Water Clarity

Water clarity is a measure of the amount of sunlight that can penetrate through the water. In general, clear water is an indicator of a healthy Chesapeake Bay.

Why is water clarity important?

Clear water is critical to bay grasses. The sun’s rays must be able to pass through the water to reach these underwater plants growing in the Bay’s shallows. Sunlight is the single most important factor determining bay grass survival.

Fish also need clear water to see their prey and avoid being eaten by predators.

What causes poor water clarity?

Nutrient and sediment pollution are the main causes of the Bay’s poor water clarity. When excess nutrients run off into the Bay, they can fuel the growth of water-clouding algae. Similarly, when soil erodes and washes into the Bay, particles of sediment can become suspended in the water.

Weather plays a large role in water clarity. Rain storms wash dirt and pollutants into the Bay, which causes the water to look muddy. During drier conditions, the water tends to be clearer. Water clarity will always fluctuate naturally depending on weather conditions.

How do scientists measure water clarity?

Scientists use a device called a Secchi disc to measure water clarity. A Secchi disc is a simple, black-and-white circle attached to a rope.

To measure water clarity:

• The Secchi disc is lowered into the water until it disappears, then pulled up until it can just barely be seen.
• Scientists mark the water line on the rope and measure the distance between the Secchi disc and the water line.
• The measurement is the water’s clarity: the depth that sunlight is able to penetrate through the water.

Watch this video to see a Secchi disc in the water: