An optics theory-based mechanistic model for Secchi disk depth (ZSD) is advanced, tested, and applied for Cayuga Lake, NY.Robust data sets supported the initiative, including for (1) ZSD, (2) multiple light attenuation metrics, most importantly the beam attenuation (c) and particulate scattering (bp) coefficients, and (3) measures of constituents responsible for contributions to bp by phytoplankton (bo) and minerogenic particles (bm).The model features two serially connected links.The first link supports predictions of bp from those for bo and bm.The second link provides predictions of ZSD based on those for bp, utilizing an earlier optical theory radiative transfer equation.Recent advancements in mechanistically strong estimates of bm, empirical estimates of bo, and more widely available bulk measurements of c and bp have enabled a transformation from a theory-based conceptual to this implementable ZSD model for lacustrine waters.The successfully tested model was applied to quantify the contributions of phytoplankton biomass, and minerogenic particle groups, such as terrigenous clay minerals and autochthonously produced calcite, to recent bp and ZSD levels and dynamics.Moreover, it has utility for integration as a submodel into larger water quality models to upgrade their predictive capabilities for ZSD.