Hypoxia, the condition of low dissolved oxygen in lakes and coastal waters, is a problem in estuaries worldwide. Hypoxia is an environmental phenomenon where dissolved oxygen concentrations in the water column fall below the minimum threshold necessary to induce respiratory distress among aquatic organisms in the system. Often caused by excess nutrients entering the estuary from agriculture and other human activities, hypoxia can stress estuarine organisms, especially benthic organisms that are immobile. Hypoxia is often defined as a concentration of dissolved oxygen below 2 mg/L.  The development of hypoxia may be a result of a number of factors, both natural and anthropogenic , but is mainly driven by excess nutrients to a system, and stratification of the water column due to salinity and temperature gradients and lack of mixing. Freshwater discharge from rivers introduces nutrient-rich water into an ecosystem. Phytoplankton, like algae and other larval species, feed on these nutrients which increase their size and biomass, causing them to sink deeper into the water column. Further transport of nutrient-rich matter occurs throughout the water column when zooplankton eat algae and other types of phytoplankton. As the phytoplankton are consumed, zooplankton excrete fecal pellets which sink to the seafloor where decomposition by bacteria breaks down the nutrient-rich fecal pellets through bacterial respiration. This process consumes most of the oxygen supply in the bottom waters, resulting in organism stress and mortality. Hypoxic waters, often termed “dead zones” are most prevalent in the coastal waters of the United States from late spring through late summer and are becoming more frequent, widespread, and persistent. In southern Louisiana there has been yearly documentation of a dead zone near the mouth of the Mississippi River.

Diagram of how bottom hypoxia develops in nearshore waters. In addition to runoff and effluent, the influx of large amounts of freshwater can also cause stratification to develop. Source: National Coastal Condition Report IV, EPA-B42-R-10-003 (April 2012).

PC Research Program

In 2008, two marine surveys fortuitously documented the occurrence of a low dissolved oxygen layer in Chandeleur Sound. PC set up a research program to investigate if the occurance of bottom hypoxia in Chandeleur sound was a common phenominom or was a rare event. Therefore, PC has been studying the hypoxia in Chandeleur Sound since 2010. In 2013, PC added Breton Sound as part of the survey after hypoxia was detected during data collection for our Blue Moon Estuary Cruise. Hypoxia is sampled in the field multiple times a year beginning in the spring and ending in the fall. The goal of our research program is to determine when the hypoxia sets up, when it dissipates and what environmental conditions contribute to both.

Hypoxia sampling locations in Chandeleur and Breton Sounds. 

To sample for bottom hypoxia a YSI meter with a 90 foot cable was used to measure salinity, temperature, and dissolved oxygen concentrations. The YSI cable was marked at two-foot intervals. First, the total depth was determined using on-board depth finders. Next, the probe on the instrument was lowered to take measurements at three depths; two feet beneath the surface, at mid-depth, and one foot from the seafloor (also called top, middle and bottom). When hypoxia was measured as part of our estuary cruise, a CTD water sampler was often used. The CTD measures salinity, temperature, dissolved oxygen and chlorophyll a continuously, with depth. All of this data combined allows us to determine if there is stratification, which is a condition necessary for hypoxia to set up, if hypoxia has set up, and at what depth the hypoxia reaches from the bottom.

Results

PC has consistently found the seasonal development of bottom hypoxia from 2010 to 2015. The extent of the hypoxia detected is not the same every year. In some years  the hypoxic area is quite small and at other times it is large. Also, since we are not sampling continuously over the hypoxic season we may not capture the full extent that the hypoxia reaches. When we surveyed more than one time during the summer in 2013-2015, we saw that the size of the hypoxia can change a great deal with in the same season. In 2011, the Louisiana Universities Marine Consortium (LUMCON) surveyed east of the river and detected bottom hypoxia that extended to the Mississippi/Alabama boarder. Their survey did not extend past the boarder but it is likely the hypoxia continued. PC’s surveys do not extend much further past the Chandeleur Islands but in many years, it is evident that the hypoxia extends further. Our vessels are not capable of navigating further. It seems that the hypoxia begins to develop after a period of low wind and calm conditions and remains until a front or hurricane produces winds string enough to induce mixing.

 

Extent of the hypoxia detected in Chandeleur and Breton Sounds from 2010 to 2017. The 2011 survey included data provided by LUMCON. The first Breton Sound survey did not take place until 2013, therefore there may have hypoxia present in Breton Sound in prior years. It was not detected there in 2014. The total area that has experienced hypoxia since 2008 is 2,341 square miles.

Major Conclusions

While the size of the hypoxic area varies on a yearly basis, in every year that hypoxia was surveyed, it was detected north of the Chandeleur Islands and into the Cat Island Channel. The development, growth, shrinking and eventual disappearance (usually due to fronts or storms) of hypoxia in Chandeleur Sound is a dynamic process and depends on current environmental conditions, water quality, tides, currents, wind and rainfall. Historical evidence suggests that the hypoxic water masses follow bathymetry very closely in that the denser, more saline water masses are trapped in the deeper regions of the Chandeleur Sound. With increases in surface freshwater discharge to the region, from Pearl River and other outlets, the bathymetry supports a suitable scenario for the development of a stratified water column, further cutting off the bottom waters for oxygen-rich surface water.

The cause of the formation of hypoxia may be a naturally induced phenomenon simply due to the lack of wind and wave energy available to mix the coastal waters with deep Gulf of Mexico water, therefore causing density driven stratification to develop on a seasonal basis. The hypoxic area is a reentrant that appears to be isolated from ocean currents prevalent in the open Gulf of Mexico. The region where hypoxia develops is bypassed by the prevailing currents in the Gulf of Mexico. The hypoxia also sets up at a time of the year when winds are low in the summer. The combination of low winds and a lack of horizontal surface currents , creates a low energy environment which allows for stratification of the water column to develop. Low wind velocity is in direct correlation with lack of vertical mixing and therefore stratification and hypoxia develops. It does not appear that the development of bottom hypoxia east of the Mississippi River is driven by the input of excess nutrients, as it is for the Dead Zone. However, the exact cause of this hypoxia has not been found and study continues.

Resources

To obtain copies of reports below email info@ScienceForOurCoast.org

Chandeleur Sound Water Quality-Oct 2010
Chandeleur Sound Water Quality-Jan 2012
Chandeleur Sound Water Quality-Sept2012
Chandeleur Sound Water Quality-July 2013