Estuary Salinity Patterns

The relatively higher density of salt water compared to fresh water means that freshwater will float on top of saltwater. In some cases, there may be mixing between the two water masses but, in other cases, little to no mixing may take place to produce a highly stratified water column with a layer of buoyant fresh water situated above a layer of salt water. The development of a well-mixed estuary or a highly stratified estuary is a function of the estuary morphology, magnitude of freshwater input, estuary-mouth tidal range, and how the tide propagates up the estuary. A classic comparison is that relatively shallow, bar-built estuaries are often well mixed and, alternatively, deep, fjord-style estuaries are highly stratified. It is also important to note that besides there being a vertical profile of salinity variation possible, there are also variations along the length of the estuary. Near the mouth of an estuary, one would expect to generally find the highest salinity water where the influence of the sea is the greatest and the most freshwater at the most inland extent where freshwater river systems enter into the estuary. These variations in salinity structures lead to differences in the distributions of salt-water and fresh-water tolerant plant and animal species within an estuary and how organisms use the estuary environments.