Hypoxia in the Gulf of Mexico: Data Collection Methods, Causes, and Future Planning

Introduction
Hypoxia refers to the condition of low dissolved oxygen content in a body of water (Rabalais, Turner, & Wiseman, 2002b). The level of oxygen at which a body of water would be described as hypoxic is not universally defined or accepted, but is most often classified within the general range of 3.0 to 0.2 ml 1‐1 (Rabalais et al., 2002b). Hypoxia develops when a stratified water column, with separation of the bottom and top water layers due to differing temperatures, salinity, or both, experiences decomposition of organic matter to such an extent that the bottom water oxygen levels are reduced (Rabalais et al., 2002b).
This condition occurs in the Gulf of Mexico on a large scale, with the hypoxic area exceeding 20,000 square kilometers (Justic, Rabalais, & Turner, 2003). This makes it the second largest hypoxic zone in area in the world, exceeded only by the Baltic basins at approximately 70,000 square kilometers (Rabalais, Turner, & Scavia, 2002a). A level of oxygen of 2 milligrams per liter or less is used as the operational definition of hypoxia specific to the Gulf of Mexico, as no shrimp or demersal fish are caught by trawlers in that oxygen range (Rabalais et al., 2002a). Hypoxia was initially reported in the Gulf of Mexico in the early 1970s, and systematic mapping and comprehensive assessment began in 1985 (Rabalais et al., 2002a).
The Gulf of Mexico is fed by both the Mississippi and Atchafalaya Rivers, and the increased levels of nitrogen and declining water quality of these sources has been blamed for the growth of the hypoxic zone (Donner, Kucharik, & Foley, 2004). The overlapping of this zone with commercially important fishing zones combined with the increasing environmental concern have made this a highly researched area of study, with an emphasis on potential human impact from nitrogen loading (Donner et al., 2004). Several “Action Plans for Reducing, Mitigating, and Controlling Hypoxia in the Northern Gulf of Mexico” have been created and endorsed by both state and federal governments (Rabalais et al, 2002a).
Mapping the size of Gulf hypoxia

Since 1985, the Gulf of Mexico has been mapped annually, usually in midsummer between the middle of July and August, in order to determine the extent of the hypoxic zone (Rabalais et al., 2002a). Results of oxygen testing of water samples show that the hypoxic water mass reaches in a westerly direction from the Mississippi River Delta, extending across the Louisiana Shelf, and within a ranging distance from the upper Texas Coast(Rabalais et al., 2002a). Hypoxic conditions were found to exist at depths of up to 60 meters, but most often between 5 and 30 meters (Rabalais et al., 2002b). The size has changed over the surveyed time, ranging between 8,000 to 9,000 square kilometers from 1985 and 1992, and after the Great Mississippi Flood increasing to between 16,000 and 21,000 square kilometers from 1993 and 2001. (Rabalais et al., 2002a).
More frequent sampling was conducted in order to determine the oxygen levels at different times during the year, and results indicated that hypoxic levels below the pycnocline existed from the end of February through until October. (Rabalais et al., 2002b). Almost continuously present from the middle of May through September, hypoxia presence was inconsistent through March, April, and May, and was the most demonstrably present and severe through the summer months of June, July, and August (Rabalais et al., 2002b).

