Ed(490), according to the definition of diffuse attenuation (Xing et al., 2011).
The influence of CDOM on the diffuse attenuation coefficient is considerably more pronounced at 412 nm than at 490 nm, owing to the exponential increase in its absorption toward shorter wavelengths; therefore the Kd(412) was better adapted to retrieve information on the CDOM content. Based on bio-optical relation- ships, a new empirical equation was established as follows:
K d ð412Þ¼ 0:01 þ 0:0676½Chla 0:686 þ 1:3ay ð412Þ ð4Þ
Here, the three terms on the right side of Eq. (4) represent the respective contributions of pure seawater, particles and CDOM to the attenuation coefficient Kd(412). From Eq. (4), once [Chla] has been retrieved (through Eq. (3)) and deriving Kd(412) from Ed(412), the CDOM absorption ay(412) can be subsequently derived. Thereafter, the corresponding coefficients (SlopeY and OffsetY) were determined via linear regression analysis between radiometry-based ay(412) and concurrently observed FluoY profiles (Xing et al., 2012).