Just an update. After your last advice, I went out and bought the cracked corn and layer pellets, but the next day he was gone. There was no evidence of any "foul" play.
Winter is now basically over here and the ice is beginning to melt around the edges. This last weekend a single swan arrived at the lake. Two days ago a pair of swans showed up and began to repair last year's nest. The single swan comes and goes, but the pair has stayed. I'm assuming the pair are last year's parents and the single swan is the stranded swan that I was concerned about last winter. Looks like we will once agin have a beautiful pond, free of algae and geese.
John