Sediment denitrification

The increasing zone of hypoxia in the Gulf of Mexico is largely attributed to the increased decomposition of organic matter due to the accelerated rate of primary production, found especially in the summer months (Childs, Rabalais, Turner, & Proctor, 2002). Childs et al. (2002) state that the increased production is driven mainly by the heightened input of inorganic nitrogen, primarily from the Mississippi River. Nitrate and nitrite can be reduced to nitrous oxide or dinitrogen through denitrification, a form of anaerobic microbial respiration, forming a sink for bioavailable nitrogen (Childs et al., 2002). The facultative anaerobes that perform this process only start to denitrify when nitrogen oxides are present and oxygen concentration is low, however at very low oxygen levels nitrate could reduce to ammonia as opposed to undergoing denitrification (Childs et al., 2002).
Childs et al. (2002) performed a study to find sediment denitrification rates, measured at 7 different stations along the Louisiana Shelf in 1999 at the peak of the hypoxic season, in order to compare with reported rates from other areas and contemporaneous water quality data. To conduct this study, the bottom water of the Gulf of Mexico was surveyed from the RV ‘Pelican’ on a transect cruise covering more than 20,000 km2 and 83 stations (Childs et al., 2002). Collections were made at each of these stations of water samples and hydrographic data, including pH, temperature, conductivity, dissolved oxygen levels, salinity, fluorescence, percent of light transmitted, and depth (Childs et al., 2002). Bottom water and midwater samples were retrieved using 5 1 Niskin bottles, and surface water level samples were collected simply using buckets (Childs et al., 2002). Intact sediment cores were taken from each of seven specific stations using a boxcoring device (Childs et al., 2002).
The water samples were then incubated, anaerobically to prevent contamination, in 160ml serum bottles (Childs et al., 2002). Included was a head space which contained a combination of nitrogen gas and ethyne gas, which was then transferred to 10ml Vacutainers and analyzed using a Shimadzu GC‐8A gas chromatograph to determine the linearity of N2O production (Childs et al., 2002). The box core samples were sub‐cored using butyrate cores that were 5cm in diameter, with a water layer being kept over the surface of the sediment in order to reduce oxygenation before analysis (Childs et al., 2002).
The results of the analysis found that the hypoxic zone in the summer of 1999 spanned over 20,000 km2, and that denitrification rates varied depending on the concentration of dissolved oxygen (Childs et al., 2002). As expected, the highest rates of denitrification occurred where the dissolved oxygen concentration ranged between 1 and 3 mg l‐1, and the lowest rates occurred where the dissolved oxygen concentration was 5.1 mg l‐1 (Childs et al., 2002). However, when the dissolved oxygen levels dropped below 1 mg l‐1, the denitrification rates were found to be about half of the rates found at levels above 1 mg l‐1 (Childs et al., 2002). The rates overall were found to be at the low end on the scale of rates reported from other systems, which was somewhat unexpected as hypoxic conditions of low oxygen levels, high carbon levels, and high amounts of nitrate should be ideal for denitrification (Childs et al., 2002). The observed rates do vary significantly and the highest rates were found in shallow waters near shore and in the more freshwater environments, whereas the other areas (likely in situ denitrification) could be lower due to the fact that Childs et al. (2002) measured the maximum potential denitrification capability of the sediment, which is not consistent among all studies.
Overall, the data collected by Childs et al. (2002) suggested a positive feedback loop existing between the severity of hypoxia and the duration of presence of fixed nitrogen, resulting in exacerbating hypoxic conditions. As the process of denitrification was suppressed under severe hypoxia, this would effectively increase the amount of bioavailable nitrogen present in the water, which is thereby associated with an increase in primary productivity‐ a factor leading to the original development of the hypoxic zone(Childs et al., 2002).

Contributing Factors

Justic et al. (2003) collected data and ran a mathematical model in order to determine correlation of hypoxia with concentration of nitrate and ambient water temperatures, specific to the Mississippi River discharge. The Mississippi River Watershed/ Gulf of Mexico Hypoxia Task Force has established a goal to reduce the area of the hypoxic zone to less than 5000 km2 by 2015, with a proposed action plan requiring a decrease of 30% in nitrogen flux (Justic et al., 2003). Justic et al. (2003) ran models in order to determine whether this would be an adequate target in order to meet that goal,and how the river discharge would be affected by the climactic impact of global warming.
Through the use of a two‐box modeling system, assuming uniform properties for both layers below and above the pycnocline, Justic et al. (2003) used oxygen flux, oxygen concentration, nitrate flux, and numerous other variables to predict the relationship between nitrate concentration, temperature, and hypoxia. The results of the study indicate that the hypoxic conditions in the Gulf of Mexico are very sensitive to variations in river discharge, nitrate flux, and water temperatures, and indicate that a nitrogen flux decrease of 30% from the Mississippi River may not actually be enough in order to meet the task force goal (Justic et al., 2003). For example, a 30% decrease in such flux was predicted to result in a 37 % decrease in the frequency of hypoxia, however an increase in discharge by only 27% was found to produce the same magnitude of increase of hypoxia under some climactic change scenarios. (Justic et al., 2003). More research and modeling would be required in order to more conclusively determine the magnitude of the correlation, but the study performed by Justic et al. (2003) did provide strong evidence for simply the existence of a correlation.

