Yeah, you. A study of nearly two millennia of microscopic growth at the bottom of the pond helps confirm an earlier report that "more than half of the summer phosphorus budget of the lake may now be attributable to urine released by swimmers."
That's potentially bad news in general and with looming climate change in particular, because the phosphorus from all that micturition promotes the growth of one kind of slimy, floating algae that effectively blocks sunlight from another form of bottom-dwelling algae that helps filters the water, the authors write:
It will therefore be prudent to further reduce the flow of anthropogenic nutrients to Walden Pond under the warmer, wetter conditions that most climate models project for New England during the 21st century. Swimmers are probably the largest source of such nutrients now, and demand for the beach facilities is likely to increase in a warmer future. Swimmer-education programs or construction of a separate swimming pool facility nearby to relieve pressure on the lake might therefore be advisable.