I've talked about coral reefs being the rainforest of the ocean, an oasis so to speak. That raised a question: Is there such a thing as a desert in the ocean? A place where you won't find a great abundance of life?
And the answer is: yes, such places exist. They are known as dead zones. Sounds dangerous. A dead zone is an area in the ocean that is oxygen-starved (Editors of e, 2008) and they are therefore also called hypoxic zones (hypo = under, oxic = with oxygen). If oxygen levels in an area drop below 2 mg/l, it qualifies as a hypoxic zone (Rabalais et al., 2002).
Dead zones occur naturally in deeper parts of the eastern Paciļ¬c Ocean, in the Arabian Sea, and off West Africa (Levin et al., 2001), and some of these parts are permanently hypoxic or anoxic (no oxygen at all). In deep water dead zones there is not as much diversity of species compared to deep oxygenated waters. You need a very specific set of traits to be able to survive in oxygen depleted zones. But then again...the animals that are able to live there tend to show up in massive amounts, so there are still a lot of creatures to be seen down there (mainly worms, crabs, star fish, some shellfish) (Levin et al., 2001).
And then there are the dead zones that occur in shallow, coastal waters. The biggest ones are found in the Gulf of Mexico, the Black Sea and the East China Sea. The dead zone in the Gulf of Mexico is being studied the most, and a lot of the numbers I've used here come from research in that area.
The number of coastal areas where hypoxia occurs is increasing and seems to be driven by human activities (Rabalais et al., 2002). Hypoxia in shallow waters is caused by an overload of nutrients in the water, mainly nutrients such as nitrogen and phosphorus. These nutrients come from fertilizers used for agriculture, effluents of sewage systems, emissions of factories and cars, and they get flushed into river systems, eventually ending up in the ocean.
The number of coastal areas where hypoxia occurs is increasing and seems to be driven by human activities (Rabalais et al., 2002). Hypoxia in shallow waters is caused by an overload of nutrients in the water, mainly nutrients such as nitrogen and phosphorus. These nutrients come from fertilizers used for agriculture, effluents of sewage systems, emissions of factories and cars, and they get flushed into river systems, eventually ending up in the ocean.
What happens next? There are 2 things required for hypoxia to occur (Rabalais et al., 2002). First of all, the water column must have different layers of water (stratified), so that the bottom layer is separate from the top layer. These layers are formed by differences in water temperature and salinity; cooler, saltier water = heavier, and forms bottom layer. If these layers have formed there is no (or little) transport of oxygen from one layer to the other. The second factor is the breakdown of organic material. The high amount of nutrients in the water causes a massive increase in phytoplankton (tiny microscopic plant like organisms), which results in more organic matter reaching the bottom sediments. There, the organic matter will be broken down by bacteria using oxygen (aerobic bacteria), leaving the water above the sediment depleted of oxygen. Decomposition of the organic matter also leads to a growth in microbial organisms, increasing the amount of oxygen needed to support the organisms (Diaz & Rosenberg, 2008). And this happens in an environment where oxygen is already scarce...
These steps are the first phase of hypoxia. In the second phase, the oxygen levels keep dropping, causing the mass mortality of benthic (on/in the bottom living) animals. These are mainly fish, crabs and oysters that need about 3 mg/l oxygen. Worms and clams in the sand only need oxygen concentrations of 1 mg/l, so they are still okay with living in most dead zones. Sometimes, even animals that can swim away won't make it. And then you might see horrible death scenes of thousands of dead bodies washed upon the beach, or floating at the surface. It's not a pretty sight, and I can only imagine the smell that must come with so many dead fish...The third phase is characterized by hypoxia becoming a seasonal event. Very often hypoxia occurs in summer, when the temperatures are higher and there is explosive growth of phytoplankton (increasing the amount of organic matter, which leads to more decomposition...etc. phase 1 all over again). If the hypoxia sustains for years, the fourth phase can be entered where the dead zone can expand and the amount of oxygen will decrease even more, leading to an anoxic (without any oxygen) area (Diaz & Rosenberg, 2008).
All these changes, deaths, and animals leaving the scene to find a better place to live, will eventually cause a change in the ecosystem structure and functioning. And that in turn will affect the way we can use these ecosystems. A lot of the dead zones we know are important economical fishing grounds. So if the animals disappear from there, or are in a bad condition because of the water quality, that for sure will affect fisheries, resulting in less fish (or bad fish) on your plate.
I always like to finish on a note of 'think about what you're doing'. This is a big problem. We can't solve it over night, but maybe just stop and think about it for a second when you fertilize you garden, or step in your car (for a 5 minutre drive to the supermarket?), or flush something chemical down the drain. Think about where it ends up, and what it does to the oceans.
I know this was kind of a tough post, oxygen levels aren't the most exciting thing to write about...but I definitely learned new stuff writing this, and I can only hope I got the information across without boring you to death!
I always like to finish on a note of 'think about what you're doing'. This is a big problem. We can't solve it over night, but maybe just stop and think about it for a second when you fertilize you garden, or step in your car (for a 5 minutre drive to the supermarket?), or flush something chemical down the drain. Think about where it ends up, and what it does to the oceans.
I know this was kind of a tough post, oxygen levels aren't the most exciting thing to write about...but I definitely learned new stuff writing this, and I can only hope I got the information across without boring you to death!
References:
Chesapeake Bay Program (2012) Dissolved oxygen. Retrieved from: http://www.chesapeakebay.net/discover/bayecosystem/dissolvedoxygen
Diaz, R.J. and Rosenberg, R. (2008) Spreading dead zones and consequences for marine ecosystems. Science, 321, 926-929
Editors of e - The Environmental Magazine (2008) Deserts in the ocean. Retrieved from: http://www.popsci.com/editors-e-environmental-magazine/article/2008-10/deserts-ocean.
Levin, L.A., Etter, R.J., Rex, M.A., Gooday, A.J., Smith, C.R., Pineda, J., Stuart, C.T., Hessler, R.R. and Pawson, D. (2001) Environmental influences on regional deep-sea species diversity. Annual review of Ecology, Evolution, and Systematics, 32, 51-93.
Rabalais, N.N., Turner, R.E. and Wiseman Jr, W.J. (2002) Gulf of Mexico hypoxia, a.k.a."The Dead Zone". Annual review of Ecology, Evolution, and Systematics, 33, 235-263.
Links:
Top picture: http://www.my-walls.net/wp-content/uploads/2012/10/fish-underwater-desert-balloon-desert-fish-lifebuoy.jpg
Map: https://nofishleft.files.wordpress.com/2010/11/coastal-dead-zones.jpg
Third picture: http://ian.umces.edu/ecocheck/forecast/chesapeake-bay/2010/methods/#_DO_-_anoxia
Dead starfish: http://www.csgc.ucsd.edu/NEWSROOM/NEWSRELEASES/2012/documents/Sonomaalgalbloom.htm
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