Hypoxia Controlled by Hydrodynamics

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. 1. General introduction Oxygen is essential for almost all marine biota, including fishes and invertebrates, to maintain their life. However, the amount of oxygen diluted in the water is limited. The saturation rate of oxygen in the water is only 5.2 mL L–1 (=7.4 mg L–1) under 1 atoms at 20°C. This concentration is significantly lower than that in the air (210 mL L–1). Therefore, marine animals develop advanced gills to take in oxygen efficiently from the seawater. In spite of the advanced organ intrinsic to marine animals, significant decrease of dissolved oxygen (DO) in the water damages them. DO concentration sometimes reduces seriously to the level which has harmful effects on marine animals especially in summer. This water mass is called hypoxia or hypoxic water. The water including nearly zero amount of oxygen is called anoxia. Oxygen depletion exerts a serious impact on marine ecosystems, although the tolerability of marine animals is different among the species. For instance, fishes such as red sea bream and yellowtail are going to die within a few days by exposure to the water with 3 mg L–1 of DO (Ishioka 1982; Yamamoto et al. 1990). In general, oxygen deficiency lower than 4 mg L–1 exerts a baneful influence upon cultured fish (Inoue 1998). On the contrary, benthic animals tend to be tolerant to low oxy118 A. Kasai / Aqua-BioSci. Monogr. 7: 117–145, 2014 doi:10.5047/absm.2014.00704.0117 © 2014 TERRAPUB, Tokyo. All rights reserved. the Seto Inland Sea, there are some regions including Osaka Bay, Harima-Nada, Hiuchi-Nada, Hiroshima Bay, Suo-Nada and Beppu Bay, where hypoxia occurs every summer (Fig. 1a). The diversity of species of macro benthic animals is low in the hypoxic regions (Fig. 1b). One of the most wide-spread hypoxia is observed off the coast of Louisiana and Texas, USA (Rabalais et al. 2002). The survey of the region shows an area of about 17,000 km2 experiencing hypoxia, which leads to large changes in bottom water marine life. This region is called the “dead zone” because of the failure to catch demersal fish and benthic animals. Not only with the objective of environments, but also fisheries are of course damaged by hypoxia, especially in enclosed euphotic bays and lakes. Demersal fishes, crabs and shellfishes are rarely observed in the bay head of Mikawa Bay in summer. Shijimi clam fisheries in Lake Shinji and Lake Ogawara, both of which are foremost Shijimi fisheries brackish lakes in Japan, are restricted in the narrow coastal areas which are shallower than 5 m depth, because the deep central areas become hypoxic in summer and shijimi clams cannot survive 3