The Optical Environment under the Sea in the Head of Osaka Bay Based on a Long-term Field Observation

Abstract

An optical environment under the sea is important for designing environmental restoration. Irradiance under the sea is affected by SS(Suspended Substance) which is variable spatially and temporally, so it is not so easy to understand the optical environment. Therefore, it is important to investigate actual phenomena in the field. This study describes a decay of incident rays of sunlight with water depth based on the observation in the head of Osaka Bay. The decay of incident rays defined as the absorption coefficient, k, was considered. In the result, monthly averaged value of k was between 0.94m^-1 and 1.15m^-1 in the summer and between 0.46 m^-1 and 0.59 m^-1 in the winter. Moreover, a compensation depth or a compensation point based on k was examined and it suggested that the photon flux density was insufficient for seaweed (or seagrass) on the sea bottom in the head of Osaka Bay.