In the October 1998 issue of Smithsonian Magazine another explanation for
mass mortalities of mollusks is offered. It seems that ocassional
upwellings of water from the outer continental shelf take place.  This
water is 30 Degrees F colder then the coastal surface water.  The
upwellings seem to be caused by strong winds out of the South and Southwest
which blow the warmer surface water out and the colder water is pulled into
replace it.  The colder water brings very high nutrient levels which can
lead to phytoplankton blooms.  These blooms lead to oxygen depletion as
bacteria graze on the microscopic plants.  In 1976 a low oxygen event
killed millions of surf clams.  This information was in an article titled
Wiring the Jersey Coast.  (That's New Jersey, USA)  For more info try
www.smithsonianmag.si.edu or your local library.