Explain how ‘dead zones’ such as appear off the U.S. Gulf coast occur. (See Essay 8.1) "The ocean is teeming with life balanced between the creation and destruction of organic carbon. Like green plants on land, algae and bacteria in the surface waters of the ocean combine nutrients, water, and carbon dioxide in the presence of sunlight to fix organic carbon in the form of biomass. This organic carbon, directly or indirectly, provides food for life in the ocean and is a key part of the carbon cycle.Excessive amounts of nutrients from agricultural and urban runoff, and treated sewage wastewater, lead to large algal blooms in many areas of the coastal ocean. As the organic carbon in the bloom sinks in the water column, photosynthesis declines or ceases but organisms continue to respire. This respiration, along with the respiration of decomposers or consumers, depletes the surrounding water of oxygen, killing benthic (bottom-dwelling) organisms and any pelagic (open ocean) organisms that cannot escape the low oxygen area" Depletion of dissolved oxygenin coastal waters via nutrient input390+ -Nutrient enrichment(nitrogen, phosphorus)Elevated primary productionAccumulation of detritusAcceleratedmicrobial (Benthic) respirationDepletion ofDissolved Oxygen (DO) in bottom watersEssay 8.1 Figure 1Process by which nutrient input leads to depletion of dissolved oxygen in bottom waters, creating a dead zone.Across large areas of the inner continental shelf affected by depleted dissolved oxrygen (DO), free-swimming animals, such as fish, shrimp, and crabs, are forced to migrate or die. Animals that cannot move elsewhere,such as mollusks, suffocate and die. Waters with less than 2 mg of DO per L are hypoxic, supporting only anaerobic bacteria and archaea species that thrive with little or no oxygen. If the oxygen concentration declineseven further, so there is no dissolved oxygen, the hypoxic zone is called an anoxic zone. Because they kill or exclude all animals, areas of hypoxic and anoxic waters are also known as a dead zones.Though hypoxic zones lack animals, they still teem with the microbial life that thrives in the oxygen-deficient waters. In the absence of oxygen, these microbial communities use nitrate to break down organic carbon.However, denitrification often releases small amounts of a powerful greenhouse gas (nitrous oxide) into the surrounding water.Since the mid-1970s, a dead zone with hypoxic bottom waters, has developed in the Gulf of Mexico every summer, spreading west of the Mississippi River delta and along the Louisiana-Texas Gulf Coast (Essay 8.1Figure 2). While it varies in area from year to year, it averages almost 15,000 km² (5800 mi.?), making it the largest in U.S. coastal waters and one of the largest worldwide. Throughout the summer, the water column isstratified with a strong vertical density gradient formed by warm, oxygenated water near the surface and cold, oxygen-depleted water at depth. The stronger winds in late fall mix the water column, replenishing thedissolved oxygen content of the bottom waters, so life can return. 3911 Mississippi River(2 Ohio RiverContinentalDivide2007 hypoxic("dead") zone3 Missouri RiverEssay 8.1 Figure 2In summer, a large volume of bottom water significantly depleted in dissolved oxygen develops in the Gulf of Mexico just offshore of the Louisiana-Texas coast, as represented by the 2007 dead zone (red on the map). Hypoxic conditionseliminate many bottom-dwelling organisms and are caused by excessive amounts of nitrogen and phosphorus fertilizer washed from agricultural land in the Mississippi River drainage basin (shown in blue). [Courtesy of the U.S.Geological Survey (USGS)]The Mississippi River and its tributaries drain a huge area (about 40%) of the coterminous United States, from the headwaters in Minnesota west to the Rocky Mountains and east to the Appalachian Mountains. Thisdrainage basin includes the Corn Belt, which is responsible for much of the runoff in the Gulf dead zone. Unless efforts to reduce nutrient loading are successful, the problem is likely to be exacerbated (creating a largerdead zone) as more land is farmed for biofuels and as world food demands increase.A 2010 Scientific Assessment of Hypoxia in U.S. Coastal Waters documented an exponential growth of seasonal dead zones in U.S. coastal waters over the past 50 years (currently a 30-fold increase). Half the 647waterways reviewed by the working group experience hypoxic conditions, including the Gulf of Mexico. The report concludes that "overall, management efforts to stem the tide of hypoxia have not made significantheadway" principally due to the steady increase in development and population growth in coastal watersheds. The report also emphasizes the “need to understand the complex underlying science of hypoxia and to predicthe range of impacts of hypoxia on ecosystems.'In order to fully understand, model, and manage the impact of hypoxia, scientists must understand their microbial communities. As they exist in nature, these communities are too complex to culture, or grow, and studyin the lab. Environmental genomics, also known as metagenomics, provides a way to capture information about the metabolic potential of indigenous microbial communities. Metagenomics enables scientists to take anenvironmental sample, in this case of hypoxic ocean water, and sample the genomes of millions to billions of microbes at the same time. The segments of genetic material are reassembled to discover the species and theirmetabolic pathways. By understanding microbial metabolism at a community level, scientists can reconstruct how carbon dioxide, sulfur compounds, and nitrates are processed and move between the ocean andatmosphere.Coastal hypoxia arising from excess nutrient input is a growing problem around the world. Once the largest in the world, the dead zone in the Black Sea, near the mouth of the Danube River, covered an area of 40,000km? (15,400 mi.?) until the Soviet Union collapsed and the price of fertilizers rose drastically. Large dead zones have developed in the northern Adriatic Sea since the 1950s, the Baltic Sea since the 1960s, and the Katteg:since the 1980s. Other locales where large rivers discharge nutrients, creating hypoxic conditions in bottom water on the continental shelf, include the eastern North Sea and the East China Sea.Every summer since 2002, large dead zones have developed off the coast of Oregon and Washington, ranking second in size to the Gulf of Mexico dead zone and third largest in the world. In this case, fertilizer is not tkey factor in causing the hypoxia. Along the Pacific Northwest, coastal upwelling occurs in summer when Ekman transport moves surface waters away from the coast, which draws up nutrient-rich waters from below thephotic zone. These coastal areas are highly productive with diverse, abundant ecosystems. However, in the middle of the North Pacific Ocean the oldest deep ocean water has been away from contact with the surface forlonger than any other water mass and the continual oxygen consumption by respiration has reduced the oxygen concentration to near zero. This pool has grown as climate change slows thermohaline circulation so the waages longer while an increase in primary production in the photic zone produces more organic carbon that descends and increases respiration. With stronger northern winds, caused by higher atmospheric temperatures,more of this pool of deep water is drawn up. When coastal upwelling now occurs in summer, it carries low oxygen water onto the continental shelf and creates the Washington and Oregon dead zones.

