The water of Walden Pond, which Henry David Thoreau described in 1854 as “so transparent that the bottom can easily be discerned at the depth of 25 or 30 feet”, is no longer quite so clear according to a new study.

The Massachusetts pond was made famous in Walden, the transcendentalist writer’s account of the years he spent next to it in order to “live so sturdily and Spartan-like as to put to rout all that was not life”. The pond has been greatly affected by human activity. Everything from forest fires in the 19th century, to wood-cutting operations, the use of pesticides in the 1960s and increasing tourism have affected the water quality, according to the paper. Over half of the phosphorus in the lake in the summer “may now be attributable to urine released by swimmers”, while a footpath to Thoreau’s cabin “caused large amounts of soil to wash into the lake”.

“In the century and a half since Thoreau and his transcendentalist colleagues wrote of humanity as somehow separate from nature, we’ve become a force of nature in our own right, powerful enough to change the chemistry and temperature of the atmosphere and the ecology of lakes and ponds worldwide,” Curt Stager, a natural sciences professor at Paul Smith’s college, New York, and one of the authors of the paper told CNET.

Stager and his fellow researchers analysed sediment from the pond to determine how the growth of algae has changed. Their research revealed that the major changes in the lake’s algae happened in the 20th century. The closure of a nearby landfill site, the stabilisation of the shoreline and the upgrading of septic facilities have mitigated the effects of humans on the lake since 2000, but the researchers warn that the pond has not returned to the state described by Thoreau.

Writing in the mid-19th century, Thoreau referred to the “crystalline purity” of the pond, to its “remarkable transparency” and its “pure and well-like character”. “Successive nations perchance have drank at, admired and fathomed it, and passed away, and still its water is green and pellucid as ever,” he wrote. “Perhaps on that spring morning when Adam and Eve were driven out of Eden Walden Pond was already in existence, and even then breaking up in a gentle spring rain accompanied with mist and a southerly wind, and covered with myriads of ducks and geese, which had not heard of the fall, when still such pure lakes sufficed them … It is a gem of the first water which Concord wears in her coronet.”

The researchers warn that the lake would likely be further affected by climate change. “The sediment darkening and high percentages of [algae] in the recent sediments of Walden Pond … indicate not only that the lake ecosystem is now quite different from that described by Thoreau but also that it may be primed for more severe reductions in water clarity in a warming future.”

Increasing temperatures in coming decades, for instance, could also increase recreational use of the lake in summer, add researchers, who suggest that a nearby swimming pool would “relieve pressure on the lake”.

“If we’re not careful, this beautiful, iconic lake … could easily become more like a murky, green stew of algae,” Stager told CNET.