Abstract:

Beginning in 1985, a team of scientists from Louisiana Universities Marine Consortium (LUMCON) and Louisiana State University (LSU), led by Dr. Nancy Rabalais and Dr. R. Eugene Turner, respectively, began documenting the area of bottom-water hypoxia (dissolved oxygen concentration < 2 milligrams per liter). Reported here are area estimations in square kilometers from 1985 to 2014.

Suggested Citation:

Rabalais, Nancy N., Smith, Leslie. 2017. Bottom-water area of Louisiana-Texas continental shelf hypoxia 1985-2014. Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/N7GF0RKZ

Publications:

Rabalais, N. N., Smith, L. M., & Turner, R. E. (2018). The Deepwater Horizon oil spill and Gulf of Mexico shelf hypoxia. Continental Shelf Research, 152, 98–107. doi:10.1016/j.csr.2017.11.007

Purpose:

These estimations of the size of the hypoxic area provide the baseline for the Mississippi River Nutrient/ Gulf of Mexico Hypoxia federal/state/tribal Task Force that has as its goal a reduction in the size of the bottom-water area of hypoxia to less than 5 milligrams per liter by the year 2035.

Data Parameters and Units:

Year: Year of sampling, (YYYY) Dates of mapping: Inclusive dates of sampling, (M/DD - M/DD) Area (km2): Estimated bottom-area of hypoxia, kilometers squared Transect Station ID Depth (m or ft) Latitude (degrees minutes or decimal degrees) Longitude (degrees minutes or decimal degrees) Location description Notes NCEI Accession #'s: 1985-1993 - 9800129 1994-1997 - 0164298 1998-2002 – 0002033 2003 – 0020956 2004 – 0032050 2005 – 0039733 2006 – 0049435 2007 – 0060060 2008 – 0069471 2009 – 0099531 2010 - 0117436 2011 - 0129417 2012 - 0162101 2013 - 0162440 2014 - 0161219

Methods:

The Louisiana and upper Texas shelf of the Gulf of Mexico has been consistently sampled in mid-summer since 1985 (with the exception of 1989 and 2016) for the presence of hypoxia and its associated environmental factors. The 5- to 7-day cruise departs from Cocodrie, LA, transits to Southwest Pass of the Mississippi River delta, then occupies shelf-perpendicular transects of stations from 4- to 5-m water depth to 30- to 40-m water depth. The transects are occupied from west to east. Hydrographic data were obtained using the R/V Pelican SeaBird SBE91+ CTD system (or equivalent for historic data), which included a SBE 43 dissolved oxygen sensor along with sensors measuring conductivity, temperature, depth, as well as other typical environmental parameters. The CTD unit included a 12-carousel rosette with 5-L Niskin bottles and was deployed from 1.5 to 2 m below the surface to within 1.5 to 2 m of the bottom. A YSI 6820 (or Hydrolab equivalent for historic data) was deployed separately to capture water quality data missed by the CTD rosette within 2 m of the surface and within 2 m of the bottom. The bottom-water area of hypoxia (km2) was determined by interpolating between sampling locations and hand-contouring parallel to isobaths over a calibrated (planimeter) grid. Bottom oxygen (mg l-1) concentration data from the YSI 6820 were used for each sampling location. It should be noted that, at times within the historical data, the bottom-water dissolved oxygen data from the CTD and the hand-held YSI 6820 equivalent were not collected together. When only the CTD data were available, then its bottom-water oxygen values were adjusted to extend from the CTD measured depth (roughly 2-m above the bottom) to a bottom depth more similar to the YSI according to relationships between the dual CTD and YSI data (Obenour et al., 2013). If these calculations changed any of the original spatially generated maps of N.N. Rabalais, then the estimate of bottom-water hypoxia was re-calculated. The percentage difference of those not changed and those changed over a 27-y period was 0.006%. The annual estimates are provided at http://www.gulfhypoxia.net. Obenour, D. R., D. Scavia, N. N. Rabalais, R. E. Turner and A. M. Michalak. 2013. A retrospective analysis of mid-summer hypoxic area and volume in the northern Gulf of Mexico, 1985-2011. Environmental Science and Technology 47:9808-9815. dx.doi.org/10.1021/es400983g

Instruments:

YSI – 6820 handheld multi-parameter instrument, 5-L Niskin bottle, oxygen titrator (Mettler Toldeo DL28), SeaBird CTD unit with 12 5-l sampling rosette, Sea-Bird Electronics SBE-9+ Underwater Units with Digiquartz Pressure Sensor, 4127 and 6885 meter depth, Sea-Bird Electronics SBE-11 Deck Unit. SBE 43 dissolved oxygen sensors (2).

Error Analysis:

The YSI 6820 Oxygen (and other) sensors were serviced and calibrated before deployment and maintained in accordance with YSI (http://www.ysi.com/) recommended procedure. The Sonde and Logger are returned to the factory at least annually for inspection and service. The Sea-Bird SBE 13-01 dissolved oxygen (and other) sensors were factory tested and calibrated at Sea-Bird (http://www.seabird.com/) recommended intervals and maintained and serviced by RV Pelican Electronic Technical support staff in accordance with Sea-Bird procedures. At the beginning of the each July Shelfwide cruise, oxygen sensors were calibrated using the procedures described in SeaBird APPLICATION NOTE NO. 13-1, Rev. D. The Winkler Titration oxygen value was determined chemically from replicate samples processed using a Mettler DL21 Titrator (http://www.mt.com/). Shipboard Winkler titrations were performed during the Shelfwide cruise to develop regressions against the YSI and SeaBird oxygen data. Both were then corrected based on the results of the regression. Winkler titrations were conducted under the supervision of Nancy Rabalais. SeaBird data post processing was done by Leslie Smith and Wendy Morrison and quality controlled by Nancy Rabalais. Winkler samples were analyzed on board the R/V Pelican with a Mettler Toledo DL28 Titrator using dissolved oxygen determination methods outlined in A Practical Handbook of Seawater Analysis by Strickland and Parsons, 1977. Winkler analyses were conducted by Wendy Morrison under the supervision of Nancy Rabalais. Horizontal_Positional_Accuracy: Cruise station positions were logged from RV Pelican's differential GPS at the beginning of sampling operations. GPS manufacturer's accuracy claim is 1-5 meters 95% of the time. Wind, currents and tidal forces may have moved the ship from the beginning position.

Provenance and Historical References:

Hypoxia surveys in the Northern Gulf of Mexico have been conducted annually since 1985. Complete data sets for each year can be found in the archives of NOAA's NCEI.

Individual Files: