Stress
Usually, but not always, a limnologist is doing science outdoors instead of indoors (at least part of the time). For a lot of people who even consider limnology, that takes away stress immediately. You know, maybe cubicle or lab work isn't their cup of tea and being outdoors seems like a relaxing change of pace.
Again, it depends on the type of limnology they do: Is it research and the gathering of samples? Is it article writing? Is it grant writing? Is it working with legislators to change laws about water safety, wildlife safety, etc.? (Working with lawmakers has got to be stressful; look at the gray hair and baggy eyes young presidents acquire in four years.)
On the other hand, working outside, limnologists are at the mercy of nature's elements (lightning near water, hurricanes, alligators in swamps...need we go on?). These things can fuel limnologists' nightmares and make them wonder if they chose the right profession.