A subwatershed is a small watershed contained within the larger Chesapeake Bay watershed. Each of the streams, creeks and rivers in the Bay watershed has its own watershed. For example, the Potomac River watershed is a subwatershed of the Chesapeake Bay watershed.

An airshed is an area of land where pollution released into the air can eventually wind up in a specific water body. Airsheds can be very large. For example, the Chesapeake Bay’s nitrogen oxide (NOx) airshed is 570,000 square miles, stretching west to Ohio and north to Canada.

Dams block shad and other anadromous fish from reaching their upstream spawning grounds. Historically, most shad and river herring spawned in freshwater areas that are now upstream of dams, road culverts and other blockages.

Scientists usually use an electronic dissolved oxygen meter to measure the amount of oxygen in the water.

The Chesapeake Bay is the largest of more than 100 estuaries in the United States.

The water at the mouth of the Chesapeake Bay is salty because it is mostly made up of salty water from the ocean. As you move north in the Bay, the water becomes less salty. Most of the Bay is brackish (a mixture of fresh and salty water).

Pfiesteria piscicida is a toxic microorganism linked to fish kills and lesions in several Chesapeake Bay rivers in 1997. Medical evidence at the time strongly suggested that human exposure to an active Pfiesteria outbreak may result in health effects. As a precaution, Maryland closed all affected rivers until the outbreaks ceased. However, there is no evidence that Pfiesteria toxins accumulate in fish flesh or that they can be passed to humans by eating seafood.

Since 1997, no major Pfiesteria outbreaks have been reported in the Bay region. Scientists continue to research the microorganism to determine how it produces toxins.

Sometimes the Bay’s deeper waters have little or no oxygen while surface waters have more oxygen. This usually happens in summer because of a combination of factors, including temperature, nutrient pollution, the way water flows in the Bay, and the shape of the Bay’s bottom.

About 20 species of bay grasses grow in the Chesapeake Bay and in lakes, streams, reservoirs and other bodies of water throughout the Bay watershed.

Plankton form the base of the Chesapeake Bay food web. All fish and shellfish depend on plankton for food during the early part of their lives, and some consume plankton their entire lives.