoxic zone at the peak time of hypoxia (July) has been well characterized and is a good indicator of the intensity of hypoxia in any given year

0

5000

10000

15000

20000

25000

0 5000 10000 15000 20000 25000

Predicted Area of Hypoxia

Obs

erve

d Ar

ea o

f Hyp

oxia

Model: Hypoxia area (km2) = 0.0998 x May NOx flux + 672 x Year -13.4 x 105 (R2 = 0.82)

Turner et al 2006

Statistical models suggest that spring/early Summer nutrient fluxes (primarily nitrate) are good predictors of mid summer size of hypoxia

Areal Extent of Hypoxic Zone – Coastal Goal Metric

LOWERMISSISSIPPI

MISSOURI UPPERMISSISSIPPI

OHIO

GULF OFMEXICO

Gulf of Mexico

LOUISIANA

AtchafalayaR

.

Mississippi

R.

C61988

1993100 km

N

94o 93o 92o 91o 90o30o

28o

29o

Gulf of Mexico

LOUISIANA

AtchafalayaR

.

Mississippi

R.

C61988

1993100 km

N

94o 93o 92o 91o 90o30o

28o

29o

Justić et al. (2002)

Hypoxic Zone Monitoring

Need for Extension of Monitoring

• Action Plan (2001): “greatly expand the long-term monitoring program for the hypoxic zone, including greater temporal and spatial data collection, measurements of macro-nutrient and micronutrient concentrations, and hypoxia…”

• Monitoring, Modeling, and Research Workgroup Report (MMR, 2004): “(monitoring) efforts need to be increased in frequency, at a minimum monthly from May through September. To develop a more complete understanding of ecosystem dynamics, selected sites should be monitored year-round. The spatial boundaries of some of these existing monitoring efforts should be expanded to collect data for defining boundary conditions in modeling efforts."

Atchafalaya River plume

Southwest Pass plume

Hypoxia 21-25 July 2004

Atchafalaya River plume

Southwest Pass plume

Hypoxia 21-25 July 2004

Causes of Hypoxia:

• Expansion of spatial boundaries

• Greater temporal resolution

Science Needs

DiMarco et al. (2006); image from N. Walker

Causes of Hypoxia:

• Benthic processes

• Hypoxic volume

Science Needs

Science Needs

• Need to trace effects of habitat loss through the food web to understand ecosystem-level effects

• Hypoxia effects in the Gulf are:

• Indirect

• Spatially-mediated responses to the environment

• Occur across multiple trophic levels

Impacts of Hypoxia:

20 m

80 m

croaker

20 m

80 m

20 m

80 m

croaker

95%75%50%25%

20 m

80 m

croaker

95%75%50%25%

20 m

80 m

95%75%50%25%

95%75%50%25%

20 m

80 m

croaker

Moderate Hypoxia

Severe Hypoxia

(1983, 1987, 1988, 2000)

(1993, 1995, 1996, 1997)

Hypoxia Effects on Atlantic Croaker Distribution

0

5000

10000

15000

20000

1983

1986

1989

1992

1995

1998

2001

Year

Are

a H

ypox

ia (k

m2 )

~33-50% habitat loss

from K. Craig

Science Needs

Support models used to:

• quantify the relationship between nutrient loading and hypoxia

• understand the causes of hypoxia

• understand the impacts of hypoxia

Hetland – ROMS model

Surface(0 - 10 m)

Pycnocline

Bottom(10 - 20 m)

Surface(0 - 10 m)

Pycnocline

Bottom(10 - 20 m) TR

FOt

NP

DO

A

A

Justic et al. (1996)

0

5000

10000

15000

20000

10 20 30 40 50 60N Load Reduction

Are

a (k

m)

2

Scavia et al 2003, 2004

10 20 30 50 70% Nitrogen Load Reduction

020406080

100%

Incr

ease

in O

xyge

nBierman et al 1994, 1999

Science Needs