Abstract
The vertical diffuse attenuation coefficient Kd (PAR) is used for calculating the euphotic zone, the first optical depth that is important for primary productivity models. Currently, Kd (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 Kd (PAR) by SD. We used irradiance profiles and SD depth (ZSD) 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 SD 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 Kd (PAR) and water type, making it ambiguous to associate compatible inherent optical properties and chlorophyll with ZSD. Although obtaining Kd (PAR) from ZSD is simple/low-cost, care should be taken in the methodology used to measure ZSD to ensure consistent results across different optical marine conditions.
Highlights
Sunlight in the electromagnetic spectrum region between 400 and 700 nm, known as photosynthetically active radiation (PAR) [1], plays an important role in the physics, biology, and chemistry of the oceans [2]
This depth is defined as the Secchi disk depth (ZSD), being inversely proportional to the amount of dissolved or particulate matter present in the water column; the ZSD reading can be used as an indicator of turbidity [9,10]
The authors of [18] mentioned that in addition to the above, a factor to consider is that when Kd (PAR) is estimated using shallow depths, this tends to be higher than values estimated for deeper layers
Summary
Sunlight in the electromagnetic spectrum region between 400 and 700 nm, known as photosynthetically active radiation (PAR) [1], plays an important role in the physics, biology, and chemistry of the oceans [2]. Primary productivity largely depends on the sunlight field in the water column, which decreases exponentially with depth. The principle for the use of the SD consists of determining the depth at which it is no longer visible to the naked eye [8]. This depth is defined as the Secchi disk depth (ZSD), being inversely proportional to the amount of dissolved or particulate matter present in the water column; the ZSD reading can be used as an indicator of turbidity [9,10]
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