Trends in oxygen variability in the northern Gulf of Mexico hypoxic zone

For over 30 years scientists have been monitoring dissolved oxygen concentrations in the bottom waters of the northern Gulf of Mexico to better understand the distribution and dynamics of hypoxia (DO <=2.0 mg/L) off the Louisiana-Texas coast. A typical seasonal pattern follows the flow patterns of the Mississippi River and annual weather patterns. Peak river discharge is in March-May and peak nitrate load follows peak discharge by one month. Low oxygen conditions lag the peak nutrient load by one month. The nutrients stimulate the growth of phytoplankton, a large portion of which settles to the seabed as senescent cells, fecal pellets and aggregates. Decomposition of the organic matter by aerobic bacteria leads to a decrease in oxygen levels, which leads to low oxygen in a stratified water column. This study explores the magnitude of oxygen variability over a 20 year period at a single site in the northern Gulf of Mexico. A primary focus of this study is the rate of change of dissolved oxygen values over small scales of hours to days and whether or not these rates change annually, seasonally, or as a result of other forcing functions. Water quality measurements made between 1989 and 2008 from a site 20 miles south of Terrebonne Bay in an area that experiences frequent occurrences of annual hypoxia serve as the source of data for this study. Annual, monthly, and daily minima, maxima, ranges, and averages were computed using a MS SQL query. MS SQL queries from the database provided the data for all further analyses. Both daily rates of change in dissolved oxygen concentrations and the rates of change within a twenty-four hour period were computed using R statistical programming. We then identified and quantified three types of events-(1) hypoxic events, defined as a period of persistent dissolved oxygen (DO) concentrations <=2.0 mg/L over a 24 hour period, (2) severely hypoxic events, defined as a period of persistent DO concentrations <=1.0 mg/L over a 24- hour period, and mixing events, defined here as a rapid increase in DO concentration (?DO>=2.0 mg/L/h). Results from both the rate analyses and the event analyses showed both seasonal variation and inter-annual variation. Monthly averages of respiration rates (?DO <=0) varied seasonally ranging from 0 to 0.27 mg/L/h. Although this pattern varied somewhat from year to year the respiration rates were lowest in the summer months of June and July (when concentrations are lowest) and highest in the fall months (exception May 2002), especially October. The number and duration of hypoxic and severely hypoxic events vary from year to year; however, these events were more persistent during summer months. Most of the mixing events occurred in March and in August while no mixing events occurred in July of any year. Although dissolved oxygen concentrations varied seasonally, these annual patterns also varied from year to year. These seasonal variations are mostly in response to annual weather patterns and variation in flow and nutrient loads from the Mississippi. The highest monthly average respiration rate was 0.27 mg/L/h in May of 2002 the same year of the largest areal extent on record. At a rate of 0.25 mg/L/h it would take only one day to bring the DO concentrations down by 6 mg/L. The mean rate over the 20 years of 0.064 mg/L/h would restore the bottom waters to hypoxic conditions in about 4 days of a re-aeration event if the mixing did not persist.