Algal blooms are one of the most common processes in the marine environment – they occur whenever nutrient levels in the water get too high and small photosynthetic algae multiply uncontrollably until the nutrient supply is exhausted. This process is called eutrophication, and while it may seem innocuous – algae are small, after all, how could they really affect the marine environment? Well, these seemingly harmless creatures can end up creating “dead zones”, huge areas of barren seascape primarily found at the outflows of freshwater rivers.
The way in which this happens starts with the first blush of the algae bloom. As the tiny protists multiply, they cloud the water until sunlight can barely penetrate, interfering with the photosynthesis of the marine plants below, which kills them and lowers the amount of oxygen available in the water. The problems are only exacerbated after the nutrient supply runs out and the algae begin to die off – as their billions of tiny bodies decompose, they take up yet more oxygen and plunge the area into a state of hypoxia (low dissolved oxygen, 3-5 mg/L) or worse, anoxia (no dissolved oxygen at all). Fish and other motile organisms flee the area, and benthic organisms that are sessile (immobile) cannot move to more oxygen-rich areas and thus suffocate. This creates a massive area devoid of all life that cannot be repopulated unless oxygen levels rebound.
These dead zones are found throughout the world wherever rivers empty into the ocean, most notably in the Gulf of Mexico, the Baltic Sea, and the southeastern Australian coast. They are caused primarily by a sharp increase in the demand for agricultural products. This demand triggers more usage of fertilizers, which ultimately start the cycle of algal growth and death. Algae blooms are a huge marine problem, decimating large swaths of the ocean annually, and they are almost cocmpletely anthropogenic – caused by our own human greed or carelessness.
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