Water clarity or transparency is an important
indicator of water quality in lakes. Clarity is measured using
a Secchi disk, a white (or black and white) plate which is lowered
into the water until it can no longer be seen: the deeper you can
see it, the better the transparency. The Secchi disk depth can
range from less than 1m in highly eutrophic lakes to more than
20m in Lake Superior.
The addition of excess phosphorus to lakes stimulates
algal growth (eutrophication) and the water becomes green and turbid
(low transparency, shallow Secchi disk depth). Transparency can
be improved by reducing phosphorus inputs to the lake.