Various daylight and light measurements for optical watermass classification and for in situ photosynthetic studies were made in the Southwest Florida Shelf ecosystem during the period 2–7 April 1982. The water masses in the test area can be subdivided into (i) a surface layer characterized by homogeneity and low concentrations of chlorophyll a and suspended matter; (ii) a thin transition zone where chlorophyll a and suspended matter increase considerably with depth; and finally, (iii) an underlying water layer extending from around the lower limit of the euphotic zone where chlorophyll a and suspended matter gradually decrease. These general features imply rather small horizontal gradients with respect to chlorophyll a and suspended matter when compared with the vertical gradients of the same two parameters. This holds true especially for chlorophyll a. Yellow substance, in terms of the penetration depth of the downward UV-B daylight irradiance, was not measurable in the test area. The colour of the surface water masses, the depth of the euphotic zone, and the Secchi-disc depth varied only slightly at those locations in the test area where these measurements were made and were almost unaffected by the differences observed in the concentration of suspended matter. An algorithm, valid for a large number of both oligotrophic and eutrophic waters, giving the depth of the euphotic zone in terms of the colour of the sea, demonstrated its validity for the water masses in the test area. A similar success was found for another algorithm involving the colour of the sea and the surface concentration of chlorophyll-like plankton pigments.