Apparent native freshwater mussel mortality event

On an overnight field trip for an ecology course taught Professor John Green of Saint Lawrence University in the fall of 1983, I went snorkeling in Cold Pond next to the Catamount Lodge, which is a retreat cabin for St. Lawrence University. The soft sediment bottom of the pond at about three to four metres depth contained a conspicuous dead shell record of what appeared to be native freshwater mussels. It appeared that they had all died in place without being removed by any predators. One end of each mussel was protruding out from the sediment in a normal orientation, but the mussels were dead. acid rain and resulting acidification of Lake water was a big topic at that time so I assumed that all of these mussels died in place relatively suddenly as the result of increased acidification of this small lake as a result of acid rain.