Ocean dead zones are defined by NOAA (National Oceanic and Atmospheric Administration) as “areas in the ocean of such low oxygen concentration that animal life suffocates and dies” (learn more here). This is because there are too many nutrients, mainly nitrogen and phosphorus, in the water. These nutrients cause the eutrophication (excessive richness of nutrients) of waterways, feeding algae spores and leading to an overgrowth of algae. This blocks sunlight to the aquatic plants below, killing them. This then prevents the production of new oxygen. After a period of time, the algae sink and decompose in the water. This process consumes oxygen, depleting the supply available to healthy marine life remaining and causing anoxia. Sometimes, this may also create harmful algae tides which can sicken or even kill humans and animals.
Dead zones are currently a prevalent issue as news suggests that they will only continue to grow, with more than 400 dead zones worldwide (read more). This threatens our ability to continue to use the ocean as a resource. 2.6 million people rely on the ocean for at least 20% of their protein. Thus, life in the sea is not something we can afford to lose. One of the worst parts is that, even though some dead zones do occur naturally, a lot of them are a direct result of human actions. This includes contributors such as fertilizer run-off from crops which add nitrogen to the seawater.
In December last year, the discovery of a new dead zone in the Bay of Bengal was announced. At the time it measured approximately 60,000 km², occupying depths of between 100m and 400m. It only contained trace amounts of oxygen, enough to keep microbial communities from removing nitrogen. Without the removal of this nitrogen, it is likely that the dead zone will only continue to grow. In the summer of last year, the Gulf of Mexico dead zone (perhaps the most famous) was predicted to average about 5,898 square miles (15,276 km²). That’s roughly 1,000 km² larger than Wales. This is not an issue that we can sweep under the rug with the largest dead zone located in the Baltic sea. In other words, right on our doorstep.
So, what can we do? Well, the solution which would be the most effective is to stop using artificial fertilisers. However, this is unlikely to happen due to farmers’ dependency on them. Therefore, a more practical solution is to use less fertiliser or, at least, only as much as the soil can take. Using natural fertilisers such as manure may also be a good idea, along with increasing crop biodiversity on land which was found to decrease watershed nitrate levels in a Louisiana State University study. Protecting coastal wetlands is also a plausible solution as they would help filter the excess nutrients before they reach the ocean. Thus, reducing the chances of a dead zone occurring in the area.
(figure 1: Image of known dead zones from https://nofishleft.wordpress.com/tag/marine-dead-zones/ )
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