Abstract:

Frequency of occurrence of hypoxia at each station sampled along the Texas/Louisiana continental shelf during annual Hypoxia surveys in the Northern Gulf of Mexico conducted annually from 1985 to 2014. 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.

Suggested Citation:

Rabalais, Nancy N., Smith, Leslie. 2017. Frequency of Occurrence of Hypoxia on the Texas-Louisiana continental shelf from 1985-2014. Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/N7BP00W2

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:

Physical, biological and chemical data have been collected as part of a long-term coastal Louisiana dataset with the goal of understanding physical and biological processes that contribute to the causes of hypoxia and to use the data to support environmental models for use by resource managers. The goal of the frequency data is to assess the areas in which hypoxia occurs most often.

Data Parameters and Units:

Station: User defined station designator Latitude: Latitude of the station in decimal degrees Longitude: Longitude of the station in decimal degrees Freq < 2 (%): Percentage of years sampled where a station had oxygen < 2 mg/L 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:

Frequency of hypoxia was examined at individual stations in terms of how often during the long-term dataset (1985-2014) hypoxia occurred at that station when that station was sampled. Note that not every station was sampled every year. In order to be included in the frequency analysis, the station had to be visited in at least half of the 29 years of sampling. Frequency of hypoxia was calculated using bottom YSI CTD measurements of Oxygen. If those data were not available, Seabird CTD data were used.

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 & 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.

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