What is Water Clarity?

Water clarity is a measure of how much sunlight penetrates though the water column. Water clarity changes depending on the amount of sediment particles, microalgae, and colored organic matter suspended in the water column. Clear water is especially important for Chesapeake Bay seagrasses since, like all plants, they need to be able to absorb the sun’s rays to grow. When there are is too much sediment, it blocks how much sunlight can penetrate to the seagrasses, limiting where they can grow. Excess sediment can also smother oysters and other bottom-dwelling creatures.

How is Water Clarity measured?

Water clarity is measured using a black and white disc called a Secchi disk, which is lowered into the water until you can't differentiate the black and white quarters of the disc anymore. This is the Secchi depth, and is an indicator of how clear the water is. Water clarity was measured at approximately 159 stations up to 11 times during the periods of interest (April to October in the tidal fresh, oligohaline and mesohaline regions, and March to November in the polyhaline regions). The proportion of time that water clarity was below the threshold at each station was calculated and then spatially averaged into reporting regions.

Threshold levels

* Lacouture et al., Estuaries and Coasts (2006) & Buchanan et al., Estuaries (2005)