Distribution of region-specific background Secchi depth in Tokyo Bay and Ise Bay, Japan

Region-specific background Secchi depth (BSD) provides valuable information on light availability in aquatic ecosystems. We estimated BSD in two eutrophic bays located in central Japan, Tokyo Bay and Ise Bay. Estimates were based on monitoring data collected in the period 1981-2015. Reliable BSD estimates were obtained for 89-96% and 67-94% of the monitoring sites in Tokyo Bay and Ise Bay, respectively. Low BSD values were obtained in the innermost sectors of Tokyo Bay (around the Nakagawa, Arakawa and Sumidagawa estuaries) and Ise Bay (around the Shonaigawa, Kisogawa and Ibigawa estuaries). BSD was positively correlated with salinity in both bays, indicating that river-supplied substances, including tripton and/or colored dissolved organic matter, strongly influenced BSD values. Although the highest chlorophyll a concentrations were measured in the innermost sectors of both bays, the proportional contribution of phytoplankton to light attenuation was surprisingly low in comparison with other sectors of the two water bodies. In both bays, phytoplankton accounted for < 40% of light attenuation through the water column, indicating that background factors played a dominant role even in these highly eutrophic waters. Determinations of Secchi depth, BSD, and the proportional contributions of phytoplankton and background factors to light attenuation will improve understanding of the aquatic light environment, which in turn will inform the development of rational coastal management practices.