Human activity in the Midwestern United States impacts not only the surrounding land but also the water in rivers and streams, which run all the way into the Gulf of Mexico. Scientists have identified several human actions, both historical and ongoing, that have contributed to major changes in the Gulf’s waters and ecosystems. Each of these manmade environmental changes has resulted in unnaturally high levels of specific nutrients, particularly nitrogen and phosphorus. This excess of nutrients in a body of water as a result of runoff is called eutrophication, and can often lead to hypoxia—the subsequent depletion of dissolved oxygen in the water.
A report written in 2000 by the National Science and Technology Council Committee on Environment and Natural Resources outlines three historical trends that have contributed to eutrophication. The first event was river channelization for navigation and flood control, which occurred prior to the 1950’s. Next was landscape alteration by humans. This includes deforestation and the expansion of agricultural drainage, both historically and currently. The problem with changing the landscape in these ways is that natural buffers for runoff are removed, which results in more nutrients entering rivers and streams. Vegetation acts as a filter for pollutants before they enter rivers or streams, and deforestation removes these plants and destroys the soil which would also help filter out harmful nutrients. The final event that the report lists, which is most important because it is ongoing and accelerating in rate, is the recent increase of nitrogen input to the Mississippi-Atchafalaya River Basin, primarily from the application of agricultural fertilizers (1).
Agriculture has caused an increase in the flow of nutrients from chemical fertilizers into bodies of water. The excess nutrients change the chemical composition of the water, impacting biological life forms in the affected areas. Sewage is another major source of nutrient flow to the Gulf. There are sections of lakes and oceans all over the world affected by eutrophication and hypoxia, and this has not only biological effects but also economic and social problems. The largest hypoxic area in the western Atlantic Ocean is found in the Gulf of Mexico (1). The biological repercussions of eutrophication, in the form of habitat alteration and entire trophic structure disintegration are devastating to the Gulf; remediation, though costly, must be put into effect in order to bring life back into the area.
The ‘Dead Zone’
The Gulf of Mexico acts as a drainage zone for all the major rivers and tributaries in the Midwestern United States. This network is called the Mississippi-Atchafalaya River Basin (MARB), and includes major rivers such as the Mississippi, the Missouri, and their tributaries, which all eventually drain into the Gulf. As shown in Fig. 1, the MARB contains water from 31 states and is the third-largest river basin in the world, behind the Amazon and the Congo basins (2). Several nutrients, the most harmful of which is nitrogen, enter streams as runoff partly from sewage but mainly as runoff from agricultural fertilizers used in the 31 states in the MARB. The exact location of the area suffering from eutrophication stretches westward from the Mississippi River delta to the upper Texas coast. This zone’s area varies between 6,000 and 7,000 square miles—nearly the size of New Jersey (3).
Why is excess nitrogen so harmful to a body of water? The main reason is that it is no longer a limiting factor to growth, particularly to algal growth. Immense blooms of algae are produced from the nitrogen surplus, and these blooms are problematic. In addition to lowering the water quality, they also lead to hypoxia; when bacteria in the water eat these algae, much of the water column’s oxygen supply is used up so that the overall amount of oxygen dissolved in the water is depleted. It should also be noted that, though nitrogen is present in the MARB in a few different forms such as dissolved organic or particulate organic nitrogen, its most common form is as dissolved inorganic nitrogen, or nitrate. The National Science and Technology Council reports, “The most significant trend in nutrient loads has been an increase in nitrate load, which has almost tripled from 0.33 million metric tons per year during 1955-70 to 0.95 million metric tons per year during 1980-96” (1).
Nitrogen in the form of nitrates is a main component of fertilizers that are used in massive quantities in the Midwest. In 2008, over 12.5 million tons of nitrogen were applied as part of fertilizers throughout the United States (4). Because farming induces it, the zone of eutrophication in the Gulf fluctuates seasonally, decreasing in size during the winter months. The nutrient flux through the MARB also varies with weather – hurricanes, flooding, or heavier rainfall increase flux by resulting in more runoff. These nutrients travel from farmlands all the way down to the Mississippi River basin, where they are emptied into the Gulf of Mexico and wreak havoc on the aquatic ecosystems.
Hypoxic waters are specifically defined as those with oxygen levels below 2 milligrams of dissolved oxygen per liter. Hypoxic areas in the Gulf are normally between 5 and 30 meters deep, but can be as deep as 60 meters or as shallow as 4 near the shore (1). Hypoxia in the area creates what is known as a ”dead zone,” or an area in the water where there are little to no living organisms. In severe cases, ecosystems can completely collapse (5). While the growth of organisms is inhibited when oxygen levels dip below 5 mg O2/L, the natural processing of nutrients, pollutants, and organic matter is also disrupted (1).
Hypoxia, anoxia, and ecosystems
Similar to hypoxia’s definition in the medical community where it refers to a deficiency in the amount of oxygen reaching tissues, a hypoxic body of water does not have enough oxygen to keep an ecosystem functioning properly (5). Because an ecosystem represents such a complex interconnected web involving all trophic levels, a change in just one species can disrupt an entire system. This is complicated by the fact that, although waters are considered officially hypoxic at around 2 mg O2/L, specific situations of depleted oxygen affect some species more than others. For example, the growth of Atlantic cod is reduced when dissolved oxygen is below 7 mg O2/L (approximately 70 percent air saturation), while the shrimp and fish that are so important to fisheries in the Gulf of Mexico avoid waters with less than 2 mg O2/L (30 percent air saturation) (5). According to Eby and Crowder, since species vary in their oxygen requirements, sensitive predators can lose access to prey. Mass emigration from hypoxic areas causes crowding in others. Crowding in more highly oxygenated refuges can result in density-dependent growth reductions or increased cannibalism (6).
