Walden Pond is not as pure as in Thoreau’s time

Posted by Curt Stager and Richard B. Primack

“Nothing so fair, so pure, and at the same time so large, as a lake, perchance, lies on the surface of the earth."

 -  Thoreau in Walden.

Walden Pond, like the rest of the world, is now quite different from what Thoreau knew 160 years ago. We have become a force of nature in our own right, and the story is written into Walden's muddy layers, as we describe in a recent article in PLOS One.
  
In Thoreau’s time, Walden Pond was a low nutrient pond of crystal clear purity. By studying six sediment cores extracted from the bottom of the lake, we showed that soil running off from disturbed shorelines, along with human wastes associated with swimmers, appear to have increased nutrient levels in the lake during the past 70 years. This has allowed algae to thrive, reducing the clarity of the water.

Efforts by the park department to protect the lake's purity have thus far kept it from turning green and murky.  But hotter summers and more swimmers will favor even more algal growth.   

Walden Pond is not as pure as in Thoreau's time

Recommendations to maintain and improve water quality include (1) reducing the swimmer-impacts through education programs, (2) stabilizing eroding banks, and (3) monitoring water quality and lake vegetation to see if there is any improvement or deterioration.