I think the lake this word is used to decribe is Glimmermere, above and behind the great keep of Revelstone.

SYLLABICATION: la·cus·trine

PRONUNCIATION: lə-kŭs’trĭn

ADJECTIVE: 1. Of or relating to lakes. 2. Living or growing in or along the edges of lakes.

ETYMOLOGY: French or Italian lacustre (from Latin lacus, lake) + –ine.

Source: Bartelby.com.