Castle Lake Limnological Research Station

Secchi Depth

The secchi disk is a white plastic disk that is used to measure the water clarity. The disk is slowly lowered by a marked rope until it is no longer visible, and the depth is noted. Then it is slowly raised until it is visible again. The secchi depth is the average of the two observations.

Secchi disks are widely used because they are durable, transportable, require no power, and--when used consistently--provide a useful measurement of water clarity. The secchi was first used extensively by the Italian astronomer and Jesuit Priest Peitro Angelo Secchi in 1865. It has been used since to measure the transparency of lakes and oceans around the world.

Castle Lake's Secchi depth varies seasonally and inter-annually with an average of 10 M. The world's record Secchi depth is 80 M, observed off Antarctica in October 1986.

Pyroheliometer

The pyroheliometer measures the amount of solar radiation illuminating the earth or, Castle Lake. It is set up next to the lake and the measurements are used in conjunction with other data to calculate the rate and amount of photosynthesis. It also provides a measure of solar heating experienced by the lake, useful in understanding the hydrodynamics of the lake.

It is a relatively simple instrument that records the changes in light
through the expansion and contraction of absorbing black and reflecting silver
metal bands. The expansion and contraction drive a pen on a recording drum.