One of The Largest Dead Zone in The Coastal Waters Around The World: The Gulf of Mexico

Coastal waters in many parts of the world are vulnerable to anthropogenic flows carried by rivers to estuaries and coastal areas at various scales. The acceleration of industrialization and modern agricultural activities has caused the symptoms of eutrophication to become increasingly worrying. The flow that lasts throughout the year with the contamination load that continues to increase in coastal waters has caused enrichment, hypoxia, anoxia, and metal contamination. Hypoxia often coexists with low dissolved oxygen (DO) and pH conditions (Gobler & Baumann, 2016). Friedrich et al. (2014) stated that oxygen depletion in waters or hypoxia occurs when oxygen consumption for the physiological needs of organisms or for chemical processes exceeds the available oxygen supply, both from adjacent water layers, from the atmosphere, or from photosynthesis.

The term "dead zone" is often used to refer to the absence of life (other than bacteria) from a habitat without oxygen or anoxia (Committee on Environment and Natural Resources. 2010). In general, organisms need oxygen to carry out physiological processes in their bodies, except for some species of bacteria and some taxa which can adapt to hypoxic and even anoxia conditions in a not-too-long time. The inability to get out of areas with low oxygen makes species with low locomotion such as oysters and clams very susceptible to hypoxia because they can cause stress and die due to inhibition of respiration and other physiological processes. Prolonged hypoxic conditions will have a significant impact on the cycle of food webs on the coast which, on the other hand, will affect the food webs in the sea, these conditions will ultimately have an impact on capture fish-based economic activities. For organisms with high locomotion abilities, they can escape when dissolved oxygen becomes too low. However, many fish species are threatened with death when dissolved oxygen concentrations drop rapidly (Rabais & Turner, 2001). Recent research seeks to illustrate how hypoxia-related habitat loss can be ecologically and economically detrimental, such as reduced growth of commercially harvested species and loss of biodiversity, habitat, and biomass (Committee on Environment and Natural Resources, 2010).

The case of hypoxia at the bottom of the northern Gulf of Mexico was revealed in 1985 when systematic mapping and oxygen monitoring activities were carried out (Rabalais et al. 1991). At that time, hypoxia in the waters was defined as dissolved oxygen below 2 mg/L which was recorded momentarily and locally. Since then, many studies have been conducted on the understanding of hypoxia in relation to seasons and annual distribution and variability, as well as the history and causes of its dynamics (Rabais et al., 2007). In the early 20th century, more than 400 hypoxic zones were identified throughout the world's coasts (Diaz et al. 2004). The number of indicated zones will continue to increase with increasing nutrient flow and increasing atmospheric temperature, so this condition must be a concern for stakeholders.?

The average dead zone covers about 5,400 miles, but those found at the bottom of the Gulf of Mexico are about 6,334 square miles or the equivalent of more than four million hectares of habitat. The exact size of the Gulf dead zone varies each year. Scientists collect water samples across the Gulf to determine the size. The dead zone can be as large as the state of New Jersey. That means millions of acres of habitat potentially unavailable to fish and bottom species. If the amount of pollution entering the Gulf isn't reduced, the dead zone will continue to wreak havoc on the ecosystem and threaten some of the most productive fisheries in the world.

NOAA says the dead zone occurs naturally, but researchers say human activity has caused the area to expand. Think of the Mississippi River as a drainage system for your street, except it connects 31 U.S. states and even parts of Canada. That’s the Mississippi Watershed. When farmers apply fertilizer, the excess nutrients such as nitrogen and phosphorus can run off during a rainstorm or snowmelt and end up in waterways that feed the Mississippi River, And farms aren’t the only source of excess nutrients or nutrient pollution. Urban runoff, such as fertilizer from lawns and golf courses, and discharges from sewage treatment plants, also feed into the Mississippi. Waste from agriculture and livestock goes into the oceans, where it stimulates the growth of algae, which in turn die and rot. During the process, the oxygen-eating bacteria decompose the algae and further cause the creation of a dead zone. According to NOAA, most marine life will die in this zone and if they live, they will soon leave the uninhabitable zone. However, even a few minutes of exposure to this biological desert can lead to negative consequences, such as changes in fish diet, growth rate, and reproduction.

The method used to detect hypoxia events in real-time is to measure DO (dissolved oxygen) conventionally or by using Kits such as autonomous underwater vehicles (AUV) (Lee et al., 2018). The use of satellites to map a wider area is also carried out in monitoring cases of hypoxia by NOAA (Committee on Environment and Natural Resources. 2010), as well as modeling using real-time oceanographic data in a long monitoring period confirmed by satellite data (Kim et al., 2020). Of all the methods that have been used, none has been able to detect events for more than 40 years, because they rely on real-time data which is closely related to the completeness of the data that has been collected and the progress of research equipment, such as the accuracy of kits for measurements in the field and the sophistication of satellites.

A variety of innovative technologies and practices are being implemented across the Mississippi River watershed to reduce nutrient pollution, such as technology that removes nutrients from wastewater, practices on the land to limit nutrients entering into waterways, and programs that help farmers implement conservation practices that protect water quality.

NOAA is also working with states to develop new runoff risk forecasting tools that help farmers determine when to use fertilizer, based on anticipated rainfall amounts. There are even steps you can take at home, such as reducing excess runoff from areas around the house, planting trees and other native plants in your yard, applying slow-release fertilizers only when needed, and minimizing food waste. Even though these efforts may take place far from the Gulf, they can still reduce the harmful impact of the dead zone!

Written by : Choerunnisa.F

Reference

Committee on Environment and Natural Resources. (2010). Scientific Assessment of Hypoxia in U.S. Coastal Waters. Interagency Working Group on Harmful Algal Blooms, Hypoxia, and Human Health of the Joint Subcommittee on Ocean Science and Technology. Washington, DC.

Diaz, R.J., Nestlerode, J., & Diaz, M.L. (2004). A global perspective on the effects of eutrophication and hypoxia on aquatic biota, p. 1-33. In G. L. Rupp and M. D. White (eds.), Proceedings of the 7th International Symposium on Fish Physiology, Toxicology and Water Quality, Tallinn, Estonia. U.S. Environmental-mental Protection Agency, Ecosystems Research Division, Athens, Georgia

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Kim, Y.H, Son, S., Kim, H.C., Kim, B., Park, Y.G., .....& Ryu, J.(2020). Application of satellite remote sensing in monitoring dissolved oxygen variabilities: A case study for coastal waters in Korea. Environment International,134:105301.

Lee, J., Park, K.T., Lim, J.H., Yoon, J.E., & Kim, I.N.(2018). Hypoxia in Korean Coastal Waters: A Case Study of the Natural Jinhae Bay and Artificial Shihwa Bay. Front. March. science. 5:70. doi.org/10.3389/fmars.2018.00070.

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Rabalais, N.N., Turner, R.E., Sen Gupta, B.K., Platon, E., & Parsons, M.L. (2007). The sediments tell the history of eutrophication and hypoxia in the northern Gulf of Mexico. Echol. application 17: S129-S143. doi.org/10.1890/06-0644.1

Rabalais, N.N., Turner, R.E., Wiseman Jr., W.J., & Boesch, D.F. (1991). A brief summary of hypoxia on the northern Gulf of Hypoxia Advances in the Gulf of Mexico 771 Mexico continental shelf: 1985-1988, p. 35-47. In R. V. Tyson and T. H. Pearson (eds.), Modern and Ancient Continental Shelf Anoxia. Geological Society Special Publication No. 58, The Geological Society, London, England.

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