An opaque disk, typically white, used to gauge the transparency of water by measuring the depth (Secchi depth) at which the disk ceases to be visible from the surface.
- ‘Temperature (in degrees Celsius) was measured at the surface and close to the bottom, and the Secchi depth was determined.’
- ‘Mean Secchi depths tended to be higher in 1996 than in 1997, but these differences also were not statistically significant.’
- ‘Further, on two dates we also measured Secchi depth and oxygen concentration in the epilimnion of each enclosure during midday.’
- ‘Although light conditions expressed as Secchi depth differed during the later part of the summer, differences over the whole season were not significant.’
- ‘Overall difference between species in use of Secchi depths varied by habitat type.’
Early 20th century: named after Angelo Secchi (1818–78), Italian astronomer.