The waters of the Chesapeake are saltiest near the mouth of the Bay and gradually become fresher northward. The force of the earth's rotation makes salt water accumulate on the Eastern Shore, so water tends to be saltier on the eastern side of the Bay at any latitude. Overall, however, the proportions of fresh and salt water in the Bay depend largely on the amount of rainfall that flows out of the Chesapeake's major rivers. Salinity is one of the most important physical features in determining what lives in a particular part of the Bay, so plant and animal populations in the Bay differ north to south, west to east, and from year to year. Temperature and bottom sediment also determine the distribution and abundance of organisms.
Estuaries teem with a variety of plant and animal life.
The constant movement of the tides sweep away wastes and circulates nutrients and food, providing a favorable situation for sedentary animals such as oysters and clams. Estuaries are important nursery grounds for fish. Indeed, many kinds of aquatic animals spend some portion of their life cycles in estuaries. The enormous biological productivity of estuaries is reflected in their immense commercial importance. The Watershed A thorough examination of Chesapeake Bay ecology includes not only the body of water itself, but also the entire drainage area, referred to as the Bay watershed .