Causes of Gulf of Mexico Hypoxia Nancy N. Rabalais 1 R. Eugene Turner 2 1 Louisiana Universities Marine Consortium 2 Louisiana State University Center for Sponsored Coastal Ocean Research, Coastal Ocean Program, NGOMEX Hypoxia Studies
.6% 58% 18% 21% [w & w 2.4%] 5 km (Goolsby et al., 1999, Rabalais 22)
(http://www.esl.lsu.edu) Mississippi River Atchafalaya River New Orleans (Photo: N. Rabalais) Hypoxic Area Overwhelming Sources of Water & Nutrients Nutrient effects are more far reaching than suspended sediment plume, esp. N dominant wind direction
Nutrients, Increased Growth, Low Oxygen Time magazine Time Magazine Coastal, No Upwelling, No OMZ
3 L. Calcasieu Atchafalaya R. Mississippi R. 29.5 29 1 28.5 7-17 Jul 1986 5 km 3 L. Calcasieu Atchafalaya R. Mississippi R. 29.5 1 29 28.5 24-28 Jul 1993 5 km 3 L. Calcasieu Atchafalaya R. Mississippi R. 29.5 1 29 28.5 21-25 Jul 1998 5 km -93.5-93 -92.5-92 -91.5-91 -9.5-9 -89.5 (Rabalais et al., 22)
Persistent, Annual, Bottom Water Hypoxia TX LA 22 * transect C (Rabalais 23) Mississippi River delta onto upper Texas coast strong salinity and temperature stratification 4-5 m nearshore to 35-45 m offshore.5 km nearshore to 1 + km offshore seasonal; most widespread and severe in May - Sep 21-25 average 15,6 km 2 in mid summer
Persistent Over Extended Periods 3. 29.5 29. 28.5 3. 29.5 29. 28.5 3. 29.5 29. 28.5 July 1-12, 1993 NECOP (Bratkovitch et al.) July 13-21, 1993 LATEX (Rabalais et al.) July 24-3, 1993 NECOP (Rabalais et al.) 95. 94. 93. 92. 91. 9. 89. (Source: N. Rabalais, LUMCON)
3. Sabine L. Frequency of Hypoxia 1985-25 L. Calcasieu Atchafalya R. Mississippi R. 29.5 transect F Terrebonne Bay 1 29. 28.5.75.5.25 75% 5% 25% 25% 5 km transect C -93.5-92.5-91.5-9.5-89.5 (modified from Rabalais et al. 22) more frequent down current from Mississippi and Atchafalaya rivers discharges distribution varies monthly; nutrient flux, discharge, currents climate driven differences
Estimated Size of Bottom-Water Hypoxia in Mid-Summer 2, Hurricanes Area (km 2 ) 15, 1, Drought Year 5, 1985 no data 1986 1987 1988 1989 199 1991 1992 1993 1994 1995 1996 1997 1998 1999 2 21 22 23 24 25 26 (updated from Rabalais et al. 22) Nutrient-enhanced carbon production leads to oxygen depletion, and the distribution and dynamics are bounded by the physics of the system.
Stratification (mid-summer) -5. -1. Salinity Temperature C 2 22 24 26 28 3 32 34 36-15. 2-2. -25. Density (sigma-t) C6B 4 6-5. -1.. 5. 1. 15. 2. 25. 3. 35. 4. Depth (m) 8 1 12 Temperature -15. -2. -25. Dissolved Oxygen (mg/l). 5. 1. 15. 2. 25. 3. 35. 4. km (N. Rabalais, LUMCON) 14 16 18 Salinity Dissolved Oxygen 2 1 2 3 4 5 6 7 8 Dissolved Oxygen (mg/l)
unexpected eastward flow strong vertical shear weak sheared flow strong unidirectional to bottom Hurricane Isadore upwelling favorable consistent with wind-driven downwelling expected from predominantly SE winds 14 March through 12 November 22 (Wiseman et al. 24)
1 st mode: nearly unidirectional vertically-sheared flow 2 nd mode: two-layered flow summer > spring or fall 82% 13% EOF (empirical orthogonal flow) analysis (Wiseman et al. 24)
Surface Salinity (psu) 1 2 3 4 5 Surface DIN (µm) 1 2 3 4 Bottom Dissolved Oxygen (mg l -1 ) C1 C3 C4 C5 C6B C6 C7 5 Surface Chlorophyll (µg l -1 ) C8 C9 1 J F M A M J J A S O N D 2 3 4 5 J F M A M J J A S O N D (Rabalais et al., in revision)
1. a 1.5 NP = -.34 + 3.93 x 1-7 (N-NO 3 ) t-1 b R =.73; P <.1.5 1. CCF. -.5 NP (gc m -2 d -1 ).5. -1. -6-5 -4-3 -2-1 1 2 3 4 5 6 Time lag (months) (Justić et al. 1997) -.5 1x1 6 2x1 6 3x1 6 4x1 6 N-NO 3 load (kg d -1 ) Prior and further analyses of timing and volume of discharge, timing and load of nutrients, particularly nitrate-n, timing and accumulation of carbon, and rates and temporal sequence of oxygen depletion are consistent with a seasonal cycle of nutrient-enhanced production, flux of organic matter and depletion of oxygen under persistent and strong water column stratification.
