The origins of quantitative water mass classification probably date to the first deployment of the Secchi disc from the papal yacht L’Immaculata Concezion on April 20, 1865. Invented at the request of the Commander of the papal navy by Father Pietro Angelo Secchi, the Secchi disc continues in widespread use (Figure 1). Secchi depth atlases provide a broad view of the variability of optical properties in natural waters (Arnone, 1985). Although these measurements represent the largest assemblage of in situ optical data, they are extremely limited in spatial and temporal coverage.

Arnone, R.A., A.M. Wood, and R.W. Gould, Jr. 2004. Science box: The evolution of optical water mass classification. Oceanography 17(2):14–15, https://doi.org/10.5670/oceanog.2004.42.

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