Science for Solutions NOAA COASTAL OCEAN PROGRAM Decision Analysis Series No. 15 Characterization of Hypoxia Topic 1 Report for the Integrated Assessment on Hypoxia in the Gulf of Mexico Nancy N. Rabalais, R. Eugene Turner, Dubravko Justic´, Quay Dortch, and William J. Wiseman, Jr. May 1999 U.S. DEPARTMENT OF COMMERCE National Oceanic and Atmospheric Administration National Ocean Service Coastal Ocean Program GULF OF MEXICO HYPOXIA ASSESSMENT This report is the first in a series of six reports developed as the scientific basis for an integrated assessment of the causes and consequences of hypoxia in the Gulf of Mexico, as requested by the White House Office of Science and Technology Policy and as required by Section 604a of P.L. 105-383. For more information on the assessment and the assessment process, please contact the National Centers for Coastal Ocean Science at (301) 713-3060. DECISION ANALYSIS SERIES The Decision Analysis Series has been established by NOAA's Coastal Ocean Program (COP) to present documents for coastal decision mak- ers which contain analytical treatments of major issues or topics. The is- sues, topics, and principal investigators have been selected through an extensive peer review process. To learn more about the COP or the De- cision Analysis Series, please write to: NOAA National Centers for Coastal Ocean Science Coastal Ocean Program 1315 East-West Highway, Room 9700 Silver Spring, Maryland 20910 phone: 301-713-3338 fax: 301-713-4044 web: http://www.cop.noaa.gov e-mail: [email protected] Cover image: Extent of bottom water with dissolved oxygen less than 2 mg/l for July 1996 (based on data of Rabalais, Turner, and Wiseman). Science for Solutions NOAA COASTAL OCEAN PROGRAM Decision Analysis Series No. 15 Characterization of Hypoxia Topic 1 Report for the Integrated Assessment on Hypoxia in the Gulf of Mexico Nancy N. Rabalais, Louisiana Universities Marine Consortium R. Eugene Turner, Louisiana State University Dubravko Justic´, Louisiana State University Quay Dortch, Louisiana Universities Marine Consortium William J. Wiseman, Jr., Louisiana State University May 1999 U.S. DEPARTMENT OF COMMERCE William M. Daley, Secretary National Oceanic and Atmospheric Administration D. James Baker, Undersecretary for Oceans and Atmosphere National Ocean Service Nancy Foster, Assistant Administrator National Centers for Coastal Ocean Science Donald Scavia, Director Center for Sponsored Coastal Ocean Research—Coastal Ocean Program David Johnson, Director This publication should be cited as: Rabalais, Nancy N., R. Eugene Turner, Dubravko Justic´ , Quay Dortch, and William J. Wiseman, Jr. 1999. Characterization of Hypoxia: Topic 1 Report for the Integrated Assessment on Hypoxia in the Gulf of Mexico. NOAA Coastal Ocean Program Decision Analysis Series No. 15. NOAA Coastal Ocean Program, Silver Spring, MD. 167 pp. i iv Characterization of Hypoxia Corresponding author information: Nancy N. Rabalais Louisiana Universities Marine Consortium 8124 Highway 56 Chauvin, Louisiana 70344 This publication does not constitute an endorsement of any commercial product or intend to be an opinion beyond scientific or other results obtained by the National Oceanic and Atmospheric Administration (NOAA). No reference shall be made to NOAA, or to this publication furnished by NOAA, in any advertising or sales promotion that would indicate or imply that NOAA recommends or endorses any proprietary product mentioned herein, or that has as its purpose an interest to cause directly or indirectly the advertised product to be used or purchased because of this publication. Contents LIST OF FIGURES AND TABLES ..................................................................................................................... v ACKNOWLEDGMENTS xi FOREWORD xiii EXECUTIVE SUMMARY xv 1. INTRODUCTION 1 2. METHODS ............................................................................................................................................3 3. DIMENSIONS AND VARIABILITY OF HYPOXIA.......................................................................................6 3.1 Mid-Summer Extent .............................................................................................................................6 3.2 Variability in Mid-Summer ....................................................................................................................8 3.3 Shelf-wide Distribution in Spring and Fall ..........................................................................................10 3.4 Cross-Shelf Spatial and Temporal Variability, Southeastern Louisiana.............................................24 3.5 Time Series........................................................................................................................................24 3.6 Average Conditions............................................................................................................................27 3.7 Extremes in River Discharge .............................................................................................................29 4. PHYSICAL OCEANOGRAPHY..............................................................................................................34 4.1 Plumes and Coastal Circulation.........................................................................................................34 4.2 Stratification .......................................................................................................................................38 4.3 Relative Importance of Physical Factors ...........................................................................................40 5. RIVER DISCHARGE AND FLUX OF MATERIALS ..................................................................................43 5.1 Freshwater Discharge........................................................................................................................43 5.2 Sediment Discharge ..........................................................................................................................45 5.3 Nutrient Flux.......................................................................................................................................46 5.3.1 Contribution of Mississippi and Atchafalaya Rivers versus other sources............................47 5.3.2 Flux and concentration of nutrients.......................................................................................48 5.3.3 Seasonality ...........................................................................................................................52 5.3.4 Nutrient ratios........................................................................................................................53 5.3.5 Anthropogenic influences .....................................................................................................56 5.4 Changes in Offshore Nutrients ..........................................................................................................59 5.5 Allochthonous Organic C and N Inputs..............................................................................................61 5.6 Other Constituents.............................................................................................................................62 iii iv Characterization of Hypoxia 6. BIOLOGICAL PROCESSES..................................................................................................................63 6.1 Dilution, Uptake, and Regeneration of Nutrients ...............................................................................63 6.2 Nutrient-Enhanced Primary Production .............................................................................................64 6.3 Nutrient Limitation of Phytoplankton Growth......................................................................................71 6.4 Phytoplankton Species Composition and Changes in Response to Nutrient Limitation....................74 6.5 Carbon Flux.......................................................................................................................................77 6.6 Composition of Sinking Material and the Relationship to Nutrient Availability...................................79 6.7 Respiration.........................................................................................................................................81 6.8 Nutrient Ratios and Implications to Biogeochemical Cycles and Trophodynamics ...........................81 6.9 Oxygen and Carbon Budgets.............................................................................................................83 6.10 Biogeochemical Processes of Low-Oxygen Environments ...............................................................87 6.11 Linkages with Mississippi River Discharge and Nutrient Flux............................................................87 7. LONG-TERM COASTAL ECOSYSTEM CHANGES................................................................................93 7.1 Historical Documentation...................................................................................................................93 7.1.1 Oxygen minimum layer .........................................................................................................93 7.1.2 Early reports of Louisiana shelf hypoxia ...............................................................................95 7.2 Data Availability..................................................................................................................................97