Salinity (PSU) Surface NO 3 -N (μm) Surface Chlorophyll (μg l -1 ) Surface NH 4 -N (μm) Bottom Dissolved Oxygen (mg l -1 ) (Rabalais et al., in revision)
9 8 7 cold 6 front 5 4 3 2 1 9 8 7 6 5 4 3 2 1 9 8 7 6 5 4 3 2 1 9 8 7 6 5 4 3 2 1 Bottom Oxygen, 2 m water depth, 1993 Apr 2 4 6 8 1 12 14 16 18 2 22 24 26 28 3 Jun 2 4 6 8 1 12 14 16 18 2 22 24 26 28 3 Aug 2 4 6 8 1 12 14 16 18 2 22 24 26 28 3 Oct cold fronts respiration Tropical Storm, Campeche 2 4 6 8 1 12 14 16 18 2 22 24 26 28 3 Day May cold front 2 4 6 8 1 12 14 16 18 2 22 24 26 28 3 Jul 2 4 6 8 1 12 14 16 18 2 22 24 26 28 3 Sep extremely low and anoxic conditions respiration 2 4 6 8 1 12 14 16 18 2 22 24 26 28 3 Nov respiration deep water intrusion 2 4 6 8 1 12 14 16 18 2 22 24 26 28 3 Day reduction of the oxygen concentration from about 6 mg l -1 to less than 2 mg l -1 is 18, 11 or 9 days, in April, May and July, respectively (Rabalais et al., in revision) (Rabalais et al. 22)
(Turner et al. submitted) (Meade 1995) (Turner and Rabalais 23)
NO3 -N (16 mt yr -1 ) Streamflow (1 3 m 3 s 1 ) 5 4.5 A 4 3.5 3 2.5 2 1.5 195-7 198-96 NO3 -N (mg L-1 ) 1.5 Cedar, IA DesMoines, IA Lower Illinois, IL Wabash, IN Minnesota, MN Muskingham, OH Lower Missouri, MO Lower Mississippi, LA http://wwwrcolka.cr.usgs.gov/midconherb/hypoxia.html 3% increase in N load 8% due to NO 3 - concentration 2% due to discharge (Donner et al. 22, Justić et al. 23)
193 23 maximum flow 27 193 23 mean flow 26 23 1 cfs 24 193-23 minimum flow http://www.mvn.usace.army.mil/eng/edhd/tar.gif
(Lohrenz et al. 1997) 5% C * More Nutrients, More Production, More Carbon Flux Decreased Water Clarity Decreased Oxygen Concentration Average Secchi Disk (m) 6 4 2 198 1984 1988 1992 1996 Year (Rabalais et al. 221) (Turner et al. 25)
Predicting Hypoxia in summer (nitrate flux in the spring, Apr-Jun, year) Similar analyses with PO4, TP, TN, Si, various Si:N:P ratios indicate that N, in the form of NO3+NO2, is the major driving factor influencing the size of hypoxia on the Louisiana shelf. (Turner et al. 26)
.75.5.25 Photo removed. Sediment cores taken from the Mississippi River bight where accumulation rates are between.5 and 2.5 cm y -1. Station I3 F35 E5 E3 E6 D5 3. Sabine L. L. Calcasieu Atchafalya R. Mississippi R. Latitude (ºN) 29.5 29. 28.5 75% 5% 25% 25% * 5 km Terrebonne Bay Mississippi River Delta Bight 1 93.5 92.5 91.5 9.5 89.5 Longitude (ºW)
Fossil and chemical biomarkers from cores (in the last half of the 2 th century): increased phytoplankton production Increased diatom biomass Increased diatoms that are less silicified as Si:N 1:1 Increased phytoplankton pigments Increase in TOC Increase in hypoxia tolerant benthic foraminiferans Loss of hypoxia intolerant benthic foraminiferans Increase in pigments of anoxygenic phototrophic brownpigmented green sulfur bacteria Worsening hypoxia stress Hypoxia has not always been present The long-term changes are consistent with the observational data on hypoxia and coincident with the well-documented increase in nitrate export from the Mississippi River.
Important Factors for Hypoxia Stratification Currents Winds, waves Nutrient-enhanced primary production High flux of surface carbon to the seabed Oxygen consumption exceeds oxygen resupply Directly proportional to N load Unimportant (or Minimal) Factors for Hypoxia Deep-water oxygen minimum layer Allochthonous river carbon Ground water Wetland erosion Estuarine nutrients Mississippi River mainstem and deltaic levees Reduced suspended sediments Upwelled nutrients Climate (not as yet) (Source: N. N. Rabalais, LUMCON)
2 Teragrams of nitrogen Period of explosive increase Total reactive N of coastal eutrophication 15 Industrially fixed 1 (mainly fertilizer) 5 N-fixing crops Fossil fuel combustion 19 192 194 196 198 2 Period of the explosive increase in coastal eutrophication around the world in relation to global additions of anthropogenically fixed nitrogen (from Boesch 22).