What makes an estuary?

Satellite photo of Cape Fear. The Cape Fear River estuary appears as a gray vertical stripe in the middle of the image. (Photo from NASA. More about the photograph)

Estuaries are defined as semi-enclosed bodies of water with intermediate salinities caused by the mixture of fresh and salt waters. That sounds simple enough, but the true qualities of estuaries are found in the interaction between the river and ocean inputs with the geology and biology of the landscape.

Estuarine organisms do not live in a static environment of diluted seawater but in an environment that is constantly evolving, be it on a time scale of hours, as the tide moves in and out, or on a time scale of thousands of years, as sea level rises or falls and estuaries move up or downriver to keep up. This constant change means that the entire estuary ecosystem must maintain a dynamic, rather than static, equilibrium. It is this ability to handle change that makes estuaries unique. A hurricane like Fran or Floyd has devastating consequences for coastal ecosystems such as estuaries as well as for residents of coastal areas. But like people, the estuary ecosystem rebuilds itself; plants establish new territory, mud dwelling creatures move to accommodate new sediments or longer periods of overlying fresh water, and fish swim further downstream to spawn.

One reason it is so difficult to understand estuary ecosystems is that we, as tourists or researchers, see only snapshots of the system, while the real action can only be captured by streaming video — and lots of it. But we do what we can.

next