If this was ocean, yes — however a freshwater lake in Antarctica might actually be pretty darn ancient. (And may have spent most of its existence being ice, not liquid water.)
Down in South America there is some sort of TINY fish/worm/bacteria that crawls in through your urinary tract (!), then grows inside of you.
If there is that kind of stuff in the “modern” world, God knows what might be in a lake that has been closed off for so long. I’m sort of on the serious side. It may be aliens in the form of some weird bacteria. Let’s hope if it is is just likes water and doesn’t go airborne. (Hmmm - I’m just thinking of a mayfly hatch and how nasty the windshield gets when you drive through one).