Water Clarity (Secchi Depth): Closure between Theory and Century-old Measurements

Water clarity is a first-order indicator of the health of an aquatic ecosystem, which is commonly represented by the Secchi disk depth (Z_SD), a practice still followed since its invention in 1860’s. There are roughly a million of Z_SD data measured in various locations of the global water ways, lakes, and the oceans in the past ~150 years, along with numerous measurements of the diffuse attenuation coefficient (K_PAR) and chlorophyll concentration (Chl) since 1900’s. General, and consistent, relationships were found for Z_SD vs K_PAR and for Z_SD vs Chl from these wide range of measurements. However, it has been a puzzle in the past 60+ years that these relationships could not be explained by the classical Secchi depth theory, thus there is no closure between theory and data for this extremely popular field observation. Recently Lee et al. (2015) put forth an innovative theory regarding Secchi disk observation and developed a new theoretical model to illustrate the relationship between Z_SD and light attenuation in water. This model is evaluated with numerical simulations and historical observations and found that the century-old relationships from field observations can be perfectly explained with this new model. Therefore, a closure between theory and data is now achieved, which further lays a solid ground to remotely sense Z_SD from satellite ocean color measurements. In addition, it opens the door to expand the utilization of Z_SD for the estimation vertical profile of solar radiation in the upper water column and the derivation of inherent optical properties.