Odds of Hanging On
If you're young, in good shape, and have no lake goop or microorganism phobias, you can stay in this field for a long time. Of course, you won't stay young and in great form forever, so you'd be wise to try to stay in shape, because limnology can be a somewhat physical job depending on the track you take (or what takes you).
If you eventually do decide to go academia (and manage to do so), be sure to work toward tenure. That way, you'll be guaranteed a job until you, yourself, are ready for a water burial.