|Vol. 7 (No. 4), pp. 117-145, 2014||doi:10.5047/absm.2014.00704.0117|
Field Science Education and Research Center, Kyoto University, Oiwake, Kitashirakawa, Sakyo, Kyoto 606-8502, Japan
(Received on December 20, 2013; Accepted on April 15, 2014; Online published on October 24, 2014)
Abstract: In summer dissolved oxygen is often depleted in the lower and bottom layers in many coastal basins all over the world. This phenomena is called hypoxia. When the oxygen consumption exceeds oxygen supply, the water becomes hypoxic. The oxygen is consumed by decomposing organic matter by bacteria (biochemical processes), while the oxygen is supplied by physical processes such as convection, advection and diffusion. The primary cause of hypoxia is the consumption of oxygen in the water column, but physical processes mainly control its generation, distribution and configuration. In addition to the vertical supply of oxygen by mixing, horizontal transport by estuarine circulation plays the major role in the formation of hypoxia in regions of freshwater influence. As the hypoxic water contains a lot of nutrients, it plays an important role for primary production, producing middle layer chlorophyll maximum in summer and inducing bloom of phytoplankton in autumn.
Keywords: circulation, dissolved oxygen, estuarine circulation, fortnightly shifts, hydrographic condition, hypoxia, tide
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