Jane Heintz asks why the salinity of the ocean might change from day to day
or year to year. It is hard for a doctor to diagnose a patient at a
distance, but here is one possibility. Is the ocean near Pawleys Island
(South Carolina) normally diluted by fresh water from the mouth of a nearby
river? If so, then consider that rivers are abnormally low in the
southeastern United States this year because of low rainfall, which is an
effect of El Nino. Yes, El Nino is to blame once again! On a day-by-day
basis, currents could redirect the plume of fresh water from your part of
the shoreline to other areas.
 
As to effects on local mollusks, some species are strongly affected by
changes in salinity and others are much less so. I don't know of any
molluscan species that can live AND BREED in both fresh and fully marine
water. For instance, the bay clam Rangia cuneata (Sowerby, 1831) thrives in
the fresh parts of Lake Ponchartrain, Louisiana, growing to an abnormally
large size there, but these giants cannot breed in fresh water. What kinds
of clams and snails are especially affected by changes in salinity, for
instance, during floods and droughts?
 
Andrew K. Rindsberg
Geological Survey of Alabama