Ocean turbidity

Ocean turbidity is a measure of the amount of cloudiness or haziness in sea water caused by individual particles that are too small to be seen without magnification.

There are a number of ways to measure ocean turbidity, including autonomous remote vehicles, shipcasts and satellites.

From a satellite, a proxy measurement of the water turbidity can be made by examining the amount of reflectance in the visible region of the electromagnetic spectrum.

For the Advanced Very High Resolution Radiometer (AVHRR), the logical choice is band 1, covering wavelengths 580 to 680 nanometres, the orange and red.

The reflectance percentage can be correlated to attenuation, Secchi disk depth or total suspended solids although the exact relationship will vary regionally and depends on the optical properties of the water.

In the Mississippi River plume regions these same reflectance values would represent sediment concentrations that are about ten times or more higher.

The absorption and scattering characteristics of a water body determine the rate of vertical light attenuation and set a limit to the depths contributing to a satellite signal.

For example, in the Bahamas, the water is quite clear and only a few metres deep, resulting in an apparent high turbidity because the bottom reflects much band 1 light.