Estuaries

An estuary is a partially enclosed body of water formed where freshwater and saltwater mix.  Estuaries are influenced by the tides, and are also known as harbors, bays, lagoons, or sounds. It is important to note that not all water bodies by those names are necessarily estuaries. The defining feature of an estuary is the mixing of fresh and salt water, not the name. Estuaries support diverse communities of plants and animals, specially adapted for life at the edge of the sea.  Many different habitat types are found in and around estuaries, including shallow open waters, freshwater and salt marshes, sandy beaches, mud and sand flats, rocky shores, oyster reefs, mangrove forests, river deltas, tidal pools, sea grass and kelp beds, and wooded swamps.

     Almost 50% of the U.S. population live in coastal areas.  Unfortunately, when populations increase, ecosystems are altered by the filling of tidal wetlands, dredging of channels, and building on our shorelines.  These activities can stress our coastal waters, resulting in a decline in drinking water quality, harmful algal blooms, loss of habitat, and other environmental degradation.

Why are Estuaries Important?

     Estuaries are critical for the survival of many species such as birds, mammals, fish, and other wildlife, who depend on estuarine habitats as places to live, feed, and reproduce. Estuaries provide ideal spots for migratory birds to rest and refuel during their journeys.  They also serve as nursery grounds for fish, shellfish, and certain species of birds.  They may serve as filters for pollution and sediments that enter the estuary from upland rivers. Plants and soils in an estuary help to absorb flood waters, and protect both people and animals from high storm surges. Salt marsh vegetation helps to prevent erosion and keeps the shoreline stable.

     Among the cultural benefits of estuaries are recreation, scientific knowledge, education, and aesthetic values. Boating, fishing, swimming, surfing, and bird watching are just a few of the numerous recreational activities people enjoy in estuaries.  The tangible and direct economic benefits of estuaries should not be overlooked.  Tourism, fisheries, and other commercial activities thrive on the wealth of natural resources estuaries supply.  The protected coastal waters of estuaries also support important public infrastructure, serving as harbors and ports vital for shipping, transportation, and industry.

(Information on estuaries adapted from the U.S. Environmental Protection Agency’s National Estuary Program)

 Density

     The density of water is affected by two factors:  temperature and amount of dissolved material (in this case, salt). The relationship between these two factors is important in determining the environment of an estuary, where fresh and salt water meet and mix.

Temperature and Density

     Most liquids become denser as they cool, but water's polar molecules change this pattern. While in liquid form, water molecules move around each other randomly. Hydrogen bonds continually form and break between different molecules depending on the energy available. Decreasing water temperatures indicate a decreasing availability of energy. When water temperatures reach 4 degrees Celsius, there is not enough energy available to break hydrogen bonds once they formed, and water reaches its greatest density.  As the temperature continues to decrease, the continual formation of hydrogen bonds forces water molecules to line up in crystalline formation. This becomes visible to us when the temperature reaches 0 degrees Celsius and ice forms. This rigid, crystalline structure forces the water molecules to take up more room then when they were moving randomly in their liquid form. So ice is actually less dense than water.  When water gets colder than 4 C, it rises to the surface and freezes, allowing life to persist in deeper waters through the winter months.

Dissolved Material and Density

     Salt water is denser than fresh water because it contains salt and other minerals.  The salinity of open ocean water is 35 ppt (parts per thousand) which means that 35 grams of salts are dissolved in 1000 grams of water.  Another way to think about dissolved material and density is: If we compare a bucket of feathers with a bucket of sand, which would be heavier?  Sand is denser (and heavier) than feathers, even though they both take up the same amount of space.