2017 Volume 64 Issue 1 Pages 29-34
Phytoplankton is a potentially important component for attenuation of light in seawater, together with seawater itself, colored dissolved organic matter, and non-algal particles. In coastal areas, relative contribution of each optically active component, including phytoplankton, to total light attenuation differs largely among the areas due to wide variety of the water quality. In this study, we examined attenuation coefficient of downwelling photosynthetically active radiation (Kd) during a three-year period at the shallow cove, Shido Bay, eastern Seto Inland Sea in Japan. In parallel, we also measured proxies for abundance of the optically active components in the water column. The objective of this study was to reveal how phytoplankton contributes to light attenuation in this shallow cove, where light reaches the sea bottom. Kd through the water column varied seasonally between 0.235 and 0.838 m-1 (mean±S.D.＝0.419±0.134 m-1) with 0.7–24％ of the surface irradiance reaching on the seafloor (6.0 m beneath the sea surface). Light attenuation due to phytoplankton (chlorophyll-a; Chl-a) only accounted for approximately 20％ of total light attenuation (Kd) on average, suggesting that phytoplankton is not a major shading component in the averaged estimation. Meanwhile, because of the high coefficient of variation, not only non-algal particles, but also phytoplankton (Chl-a) contributed largely to the temporal variability in Kd. In the Seto Inland Sea, including its shallow areas such as Shido Bay, Secchi disk depth has increased gradually during the last three decades. Moreover, the biomass of seaweeds tends to increase recently, probably in response to the improvement of light environments in the overlying seawater. We suggest that even though phytoplankton is a relatively minor as a shading component, recent changes in underwater light climate in the Seto Inland Sea may be significantly related with changes in the phytoplankton biomass in relation to the reverse trend of eutrophication. If so, the abundance of benthic plants in the shallow area of the Seto Inland Sea should be indirectly controlled by phytoplankton through light shading.
