It's a new word to me and plural as you show it according to MW:

"Main Entry: po·lyn·ya
Pronunciation: "pä-l&n-'yä
Function: noun
Inflected Form(s): plural polynyas also po·lyn·yi /-'yE/
Etymology: Russian polyn'ya
Date: 1853
: an area of open water in sea ice


Reminds me of Polyanna.