Shift in apparent contrast of disc at Secchi disc depth in coastal sea areas.

The relationship between Secchi disc depth and amount of suspended material in seawater varies depending on the particular marine area. To identify the cause of this dependence, we calculated the apparent contrast (C (SD)) at each Secchi disc depth in different coastal sea areas. When the turbidity from the surface to the Secchi disc depth was uniform, the C (SD) was distributed in the range of 1.3 to 0.001 for a Secchi disc depth (Z (SD)) of 2-18 m. Z (SD) tended to decrease as C (SD) became larger. The dominant wavelength for the sea color was 475-500 nm for a Secchi disc depth of 13-18 m, and 500-575 nm for a Z(SD) of 2-6 m, shifting to longer wavelengths as the Secchi disc depth increased. That is, when Z (SD) decreased, the dominant wavelength of the sea color, and the C (SD) increased simultaneously. This phenomenon seems to occur because the contrast threshold for the human eye is higher at longer wavelengths. In other words, the contrast threshold is visibly indistinguishable when the apparent contrast in ocean waters with low Secchi disc depths is high. This phenomenon occurs because the human eye is affected by the color of the sea.