Secchi depth ("visual depth")

The secchi depth (visual depth) depends on how deep  sunlight can penetrate the water. A shallow secchi depth means that the penetrating light is greatly absorbed or effectively spread. The secchi depth is a measure of the water's content of particles. Secchi depth is measured in metres with a Secchi disc (for more information and figures, see the pdf version). The biological primary production in the ocean can occur down to the depth where about one percent of the light in the surface remains (compensation depth). In the open ocean this can be as deep as 200 metres. In coastal areas the secchi depth will often give a good estimate of the biomass of phytoplankton. There is, however, no direct correlation between secchi depth and phytoplanktonic biomass in areas with high concentrations of yellowing matter, such as in the Gulf of Bothnia. The secchi depth in the open Baltic sea can be up to 25 metres. It is significantly less near the coasts. On the Swedish west coast, it varies from 0.5 to 10 metres. The secchi depth in the open sea is more than 50 metres. 

Methods of determination
A white disc (secchi disc), with a diameter of 20-25 cm attached to a non-stretchable twine is lowered until it can no longer be seen and the depth is read. Then the disc is raised again until it appears, and the depth is read again. The secchi depth is the mean of the two readings.

Notice
The secchi depth is affected by the weather, which is a drawback. Therefore, always work in daylight. Measure on the shaded side of the boat, avoiding any disturbing water reflections, or better, use an aquascope. Mention in the sampling protocol how the secchi depth has been determined.