The tidal portion of the Chesapeake Bay watershed is subject to the ebb and flow of ocean tides. This area encompasses all of the mainstem Bay and the area north and east to the fall line. The fall line is a physical barrier on the Bay’s larger tributaries marked by waterfalls and rapids.

One of the most important characteristics of the tidal area is the wide range of salinity from the fall line to the Bay’s mouth at the Atlantic Ocean. Salinity is measured as the number of grams of dissolved salt in 1000 grams of water, and it’s expressed in parts-per-thousand (ppt). Salinity changes gradually from the fall line (low salinity) to the Bay’s mouth (high salinity) and has an impact on the habitats of living resources. Salinity also affects the physical processes in the Bay.

As freshwater from the tributaries flows toward the Bay, it mixes with saltwater. That generally means that the closer water is to the Bay’s mouth and the Atlantic Ocean, the saltier it will be. Saltwater also is denser than freshwater. The different salinity zones are as follows:

• Tidal Fresh – 0.0 – 0.5 ppt
• Oligohaline – 0.5 – 5.0 ppt
• Mesohaline – 5.0 – 18.0 ppt
• Polyhaline – 18.0 – 35.0 ppt

These zones can shift throughout the Bay depending on freshwater flow and the season. Flows tend to higher in the spring and lower in the summer.

In the summer, the difference in density is enhanced as warm air heats surface waters; bottom waters remain relatively cool. In deeper areas of the Bay, this difference in density can lead to the formation of a pycnocline, which is a layer in the water column separating two areas of different density. The pycnocline can be a physical barrier to exchange between surface and bottom waters and is a contributing factor in the depletion of dissolved oxygen from bottom waters.

High spring freshwater flows into the Bay not only shift salinity zones, they also carry nutrients (nitrogen and phosphorus) and sediments from land. This natural process has been occurring in the Bay for thousands of years. However, as the actions of more than 15 million people stress the watershed, the amount of nutrients and sediments (suspended solids) washed into the rivers and the Bay increases.

Nutrients are a necessary component of the Bay’s food web. Plankton (microscopic plants and animals) needs nutrients to grow; in turn, fish and other living resources feed on plankton. However, when the Bay contains excess nutrients, phytoplankton (plants) becomes overabundant. Excess phytoplankton dies, consuming oxygen needed by other living resources. Many species of fish and shellfish cannot tolerate low oxygen for a great length of time. So, areas below the pycnocline of low to no oxygen can become useless, reducing available habitat in the Bay. Also, when dissolved oxygen is depleted from bottom waters, complex chemical and biological interactions cause nutrients to be released from sediments. Disruptions to the pycnocline by wind allow nutrient-rich bottom waters to escape to the surface, where they fuel further phytoplankton growth and start the process again.

The Chesapeake Bay Program keeps tab on this process by monitoring nitrogen, phosphorus and dissolved oxygen levels in the Bay. Because nutrients contribute to phytoplankton growth, the Bay Program also measures concentrations of chlorophyll (chla), which is a pigment used by plants in photosynthesis. The concentration of chlorophyll in the water gives an indication of whether phytoplankton is growing at an unhealthy rate. The Bay Program also monitors water clarity by measuring the total amount of solids suspended in the water (TSS) and Secchi depth. Water clarity is important because it’s linked to the health of underwater Bay grasses (Submerged Aquatic Vegetation), which need plenty of sunlight to grow. Bay grasses, an important habitat for young fish and shellfish, tend to decrease in areas with poor water clarity.