The recent rapid eutrophication of the coastal seas of Japan has had a remarkable effect on the turbidity and transparency of the sea water, hence on the attenuation of underwater irradiance, which in turn influences the phytoplankton communities and primary productivity of the area.

The present study deals with the continuous three years observation of the total short-wave radiation, direct short-wave radiation, diffused short-wave radiation and photosynthetically active radiation of tlle Seto Inland Sea. Along with these observations, reflected short-wave radiation from the sea and transmitted short-wave radiation into the sea were determined. The availability of solar radiation for primary production, vertical distribution of spectral irradiance and attenuation coefficient were also discussed in relation to the optical water type of the region.

1. A typical seasonal variation in the monthly mean daily solar radiation (total short-wave radiation) was observed, with a maximal value of 17.0 MJ・m⁻² in July and minimal values of 7.4 to 7.5 MJ・m⁻² through November to January.

2. Seasonal variation of direct short-wave radiation was nearly identical to that of total short-wave radiation, with 9.3 MJ・m⁻² at maximum and 4.1MJ・m⁻² at minimum. Diffused short-wave radiation increased in June and decreased in January. The ratio of diffused short-wave radiation to total short-wave radiation ranged from 394000 62% with an average of 49%.

3. Seasonal variation of photosynthetically active radiation was very similar to that of direct short-wave radiation, with values of 7.3 MJ・m⁻² in July and of 3.3 MJ・m⁻² in December.

4. The albedo at the sea surface changed according to the incidence angle and surface conditions. Average daily values ranged from 2.9% on a fine summer day to 10% on an overcasted day in winter.

5. Underwater irradiance at a depth of 50cm varied widely according to such parameters as turbidity and the surface condition of the water. Observation revealed a remarkable decrease of irradiance in the upmost 50cm of the water column showing an inverse relationship between the intensity of underwater irradiance and turbidity.

6. From data on the vertical distribution of underwater irradiance, the attenuation coefficient for lux was calculated at 0.53 to 0.70, while that for quanta at 0.37 to 0.52.

7. As a result of spectral irradiance determination, a characteristic decrease in the longwave range was observed, especially in polluted areas. The optical water type of the region, as proposed by Jerlov, was classified into the Type 3 to Type 7 class for coastal waters.

8. From the simultaneous determination of net primary production by the phytoplankton community, average availability of total short-wave radiation and that of photosynthetically active radiation for primary production were estimated to be only 0.11% and 0.25%, respectively.