Dead zones in the Ocean
 

In places where great quantities of organic material are discharged into the ocean, dead zones are formed. They begin to form in the estuarys of major rivers like for example, the Mississippi or the Yangtze river, where water, due to bacterial decomposition of the organic sewage, does not contain oxygen.

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Page author: Stefan Austermühle 

Sometimes, these dead zones are caused by the explosive growth of microscopic algae (bloom). This algae bloom gives a brown or reddish color to the sea water, which is why the phenomenon is called “red tide”. In Peru, with its natural upwelling of nutrients from the deep sea, red tides can have natural causes. Nevertheless, the increasing frequency of algae blooming, especially when at contaminated sites like the Bay of Paracas, cannot be explained by natural reasons, but has its origin in the discharge of contaminating organic matter.

In order to determine the level of contamination caused by organic sewage, one measures the Biologic Oxygen Demand (BOD) of the contaminated water. This is the amount of oxygen consumed by bacteria in order to decompose the contaminatic organic substance and metabolize it to CO2 and H2O
 

BOD5 is the amount of oxygen needed during the first 5 days at a water temperature of 20 °C. Urban sewage generally has a BOD5 of around 200 mg O2/liter. Industrial sewage may have BOD values of several thousands of mg O2/liter. The contaminated water for example, discharged by the fish meal production plants of Paracas does have a BOD5 of 11,000 mg O2/liter.

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Page author: Stefan Austermühle