Water pollution is the addition of harmful substances to water, which can be chemicals or…

Answered: Explain how ‘dead zones’ such as appear…

Applications and Investigations in Earth Science (9th Edition)

9th Edition

ISBN:9780134746241

Author:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa

Publisher:Edward J. Tarbuck, Frederick K. Lutgens, Dennis G. Tasa

Chapter1: The Study Of Minerals

Section: Chapter Questions

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Explain how ‘dead zones’ such as appear off the U.S. Gulf coast occur. (See Essay 8.1)

"The ocean is teeming with life balanced between the creation and destruction of organic carbon. Like green plants on land, algae and bacteria in the surface waters of the ocean combine nutrients, water, and carbon dioxide in the presence of sunlight to fix organic carbon in the form of biomass. This organic carbon, directly or indirectly, provides food for life in the ocean and is a key part of the carbon cycle.
Excessive amounts of nutrients from agricultural and urban runoff, and treated sewage wastewater, lead to large algal blooms in many areas of the coastal ocean. As the organic carbon in the bloom sinks in the water column, photosynthesis declines or ceases but organisms continue to respire. This respiration, along with the respiration of decomposers or consumers, depletes the surrounding water of oxygen, killing benthic (bottom-dwelling) organisms and any pelagic (open ocean) organisms that cannot escape the low oxygen area"