Solved by verified expert:learned about how excess nutrient supply to surface waters due to fertilizer runoff can cause Dead Zones is coastal waters. Basically, the excess nutrients encourage the growth of phytoplankton near the surface. When these phytoplankton die, their remains sink to the bottom and the organic compounds in them are oxidized, consuming the dissolved oxygen in the water. Without oxygen, life cannot be sustained, and ultimately the food web collapses. So ultimately the fertilizers that were initially used to increase food production on land end up destroying offshore fisheries that are critical for maintaining world food supplies. This is yet another of the difficult quandaries we’ve considered this semester.What is your take on the trade-off between the need to apply chemical fertilizers in the upper mid-western US vs. the deteriorating seafood catch in the Gulf of Mexico? You could argue this from an economic standpoint, or environmental justice point-of-view, or any other aspect that you think is important. As always, you are expected to use one or more references to information from the module or assigned reading to support your argument.use the following attached document to answer the question, no need to look up other sources. Also doesn’t need to be a really long, drawn out answer. Just a couple paragraphs or so.
dead_zones.docx
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A major issue in pollution of surface waters is the role that excess nutrient flows from polluted waterways
into lakes, bays, and coastal zones play in creating excess biologic production in surface waters and
dissolved oxygen at depth. In most cases, this nutrient-rich runoff results from agricultural operations,
including the application of fertilizer to crops. Of course, such issues have already been briefly highlighted
for the Chesapeake Bay in Module 1, but such so-called “Dead Zones” are globally widespread. It is,
perhaps, easier to understand impacts on more restricted bodies of water (lakes, bays) with high fluxes of
water from nutrient-laden rivers (such as the Chesapeake Bay setting). But, such issues also plague some
coastal zones characterized by high river discharges. For example, the Gulf Coast “dead zone” has been
recognized for over a decade and is attributed to high rates of nitrogen (and phosphorus) discharge
through the Mississippi River system. During summer, 2014, this area of hypoxia (less than 2 ppm
dissolved oxygen in the water column near the bottom on the shelf) along the Louisiana and Texas coast
was just over 13,000 km2 (>5000 mi2), somewhat smaller than that in 2013. Figure 6 illustrates the extent
and severity of oxygen deficiencies during mid-summer, 2013. Coastal currents flowing westward mix and
transport nutrients flowing from the Atchafalaya and Mississippi Rivers into the ocean.
Figure 6. Contours of dissolved oxygen near the bottom on the Louisiana and Florida shelves,
June 7-July 19, 2013. Note the predominance of orange colors nearer shore in coastal Louisiana
indicating widespread hypoxia there.
Source: NOAA(link is external)
But how do high nutrient fluxes promote oxygen deficiency in coastal regions? The availability of
nutrients in shallow sunlit waters near the coast allows prolific blooms of marine plankton (primary
photosynthesis) which produces large amounts of organic matter. Nutrients can be a good thing and can
benefit the entire food chain unless the fluxes of N and P reach an extreme termed “eutrophic” conditions.
As the organic matter sinks to the bottom, it is a food source for consumer organisms (both in the water
column and on the bottom), including bacteria. Shrimp, bivalve, and fish catches can increase to a point.
In the extreme, the metabolism of fish, bivalves, bacteria and other critters consumes available dissolved
oxygen in the water column faster than it can be replenished by mixing from above or laterally by
currents. Also, because the coastal waters are warming during summer, they can hold less dissolved
oxygen initially. As long as high nutrient fluxes continue the hypoxia expands and the organisms that
depend on oxygen to survive either flee, if they can swim, or die if they are more sedentary.
Observations over a number of years indicate that the extent of hypoxia can wax and wane from year to
year. In 2012, Louisiana coastal hypoxia was much less extensive and less intense (Fig. 7, contrast with
Fig. 6). As you may recall, 2012 was a severe drought year in the mid-continent U.S. The flow of the
Mississippi River system was much reduced, and nutrient fluxes decreased commensurately.
Figure 7. Contours of dissolved oxygen near the bottom on the Louisiana and Florida shelves,
June 7-July 15, 2012. Note the predominance of orange colors nearer shore in coastal Louisiana
indicating widespread hypoxia there.
Source: NOAA(link is external)
Figure 8. Area of northern Gulf of Mexico mid-summer bottom -water hypoxia 1985-2013 (data
from N. Rabalais, Louisiana Universities Coastal Consortium). Note the smaller areal extent in
2012.
Source: US EPA, 2014(link is external)
Previous research established a connection between runoff from agricultural operations in the midcontinent region into the Mississippi River drainage and development of hypoxia. Wet years (Fig. 9
correspond to higher flow rates for the Mississippi River and greater delivery of dissolved nitrogen to the
coastal region. Note that 1987-89 were years of low nitrate flux (Fig. 9), which correspond to low area of
Gulf of Mexico hypoxia (Fig. 8)
Figure 9. Nitrate flux from the Mississippi River scaled to Mississippi River flow rates (right y-axis
in millions of cubic meters/y). Overall, there is a correlation between the two factors, particularly
after about 1970. This study found a strong correlation between nitrate flux to the Gulf of Mexico,
annual discharge to the Gulf, and fertilizer application over the entire drainage basin during the
previous two years (r2=0.89).
Source: From Goolsby and Battaglin, 2000, USGS Fact Sheet 135-00
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