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Physical Characteristics: Salinity

NOAA salinity chart

Image from Chesapeake Bay Coastal Prediction Center

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Salinity defines the relative proportions of seawater and freshwater found in different parts of the estuary. Areas close to the oceans have higher salinity, while areas that are closest to river inputs have lower salinity. Salinity is the concentration of salt in water, usually measured in parts per thousand (ppt).

The salinity of seawater in the open ocean is remarkably constant at about 35 ppt. Salinity in an estuary varies according to one's location in the estuary, the daily tides, and the volume of fresh water flowing into the estuary.  Actual salinities at a given location vary throughout the tidal cycle. Salinity in an estuary typically declines in the spring when snowmelt and rain increase the freshwater flow from streams and groundwater. Salinity levels usually rise during the summer when higher temperatures increase levels of evaporation in the estuary.

Salinity also affects chemical conditions within the estuary, particularly levels of dissolved oxygen and dissolved inorganic phosphorus in the water. The amount of oxygen that can dissolve in water, or solubility, decreases as salinity increases. The solubility of oxygen in seawater is about 20 percent less than it is in fresh water at the same temperature.  Phosphorus, which sticks to particles in freshwater, is released as salinity increased. In tidal freshwater or low salinity reaches of estuaries, dissolved phosphorus is not readily available and tends to limit phytoplankton production.

Salinity affects the physical structure of estuarine waters and influences patterns of circulation. Because salt water is denser than freshwater, layers of different salinities can form resulting in stratification of the water column. Stratification impedes mixing in estuaries, and exacerbates problems such as low dissolved oxygen at the bottom.

Salinity tolerance leads to zonation in estuarine plants and animals. Estuarine organisms have different tolerances and responses to salinity changes. Many bottom-dwelling animals, like oysters and crabs, can tolerate some change in salinity, but salinities outside an acceptable range will negatively affect their growth and reproduction, and ultimately, their survival.  Some groups of animals, such as the echinoderms, which include animals such as sea stars, brittle stars and sea cucumbers, have very few species living in estuaries because of their low tolerance of reduced salinity.

Because there is a long and relatively stable salinity gradient within the Chesapeake Bay, compared to many smaller estuaries, there are many ecological niches available to organisms.  This results in relatively high diversity for an estuary habitat.  Smaller estuaries, and shallow water areas subject to direct freshwater runoff from land, may have more variable salinity, and tend to have lower diversity. 

Human alterations of the watershed, especially due to deforestation and the increasing amount of impervious pavement, have been shown to change the hydrologic cycle of estuaries, especially the timing and amount of freshwater inputs from surface runoff and groundwater.

For further information about salinity and its effects in estuarine and coastal marine habitats refer to the following:

Levinton, J. S. 2001. Marine Biology. Oxford University Press; Chapter 4 – The chemical and physical environment.

Mann, K. H. 2000. Ecology of coastal waters, with implications for management. Blackwell Publishing; Chapter 2.3 – Physical structure and functioning of estuaries; Chapter 2.5 – The biological consequences of changing freshwater runoff

OzCoast and OzEstuaries website



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