Water Quality in Osaka Bay Based on Simultaneous Water Quality Survey

概要

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A simultaneous water quality survey in Osaka Bay has been conducted in cooperation with private sectors, universities and governments every summer since 2004. The survey is characterized by almost the same sampling date and sampling depth and by widespread sampling from the shore to the bay mouth. In this report, we describe an overview of the survey and investigate the relationships between climate conditions and the distributions of bottom dissolved oxygen (DO) and surface chemical oxygen demand (COD) as well as the distributions of nitrogen and phosphorus from the shore to the bay mouth. Bottom DO and surface COD, which are indexes of water quality targets in the Action Plan to Revive Osaka Bay, do not reach their target values in the inner bay. These distributions vary according to climate conditions. The dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) concentrations at the upper layer, which are 0.34 mg·L-1 and 0.032 mg·L-1 in the inner bay, decrease to 0.03 mg·L-1 and 0.008 mg·L-1 in the bay mouth, respectively.