Contaminant Example 2: "Dead Zones" and Excess Nutrient Runoff

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.

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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

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.

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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

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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

Previous research established a connection between runoff from agricultural operations in the mid-continent 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)

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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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Figure 10. An estimated 5.5 million metric tons of nitrogen fertilizer were applied to croplands in the Mississippi River Basin during 1991.
Source: From Goolsby and Pereira, 1995; USGS Circular 1133

It is also clear from Figure 10 that very high rates of fertilizer application characterize the Mississippi River Basin. Think back to the section called Contaminant Example: Arsenic in Groundwater when you examined nitrate concentration variation in Iowa streams at present. It should be apparent that fertilizer applications and runoff are the main culprits in hypoxia in the Gulf of Mexico.

• Formative Assessment 2: Dead Zones