Physical Danger
Limnologists don't risk their lives like say, border patrol agents, oil rig workers, or editors (you have no idea how scary some writers can be after you hand back their stuff that you've edited; ego clashing that loud is enough to warrant a trip to the audiologist) do. Still, there are a few things that can get them if they're not careful.
The dangers associated with limnology are pretty minimal, but you'll be exposing yourself to various plants, animals, chemicals (such as phosphates), and lots and lots of water. Your feet will probably get wet—a lot. Pruny feet are no fun, and neither are calluses, blisters, fungal infections, and trench foot. (What, you thought that was just a World War I problem?)