What are Hypoxic Zones?
Hypoxic zones in the ocean, also known as "dead zones" are areas of the ocean, at around 100m depth, which have oxygen concentrations below 2ml of oxygen per liter. This is, they are basically depleted from oxygen. These zones can occur naturally or be created or enhanced by human activity.
How are hypoxic zones created?
An excess in the nutrients influx to the ocean, for example, from agriculture fertilizers, leads to a process called eutrophication. Which is an increasing rate of primary production and organic carbon accumulation in excess of what an ecosystem is normally adapted to processing. The ecosystem responds to this nutrient enrichment by inducing an excessive growth of algae and vegetation in coastal areas. This excess causes an increased flux of organic matter to the sea bed. When this organic matter decomposes it consumes oxygen, creating a dead zone.
Organisms living in the ocean are very sensitive to changes in oxygen concentration, and thus can be greatly impacted by small changes of it. Oxygen is not very soluble in water: for Fresh water at 20C only 9.1 mg of Oxygen dissolve in 1 liter. Therefore, a change of 1mg of oxygen per liter implies a 10% difference on the overall ocean oxygen concentration. Moreover, oxygen solubility is very sensitive to changes in temperature and salinity, and thus it can be easily impacted by climate change (Benson and Krause 1984). The effects of hypoxia on marine life became critical around the 80’s when large areas of low dissolved oxygen started to appear with associated mass mortalities of fishes.
Hypoxic zones can occur naturally, for example, coastal upwelling zones along the western side of the continents are highly productive, thus respiration of fluxes of organic matter can produce severe hypoxia. There are several persistent oxygen minimum zones created by natural conditions, such as the one in the Pacific Ocean. The most important difference between natural and human induced hypoxia, is that in natural hypoxic zones the marine life has adapted to the low oxygen conditions, whereas in human created dead zones most of the marine life of the area cannot survive.
The Gulf of Mexico Hypoxic Zone
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 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.