Abstract
Hypoxia is a serious environmental problem in most enclosed coastal seas. The duration of hypoxia must be known to evaluate the impact of hypoxia on the benthos or surrounding environment. In Osaka Bay, continuous dissolved oxygen (DO) data have been available for several observation points on the website of the Kinki Regional Development Bureau, Ministry of Land, Infrastructure, Transport and Tourism since April, 2010. Using these hourly data, we have calculated the monthly average, the standard deviation of DO, and the durations of hypoxia and anoxia. In summer, the daily variation (daily maximum - daily minimum) of bottom DO values exceeded 1 mg L-1 at the observation points in the coastal area. The monthly standard deviation of bottom DO values is also over 1 mg L-1. Strong wind caused intermittent DO rises in the southeastern part of Osaka Bay. As a result, hypoxia with a duration shorter than 24 hours occupied half the total period of hypoxia in one month in the southeastern part of Osaka Bay in summer, except August, when hypoxia is most severe. On the other hand, the duration of hypoxia in the northern part of Osaka Bay is longer than that in the southeastern part of Osaka Bay.
