Winter got my Arum Lily's

Hi,
Noobie here.

5 years ago I planted some Calla lily in my large pond. They came up lovely. Over the past few years they have thrived and seem to have multiplied in huge amounts. It was the most gorgeous site. This year, they have not shown up at all. Just the usual mush that's left after winter. I know we have a particularly severe winter this year but I am so sad to see them go.
Anyone else had this problem? What did I do wrong? Any suggestions?

 

I think they need to be below the level that freezes.

In other words they are not really hardy, so if the corm freezes they die.

They can grow in fairly deep water, a foot is OK, not sure how deep yours were.

 

I had several clumps. All but one died. At least I have some left. Mine were not in the pond but on the edge