I may as well throw in my two-cents....

Although I've found usnea in fresh water, there is always some tidal
influence down near the bottom of the river due to the salt wedge.  Salt
water is heavier than fresh and as the tide comes in, the salt water slides
in under the fresh water.  So although the water is fresh at the surface,
it is saltier near the bottom.....where many of the usnea live (but not
all...as they are often found on tree trunks, etc. above the salt wedge).

I've noticed that the river topography influences how far usnea extends
inland.  If the land is very flat, the salt wedge extends further inland,
and so does usnea.  A steeper gradient keeps salt water and usnea confined
closer to the sea.

Limestone is very shallow along the west coast of north central
Florida.  In Dixie County the limestone is virtually right at the surface
and fresh water seeps out of this limestone at low tide.  I have found
hundreds of usnea lined up along the limestone in the fresh water at low
tide, with none even a couple meters above the fresh water intrusion....
This zone is covered with tidal water at high tide.....

Kurtles

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