...
In the Gulf of Mexico, every summer, a hypoxic zone appears as a consequence of the massive amounts of nitrogen and phosphorous coming from the Mississippi River. This zone has an average size of 15,000 km2 and has been measured from 1985 to the present. It is the second world largest hypoxic zone. This zone presents a strong seasonal dependence, it is created every spring, when the Gulf receives a massive amount of nitrate flux, and factors such as rainfall, warmer temperatures, and sunlight favor algal growth. Moreover, the waters are calmer, which prevents vertical mixing of the ocean. This lack of vertical mixing means that colder, oxygen depleted waters cannot interact with the fresher, warmer waters of the surface, thus maintaining the hypoxic zone. Then, the hypoxic zone is breaks early September when hurricanes and tropical storms mix the ocean.
How can are the nitrate flux and the size of the hypoxic zone related?
...
In Figure 1, the observed spring nitrate flux from 1985 to 2017 is plotted. The 2019 and 2020 measurements are not included in the plot, but their values are within the 10 largest nitrate flux ever observed. This indicates that the excessive load of fertilizers to the ocean is a persistent problem.
In the figure, the 3 years with highest nutrient load are signalized with red arrows: 1993, 2008, and 2013. The 3 years with lowest nutrient loads are signalized with blue arrows: 2000, 2005, and 2012. In Figure 2, the same years are marked with arrows. It can be observed that although 1993, 2008, and 2013 are among the years with largest hypoxic zone areas, they do not represent the 3 largest hypoxic zones. The same is true for the years with low nutrient loads: they correspond with smaller than average hypoxic zone sizes, but they are not the smallest zones observed. These observations indicate that there are
Figure 1: May Nitrate Flux from the Mississippi River in Metric Tones.
Figure 2: Observed hypoxic zone size in kilometers squared.