Greenwich Bay

Greenwich Bay is an estuary, an area where salt water and fresh water meet.

Salinity is a measure of the total salts found in water. It is measured in parts per thousand. For example, the open ocean is 35 parts per thousand (ppt). This means that if you had 1,000 buckets of sea water, and could put salt and water in separate buckets, 35 of the buckets would be filled with nothing but salt.

The red area below represents 35 parts out of 1,000:

The salinity in estuaries is often lower than that of the open ocean, because fresh water entering from streams dilutes the salt water. In Greenwich Bay, the salinity ranges from about 24 ppt to 30 ppt. Salinity depends on whether the tide is high or low (salinity may be higher when water is entering from Narragansett Bay), how much rain there has been lately, and how much runoff there has been.

Table salt—sodium chloride, or NaCl—usually comes to mind when we think of salt. In fact, sodium and chloride ions are the most common constituents of sea water. Together, they make up over 85 percent of dissolved substances.

Animals in Greenwich Bay must either be able to tolerate changes in salinity, or they must be able to change the salinity in their bodies to balance their internal salt concentrations.