Abstract
Various empirical relationships have been developed in the past nine decades to link the Secchi‐disk depth ( Z SD ) with the diffuse attenuation coefficient ( K PAR ), the euphotic zone depth ( Zeu ), and chlorophyll (Chl) concentration, where the latter two are important for the quantification and evaluation of photosynthesis in aquatic environments. There was also a classical theory regarding Secchi‐disk sighting, but large gaps existed between the observations and model outcomes. These gaps have puzzled the ocean community for > 60 yr and have resulted in contradictory conclusions regarding the interpretation and usefulness of Z SD . Here, we compare these measurements with data simulated based on an innovative theory and model regarding Z SD , and we found remarkable agreements between theoretical predictions and these century‐long observations. The results not only resolve the long‐standing puzzles associated with these observations, but also unify the relationships published in the literature. In particular, the ratio of Zeu to Z SD is found to be ∼ 3.5 for all waters, which is ∼ 45% greater than the consensus value of ∼ 2.4 suggested in the past for clear waters. In addition, the new model validates an empirical relationship between Z SD and Chl developed for global oceanic waters, thus providing strong support for using historical Z SD data to study changes of phytoplankton in global oceans in the past century.