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 underwater grasses. Sunlight is the single most important factor determining the survival of underwater grasses, and the sun’s rays must be able to pass through the water to reach these underwater plants growing in the Bay’s shallows.

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

What causes poor water clarity?

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

Water clarity fluctuates naturally depending on weather conditions. Storms wash dirt and pollutants into waterways, which can muddy the water. Because of this, waters tend to be clearer during drier conditions.

How do scientists measure water clarity?

Scientists use a device called a Secchi disc—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.

To see water clarity levels throughout the Chesapeake Bay, visit Eyes on the Bay (for Maryland waters) or Virginia Estuarine and Coastal Observing System (for Virginia waters).