Optically active components and light attenuation in an offshore station of Harima Sound, eastern Seto Inland Sea, Japan

Abstract

In the eastern Seto Inland Sea, Japan, phytoplankton abundance in the surface water has gradually declined, whereas Secchi depth has risen in recent years, particularly in offshore areas. Therefore, it may be hypothesized that phytoplankton dominate light attenuation in the offshore area, and that other constituents are less important. To test this hypothesis, we examined the roles of seawater, colored dissolved organic matter (CDOM), non-algal particles (tripton), and phytoplankton in the light attenuation at an offshore station of Harima Sound in the eastern Sea. The magnitude of light attenuation was then determined from the attenuation coefficient of photosynthetically available radiation (PAR) through the water column (K d). During a 13-month period, K d ranged from 0.179 to 0.507 m−1, with a mean of 0.262 m−1. The mean relative contributions of seawater (15%) and CDOM (13%) to K d were small, while the most dominant K d constituent was tripton (45%). The mean contribution of phytoplankton to K d (27%) was consequently less than that of tripton. However, 75% of the temporal variability in K d was attributed to phytoplankton, measured as chlorophyll a. Our results emphasize that the main component of light attenuation does not always govern the temporal variation of light attenuation in coastal regions.