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Hypoxic (dead zones)

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Hypoxia in aquatic systems refers to low levels (below 2mg/L) of dissolved oxygen in the water. In such environments most aquatic animals or plants will leave the area, become stressed or die. This problem is quite severe in the northern Gulf of Mexico where the Mississippi river drains into the ocean. The available scientific evidence points to eutrophication as the main culprit in creation of dead zones.

Eutrophication occurs when organic matter, usually algae and phytoplanktons, increase in the aquatic system in response to high levels of available nutrients.  When the overabundance of algae die they are decomposed by microorganisms, a process which consumes oxygen and leaves little available for other aquatic life. Biodiversity drops and with it the resilience of the entire system, leaving it vulnerable to other compounding factors. For example, algae blooms on top of the water cut off sunlight to aquatic plants at the bottom, preventing them from releasing oxygen as part of photosynthesis. As these plants die the aquatic floor becomes more easily disturbed by currents, further clouding the water and preventing sunlight or even covering delicate organisms like coral. Ultimately the particulate matter and oxygen content change water density and create stratified layers that fail to mix, preventing the system from correcting the imbalance until major current changes occur. This is why we see major algae blooms begin with the spring runoff, grow through summer and finally break up with fall and winter storms only to begin again with the next spring runoff.

So where do the excess nutrients come from? Nitrogen and phosphorus are usually the product of fertilizer runoff from human activities like farming and landscaping. Most rivers, the Mississippi included, cut through heavily agricultural regions. For more analysis of how dead zones occur and how to prevent them see the writings below.

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