Falling ocean oxygen levels due to rising temperatures and influence from human activities such as agrochemical use is an increasingly widespread problem. Considering that the sea floors have taken more than 1,000 years to recover from past eras of low oxygen, according to a recent University of California study, this is a serious problem.

Ocean regions with low oxygen levels have a huge impact on aquatic organisms and can even destroy entire ecosystems. Areas of extremely low oxygen, known as oxygen minimum zones or “dead zones”, are estimated to constitute 10% and rising of the world’s ocean.

This expansion has been attributed to a warming climate, which increases water temperature, changes ocean circulation, and decreases the solubility of oxygen in sea water. At the same time fertiliser and pesticide run-off from farming and other human activities leads to rising levels of nutrients such as nitrogen and phosphorous reaching the sea.

Together, these two processes speed up the release of chemicals from ocean sediments and promote algal blooms. Subsequent algal death and decay result in increased consumption of oxygen in the water. The result is that other aquatic species such as invertebrates on the seafloor and fish suffocate for lack of oxygen.

Due to circulation and runoff effects, dead zones are especially severe around large cities on the western continental coasts such as the coast of Peru, and within enclosed or semi-enclosed regions like the Baltic Sea or Gulf of Mexico.

Read more here http://theconversation.com/ocean-dead-zones-are-spreading-and-that-spells-disaster-for-fish-39668