Abstract
Areal surveys were conducted in August 1995, February 1996 and July 1996 in the vicinity of the planned tidal flat at the Port of Sakai-Semboku, Osaka Bay to clarify characteristics of the seawater and bottom sediment qualities and occurrence of benthic animals. Laboratory experiments were also carried out to examine the response of a benthic fish to hypoxia, using young marbled sole Pleuronectes yokohamae which has lower tolerance to hypoxia among benthic animals in the study area. Hypoxia and/or anoxia developed in summer at the lower seawater layer in the port, and this led to an increase in sulfide content and a decrease in the oxidation-reduction potential of the bottom sediment. As a result of such environmental deterioration, the number of benthic animal species was reduced (macrobenthos to less than 11 species ; megabenthos to less than 8 species) and no benthic animals were found at the innermost part of the port. In contrast, improvements of dissolved oxygen saturation and bottom sediment quality, together with the restoration of benthic fauna were observed in winter. Concentrations of total sulfide and pheophytin of the bottom sediment tended to increase with increasing depth of water. In addition, the total organic carbon and total nitrogen concentrations of the sediment showed the same trend as total sulfide in the areas shallower than ca. 12m. Both numbers of species and individuals of macrobenthic animals tended to increase with decreasing depth of water, except for shallow bottoms in the mouth of the Yamato River and Dejima Fishing Port. Laboratory experiments revealed that the mortality rate of young marbled sole was 0%, 50% and 100% when exposed at 22.4-24.3°C for 24 hrs to oxygen saturations of 30%, 25% and 19%, respectively. Breathing frequency of the young marbled sole increased at 40-50% saturation, compared to 92-100% oxygen sufficient seawater. Based on the results of field surveys and laboratory experiments, we concluded that dissolved oxygen should not drop below 30% for longer than 1 day and be kept higher than 50% on average in summer to maintain sound benthic populations in the study area.
Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
