Salinity
The salinity of estuaries fluctuates dra-matically both from place to place andfrom time to time. When seawater, aver-aging about 35‰ salinity, mixes withfresh water (nearly 0‰) the mixture has asalinity somewhere in between. The morefresh water that is mixed in, the lower thesalinity. Salinity therefore decreases asone moves upstream (Fig. 12.3).Salinity also varies with depth in theestuary. The salty seawater is more denseand stays on the bottom (see “Salinity, Temperature, and Density,” p.48). Itflows in along the bottom in what is fre-quently known as a
salt wedge.
Mean- while, the fresher, less dense water fromthe river flows out on the surface. The salt wedge moves back andforth with the daily rhythm of the tides(Fig. 12.4). It moves up the estuary on therising tide, then recedes as the tide falls. This means that organisms that stay in oneplace are faced with dramatic fluctuationsin salinity. They are submerged under thesalt wedge at high tide and under low-salinity water at low tide. If the area has a
diurnal tide,
the organisms are subjectedto two shifts in salinity every day: one asthe tide moves upstream and a second as itretreats. In an estuary with
semidiurnaltides,
salinity changes four times a day.
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Estuaries are subject to wide fluctuations insalinity.
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The behavior of water masses in es-tuaries is not always this simple. Theshape of the estuary and its bottom, the wind, evaporation of water from the sur-face, and changes in the tide all influencethe distribution of salinity. Also of im-portance are seasonal variations in fresh- water runoff from rivers as a result of droughts, rains, or snowmelt. Currentsare especially important. Because mostestuaries are long and narrow, the tidedoesn’t just rise, it rushes in, often creat-ing strong
tidal currents.
In a few placesthe tide actually comes in as a nearly ver-tical wall of water known as a
tidal bore