Hypoxia also affects ecosystems’ energy flows. In a hypoxic area, much of the system’s energy is diverted from invertebrates to microbial decomposition, and the energy flows through in pulses, favoring species with shorter lives. Since the longer-lived species are eliminated by the lack of oxygen, the decrease in overall biodiversity and biomass is associated with these pulsed energy systems (1).
In the dead zones created by hypoxic conditions, creatures such as fish, shrimp, zooplankton, and others either become less abundant or die out. The entire food web becomes muddled when bottom-dwelling (benthic) organisms die and larger, longer-living organisms are eliminated (1). Affected areas experience overall reduced species diversity and dominance of gelatinous organisms, like jellyfish (5). Severely low oxygen levels cause all trophic levels to suffer.
Water completely devoid of oxygen is considered anoxic. Significant biological and geochemical shifts accompany the transition from oxic to anoxic conditions. Bioturbation—the disturbance of sedimentary deposits—declines among benthic organisms. Organic carbon continues to accumulate at the seabed as organisms die and sink, and redox occurs at the sediment-water interface as sulfate respiration replaces oxygen respiration. This chemical process results in the creation of the compound hydrogen sulfide (H2S). H2S devastates biological communities because it is toxic to most macrofauna and contributes to the death of benthic organisms. These shifts in the benthic microbial community are evident through the existence of bacterial mats at the sediment-water interface. In addition to these bacterial mats, another indication of near anoxia is the fact that the water becomes almost completely black, and there are no signs of aerobic life (5).
Human health and livelihood
In addition to biological problems, eutrophication also has effects on peoples’ health and livelihoods. Excess nutrients can directly and indirectly lead to a decrease in local drinking water quality. The National Science and Technology Council’s report states, “Review of state assessments submitted to the U.S. Environmental Protection Agency under section 305b of the Clean Water Act indicates that most states in the MARB have substantial numbers of river miles impaired by high nutrient conditions” (1). When pregnant women drink water with too much nitrate, their infants may have insufficient oxygen levels in the blood and suffer from “blue baby syndrome” (Methemoglobinemia) (1). Algae blooms can also clog pipes, cloud the water, and interfere with recreational activities. And, on top of all this, the decay of algae gives off foul odors. Clearly, eutrophication in the Gulf of Mexico has a negative impact on humans as well as the organisms living in the water. Whether the current state of the Gulf is more detrimental to marine biology or to humans it does not matter. What is important to grasp is that the situation is affecting many lives and must be stopped as soon as possible.
Prevention and remediation
In order to tackle the eutrophication problem in the Gulf, it must first be confronted at its main source—agriculture. There must be limits and reductions to waste discharged in the form of nutrients, chemicals, and organic matter by farms and industries. Major changes in farming practices and techniques are necessary to remediate eutrophication in the Gulf. Experts have outlined several strategies that farmers can and should undertake in order to reduce the amount of nitrogen and phosphorus draining off their farms and entering the MARB. First, the application of fertilizer and manure should be as efficient as possible, both in the amount applied and the time applied. Better timing of fertilizer and manure applications can help keep runoff from being overloaded with nutrients. Farmers can also switch from fall to spring when applying fertilizers. They can plant special cover crops that absorb nutrients in the fall and winter. Finally, to effectively have as much nitrogen and phosphorus filtered out before runoff reaches major MARB rivers, farmers should reroute soil drainage systems through wetlands, grass buffer strips, or forests adjacent to rivers and streams (1).
According to the National Science and Technology Council, a 40 percent reduction in the total nitrogen flux to the Gulf would be needed in order to return nitrogen levels back to normal and alleviate the dead zone (1). If nutrient levels remain at current levels, the situation in the Gulf will probably stay the same, with the hypoxic area fluctuating a bit seasonally. The Gulf would continue to suffer from a loss of biodiversity and biomass. Scientists are not able to predict exactly what would happen in the Gulf if nutrient levels were to increase, which is likely to happen in the future due to population increase, higher food production demands, and climate change. But judging by similar situations of eutrophication in places like the Baltic and Adriatic Seas, even higher nutrient levels will almost certainly lead to the abrupt decline of ecologically and commercially valuable fisheries. The major issue with any of the suggested remediation efforts has to do with the major investments involved. Changing agricultural methods is very costly and requires great effort. To reduce nitrogen loads would result in higher equipment and material costs, as well as higher agricultural management costs. Diverting rivers and streams would also probably lower the total area of productive farmland. In order to improve sewage management, this would require an increase in treatment costs for municipalities and industries. However, the positive aspects to changing agricultural and sewage management are significant; in addition to the obvious remediation of eutrophication in the Gulf, the area would enjoy improved water quality, less soil erosion, and cost-effective flood damage reduction (1). These benefits should provide incentives for farmers to alter their practices, because living conditions will be improved locally as well as for those who live further south along the Gulf. Though expensive, remediation is necessary in order to save the Gulf of Mexico and revive it from its “dead” state.
References
1. Integrated Assessment of Hypoxia in the Northern Gulf of Mexico (National Science and Technology Council Committee on Environment and Natural Resources, 2000).
2. The Mississippi-Atchafalaya River Basin (2011). Available at http://water.epa.gov/type/watersheds/named/msbasin/marb.cfm (11 December 2011).
3. M. Bruckner, The Gulf of Mexico Dead Zone. Available at http://serc.carleton.edu/microbelife/topics/deadzone/ (11 December 2011).
4. Fertilizer Use and Price (2011). Available at http://www.ers.usda.gov/Data/FertilizerUse/ (11 December 2011).
5. R. J. Diaz, Advancing the Aquaculture Agenda (OECD Publishing, 2010), pp. 275-318.
6. L. A. Eby, L. B. Crowder, Estuaries 27, 342-351 (2004).