Fauna in deep great lakes
Contrary to oceans, deep lakes are always isolated from other deep lakes by land. Therefore, one might expect different fauna in every lake, with possibly a lot of endemic species. So it may be hard to study deep lakes in general.

The ancient and deep lake Ohrid, in the former yugoslav republic of Macedonia, has been researched relatively well. Here's a reference which will get you started. Or search "lake ohrid fauna".