Resolving the long‐standing puzzles about the observed Secchi depth relationships

Abstract

Various empirical relationships have been developed in the past nine decades to link the Secchi‐disk depth (ZSD) with the diffuse attenuation coefficient (KPAR), 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 ZSD. Here, we compare these measurements with data simulated based on an innovative theory and model regarding ZSD, 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 ZSD 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 ZSD and Chl developed for global oceanic waters, thus providing strong support for using historical ZSD data to study changes of phytoplankton in global oceans in the past century.

Centennial decline in North Sea water clarity causes strong delay in phytoplankton bloom timing

“…Central to our analysis is the relationship between Secchi disk depth ( S , m) and optical properties as recently described by Lee, Shang, Du, and Wei () and Lee et al (),where K S , the attenuation coefficient of downwelling irradiance (m −1 ) and Γ is a coupling constant found to be 1.48 (Lee et al, ). Thus, the reciprocal Secchi disk depth ( S ) is an optical property with unit/m, which enables the estimation of the composite attenuation, K S .…”

“…Equation (1) deviates from previous Secchi disk theory (Preisendorfer, 1986) in that the beam attenuation coefficient is not part of the denominator. Both theoretical and empirical evidence for Equation (1) are found in Lee et al (2015Lee et al ( , 2018.…”

“…Central to our analysis is the relationship between Secchi disk depth (S, m) and optical properties as recently described by Lee, Shang, Du, and Wei (2018) and Lee et al (2015),…”

“…where K S , the attenuation coefficient of downwelling irradiance (m −1 ) and Γ is a coupling constant found to be 1.48 (Lee et al, 2018).…”

Color, chlorophyll a, and suspended solids effects on Secchi depth in lakes: implications for trophic state assessment

“…, Heiskary and Egge ), and it also is useful to estimate other optical properties, such as the diffuse attenuation coefficient for photosynthetically active radiation, K PAR (Lee et al. ).…”