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Secchi Depth
Secchi depth measurements of water clarity are also made at each station, essentially by
lowering a white “Secchi disk” into the water until it cannot be seen by an observer on the
surface; the depth at which this occurs is recorded.
The Secchi depth is a measure
of the penetration into the water
column of light that can drive
photosynthesis, and is affected
by the amount of particulate
matter in the water column.
Particulate matter can either be
living (i.e., phytoplankton), or
nonliving, mostly clays and fine
sediments that enter the strait in
the freshwater outflow of Fraser
River. Roughly speaking, we
expect phytoplankton in coastal
areas to be growing in depths
shallower than about 2 Secchi depths.
Because of its reliance on human perception, Secchi disk datasets are often very ”noisy”, but
Secchi depths are a cheap and relatively simple measurement with a long history of use.
