Plenty of people have taken a whiz in Walden Pond since writer Henry David Thoreau sat by its shores 160-plus years ago lost in contemplation. And that, according to a new study, is just one way humans have taken a toll on the iconic body of water.
"The sediments of Walden Pond record major ecological changes to this iconic lake since the time of Thoreau," said Curt Stager, a professor of natural sciences at Paul Smith's College in New York and one of the authors of the research published Wednesday in the open-access journal PLOS One. "They also warn of more changes to come in a warming future."
The Massachusetts pond, best known as Thoreau's home for two years and an inspiration for his famous 1854 book Walden, or, Life in the Woods, has become a popular recreational site for swimmers and anglers. Visitors also like to stop by a replica of the small cabin Thoreau built near Walden Pond on wooded land owned by his friend and mentor, writer Ralph Waldo Emerson.
All that human activity has made the pond far less pristine, Stager and his colleagues conclude. Sediments running off from shorelines disturbed by urban development and foot traffic, along with human pee, they say, may have contributed to shifts in algal nutrient levels in the lake, allowing for certain kinds of phytoplankton to thrive and thus reduce the clarity of the water.
"If we're not careful, this beautiful, iconic lake ... could easily become more like a murky green stew of algae," Stager told me.
Stager and team studied sediment from the bottom of the pond near Concord -- specifically the remains of preserved algae found in the soil samples. Since these algae require sunlight, they can indicate water depth and clarity. The researchers used radio-isotopic dating to determine the age of the sediment cores, and document how the growth of algae in the lake has shifted over time.
This isn't the first time scientists have examined sediment from Walden Pond. A 2001 study conducted by the US Geological Survey surveyed the science of the lake in depth. And the sorts of changes seen in the water can be observed in lakes around the world.
Still, Walden's cultural and historical significance make it a high-profile case study of cultural eutrophication. That term refers to the way human activity speeds up the natural process by which a body of water becomes enriched in chemical deposits -- like detergents, fertilizers, or in the case of Walden Pond, the phosphorus found in urine -- that stimulate the growth of aquatic plant life.
"That unique link makes the science far more interesting to a wide audience, and also to me," Stager said. "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."
Since Thoreau's time, the area around Walden Pond has housed a wood lot, an amusement park in the late 1800s, and a county park after 1922. In 1975, the area was designated a Massachusetts State Reservation.
To step up Walden Pond conservation efforts, the authors of the paper published this week recommend lake managers pay closer attention to unseen allies like aquatic vegetation that helps trap phosphorus in the sediments, out of reach of the plankton. They also suggest swimmer-education programs or the construction of a separate swimming pool nearby to take some of the pressure off Thoreau's treasure.
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