The classification of ocean waters into “Case 1” and “Case 2” began with Morel and Prieur (1977). They wrote that

…two extreme cases can be identified and separated. Case 1 is that of a concentration of phytoplankton high compared to other particles…. In contrast, the inorganic particles are dominant in case 2.… In both cases dissolved yellow substance is present in variable amounts.… An ideal case 1 would be a pure culture of phytoplankton and an ideal case 2 a suspension of nonliving material with a zero concentration of pigments.

Morel and Prieur emphasized that these ideal cases are not encountered in nature, and they suggested the use of high or low values of the ratio of pigment concentration to scattering coefficient as a basis for discriminating between Case 1 and Case 2 waters. Although no specific values of this ratio were proposed to serve as criteria for classification, their example data suggested that the ratio of chlorophyll a concentration (in mg m-3) to the scattering coefficient at 550 nm (in m-1) in Case 1 waters is greater than 1 and in Case 2 waters is less than 1. Importantly, however, Morel and Prieur also showed data classified as “intermediate waters” with the ratio between about 1 and 2.2.

Mobley, C.D., D. Stramski, W.P. Bissett, and E. Boss. 2004. Optical modeling of ocean waters: Is the Case 1 - Case 2 classification still useful? Oceanography 17(2):60–67, https://doi.org/10.5670/oceanog.2004.48.

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