What is an ocean dead zone?
A dead zone is a region of the ocean in which the water contains lower than normal amount of dissolved oxygen levels. We use the term hypoxia to describe waters with have oxygen levels below average but are above zero, whereas the term anoxic to define waters with zero percent dissolved water. For example, fully oxygenated waters may contain oxygen concentrations of 10 parts per million (ppm). At a concentration of 2-5 ppm, fish and other marine life have trouble breathing and will swim away towards a more oxygen rich area. This movement forces them to crowd with other local species where they are also more prone to attack by different predators. Animals that live on the sea floor cannot escape the hypoxic zone as easily and begin to die at 1.5 ppm.
what processes cause oxygen levels to decrease?
During the process called eutrophication ocean waters are supplied with an excess of nutrients that stimulate plant growth. These plants serve as a source of nutrition for microscopic organisms called phytoplankton. Plant growth therefore stimulates rapid population growths of phytoplankton known as algal blooms. These organisms produce their necessary oxygen in the daytime via photosynthesis, but the absence of sunlight overnight forces them to respire using the available surrounding oxygen. Since the fish already evacuated the area, there are not enough predators to consume the phytoplankton before it dies. These dead organisms then settle to the sea floor where oxygen is used up further during bacterial decomposition.
Natural causes include coastal upwelling in which changes in wind patterns cause deeper waters to be pushed upwards and towards the shallower shores. These deeper waters are very nutrient rich and cause the algal blooms to occur in the fish-rich shallower waters. Climate change and global warming may be contributing to an increased frequency of these wind patterns.
Second, a phenomenon called stratification can create ocean dead zones. This process occurs when fresh water from the rivers forms a separate layer on top of the denser ocean salt water. The density difference prevents mixing of the layers, so when air adds oxygen to the top layer, it is unable to travel through to the bottom saltwater layer, creating a dead zone. Again, climate change could be contributing to the negative effects as it leads to increases in river water runoff.
Natural causes include coastal upwelling in which changes in wind patterns cause deeper waters to be pushed upwards and towards the shallower shores. These deeper waters are very nutrient rich and cause the algal blooms to occur in the fish-rich shallower waters. Climate change and global warming may be contributing to an increased frequency of these wind patterns.
Second, a phenomenon called stratification can create ocean dead zones. This process occurs when fresh water from the rivers forms a separate layer on top of the denser ocean salt water. The density difference prevents mixing of the layers, so when air adds oxygen to the top layer, it is unable to travel through to the bottom saltwater layer, creating a dead zone. Again, climate change could be contributing to the negative effects as it leads to increases in river water runoff.
where are excess nutrients coming from?
Chemical nutrients, in particular nitrogen and phosphorus, seep into the soils and through to the oceans primarily from farm fertilizers but also from sewage and factory emissions. Another major source of excess nitrogen is created from burning coal, oil and natural gas.
Anything else?
As oxygen is no longer available, certain organisms can switch to using sulfides as their terminal electron receptor in respiration. As a result, hydrogen sulfide, which is lethal to most organisms, is released into the open ocean and can negatively effect animals far from the dead zone. Also, high concentrations of methane gas can adversely effect our oceans as the methane can chemically take the place of oxygen but can not be used in animal respiration. These high concentrations of methane have been found in the waters around the BP oil spill.