Effect of Nitrogen Fertilizer

A study by Donner et al. (2004) examined how agricultural practices, combined with climate influences, affect the input of nitrogen into the Mississippi River Basin and therefore the Gulf. A modeling system called HYDRA uses the IBIS model predicted runoff, surface runoff, groundwater drainage, and nitrate leaching in order to simulate river discharge and nitrate export, which has been widely blamed in popular media for the growth of the hypoxic zone in the Gulf of Mexico (Donner et al., 2004). This study was also the first time that a process‐based and dynamic modeling system had been used to predict and simulate the combined influence of the use of fertilizer, land management, and climate on the nitrogen cycle and the export into the river basin (Donner et al., 2004). The simulations from the model indicated that the factors leading to the doubling of nitrate export into the Mississippi River since 1960 can be attributed to an increase in fertilizer use, especially on maize, the recent increase in popularity of soy and the subsequent expansion of soybean cultivation, and an increase in runoff into the basin (Donner et al., 2004). Donner et al. (2004) presented findings that suggest that up to 90% of the nitrate found in the river could be attributed to fertilized crops, with the majority appearing to originate from an area known as the “Corn Belt,” which is a stretch of land across Iowa, Illinois, and Indiana.

Historical Analysis

The Gulf of Mexico has been the focus of observation and studies for over 30 years, as early as the 1970s as indicated by attached Table 1 (Rabalais et al., 2002a). As previously mentioned, the hypoxic zone has been increasing, and more largely affecting commercial fisheries and economics, and combined with the growing environmental concern the amount and intensity of studies conducted on this area have also increased within the last several years (Rabalais et al., 2002a). In 1990, continuously recording (at 15 minute intervals) oxygen meters were deployed at a 20m depth along Terrebonne Bay in the Gulf (Rabalais et al., 2002b). Since this time, the emerging pattern shows a gradual decline of oxygen concentrations in bottom water during spring and summer, with persistent hypoxia for extended periods from May through September, and subsequent wind mixing in the fall that is sufficient to prevent bottom water hypoxia for prolonged periods of time (Rabalais et al., 2002b). As water column data before the 1970’s is not available, sediment records for paleoindicators of long term transitions relating to concentrations beneath the Mississippi River, specifically oxygen conditions, can be used (Rabalais et al., 2002b). Sediment cores from both within and beyond the hypoxic region were found to contain both biological and chemical remnants that can reflect the conditions of both the bottom and surface waters at the time of sediment deposition, which can therefore provide evidence of changes that occurred as long as a century ago (Rabalais et al., 2002b). The sediment cores analyzed by Rabalais et al. (2002b) indicate, using accumulated amounts of diatom remains and marine‐origin carbon, an overall increase of hypoxic conditions has occurred since the turn of the century, and an increase in severity since the 1950s, when the nitrate flux to the Gulf of Mexico from the Mississippi River tripled. Since no significant increase in organic carbon or silica in rivers since 1950 has been found, it is reasonable to infer that any increase in sediment biologically bound silica since then can be attributed to in situ production of marine diatoms, meaning it is an excellent indicator for this type of production (Rabalais et al., 2002a).
 
Conclusion

With the increase of studies focusing on the situation in the Gulf of Mexico, the numerous conditions that contribute to the worsening situation can be better understood and a more accurate plan of action can be developed. Integrated assessments are being developed and updated, and the Mississippi River/Gulf of Mexico Watershed Nutrient Task Force has published extensive reports in both 2001 and 2008 (Rabalais et al., 2002a). Long term data and paleoindicators both verify and strengthen the relationship between the amount of nitrogen in the Mississippi River and the extent of the Gulf hypoxia (Rabalais, Turner, Sen‐Gupta, Boesch, Chapman, & Murrell 2007). With additional surveys in shelfscale areas, and higher frequency observations at specific and key locations, and other measures of hypoxia including volume and total deficiency of oxygen would increase understanding in the processes that support both the initial formation and the long term maintenance of hypoxia (Rabalais et al. 2007). The recent findings of Rabalais et al. (2007) reinforce the science supporting the increase of hypoxia over the last century and correlation to increased nutrient loading, and support the current Mississippi Task Force Action Plan to focus on reduction of nitrogen addition as a means of reducing hypoxia in the Gulf of Mexico.


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