| Summary: | Ocean color is the result of absorption and scattering, as light interacts with the water and the optically active constituents. The measurement of ocean color changes enables monitoring of these constituents (dissolved or particulate materials). The main objective of this research is to use digital images to estimate the light attenuation coefficient (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>K</mi><mi>d</mi></msub></mrow></semantics></math></inline-formula>), the Secchi disk depth (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>Z</mi><mrow><mi>S</mi><mi>D</mi></mrow></msub></mrow></semantics></math></inline-formula>), and the chlorophyll a (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><mi>C</mi><mi>h</mi><msub><mi>l</mi><mi>a</mi></msub><mo stretchy="false">)</mo></mrow></semantics></math></inline-formula> concentration and to optically classify plots of seawater using the criteria proposed by Jerlov and Forel using digital images captured at the ocean surface. The database used in this study was obtained from seven oceanographic cruises performed in oceanic and coastal areas. Three approaches were developed for each parameter: a general approach that can be applied under any optical condition, one for oceanic conditions, and another for coastal conditions. The results of the coastal approach showed higher correlations between the modeled and validation data, with <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>r</mi><mi>p</mi></msub></mrow></semantics></math></inline-formula> values of 0.80 for <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>K</mi><mi>d</mi></msub></mrow></semantics></math></inline-formula>, 0.90 for <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>Z</mi><mrow><mi>S</mi><mi>D</mi></mrow></msub></mrow></semantics></math></inline-formula>, 0.85 for <i>C</i><i>h</i><i>l<sub>a</sub></i>, 0.73 for Jerlov, and 0.95 for Forel–Ule. The oceanic approach failed to detect significant changes in a digital photograph. The most precise results were obtained when images were captured at 45° (n = 22; <inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><mrow><msub><mi>F</mi><mrow><mi>r</mi><mo> </mo><mi>c</mi><mi>a</mi><mi>l</mi></mrow></msub><mo>=</mo><mn>11.02</mn><mo>></mo><msub><mi>F</mi><mrow><mi>r</mi><mo> </mo><mi>c</mi><mi>r</mi><mi>i</mi><mi>t</mi></mrow></msub><mo>=</mo><mn>5.99</mn></mrow></semantics></math></inline-formula>). Therefore, to ensure precise results, the angle of photography is key. This methodology can be used in citizen science programs to estimate <i>Z<sub>SD</sub></i>, <i>K<sub>d</sub></i>, and the Jerlov scale.
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