A New Algorithm to Estimate Diffuse Attenuation Coefficient from Secchi Disk Depth.

The vertical diffuse attenuation coefficient K d ( PAR ) is used for calculating the euphotic zone, the first optical depth that is important for primary productivity models. Currently, K d   (PAR) can be estimated using an irradiometer or a Secchi disk ( SD ). The main objective of this work is to define a model that can be applied to a wide range of optical marine conditions to estimate K d   (PAR) by SD . We used irradiance profiles and SD depth ( Z S D ) from 679 stations in various marine regions. Three parametric models were developed, and their statistical performance was evaluated in view of previous approaches reported and remote sensing data. The best results were obtained with an adaptive model representing three cases: clear-water, turbid-water, and a transition zone (R² = 0.965, MAE = 0.083, RMSD = 0.239, BIAS = 0.01, and MPI = 0.854). Previous models considering a single optical depth figure at which the S D disappears did not capture the marine optical complexity. Our classification of 113 stations with spectral absorption data into Jerlov water types indicated that no unique correspondence existed between estimated K d (PAR) and water type, making it ambiguous to associate compatible inherent optical properties and chlorophyll with Z S D . Although obtaining K d ( PAR ) from Z S D is simple/low-cost, care should be taken in the methodology used to measure Z S D to ensure consistent results across different optical marine conditions. [ABSTRACT FROM AUTHOR]