I am doing worldbuilding on a region centered around a gigantic lake in the sub-tropical region in the northern hemisphere of a planet. I want the lake to incredibly huge, like around 2 million square kms. My question is, would it make sense for this lake to be freshwater at all? If no, would this now saltwater lake need to be connected to the sea/ocean and can it be underground?
I tried searching for any answers, but I was not able to find any answer to if it is possible for it to be freshwater at all. The idea for the biomes would be wetlands next to the lake itself, followed by jungles which turn in shrubland and then into a sort of savannah followed by a desert as we go further away (I am talking about a 5000 kms * 6000 kms sort of area). Would this make sense at all? I hope someone can help me with this dilemma!
Thanks to all who take time to read this. Cheers! :)