Paleolimnological assessment of human-induced impacts on Walden Pond (Massachusetts, USA) using diatoms and stable isotopes

Abstract

Multi-proxy analysis of a sediment core spanning 1600 years from Walden Pond, Massachusetts (USA), reveals substantial changes in the nutrient status over the past ∼250 years resulting from anthropogenic impacts on the lake and watershed. Following a period of environmental stability from about 430 AD to 1750 AD, the abundance of the diatom Cyclotella stelligera increased, the chrysophyte cyst to diatom ratio decreased, organic content declined, bulk organic δ¹³C decreased, and bulk organic δ¹⁵N increased. These changes coincided with logging in the watershed, and are mainly attributed to an increase in detrital input of inorganic sediment and delivery of dissolved soil decomposition products from the watershed. With the beginning of intensive recreational development of Walden Pond in the early 20th century, oligotrophic diatom species were largely replaced by disturbance indicators and the diatom-inferred lake pH increased by 0.5 units, while the bulk organic carbon and nitrogen stable isotope composition markedly shifted to lower and higher values, respectively. These changes reflect inorganic inputs from erosion related to trails, beaches, and construction, as well as increased nutrient inputs by wastewater seepage into groundwater and extensive recreational usage. Diatom-inferred total phosphorus increased only slightly, probably because oligotrophic species still persist during spring and autumn, when Walden Pond has lower nutrient concentrations due to reduced recreational activity. During the last 25 years, diatom assemblages stabilized, suggesting that management measures have been effective in reducing the rate of eutrophication. Notably, the changes observed over the past 250 years are well beyond the range of natural variability of the past 1600 years, yet the pre-disturbance record provides a useful target for developing additional restoration and conservation measures to ensure future environmental protection of this historical site.

Keywords:

• New England
• chrysophytes
• pH
• total phosphorus
• land-use history

Acknowledgements

This study was supported by NSF and NSERC grants to D. Foster and to R. Pienitz, respectively, and by logistical support from the Centre d'études nordiques and Harvard Forest. We are grateful for the very helpful comments of the members of the Paleolimnology-Paleoecology Laboratory at Université Laval and of several anonymous reviewers on earlier versions of the manuscript. We thank Susan Clayden for counting the pollen and John Smol for providing information about the chrysophytes.

Notes

^*Including ammonia.

^**Excluding ammonia.

n.d. = no data.

WA inv. = Weighted averaging with inverse deshrinking; RMSE = root mean squared error of prediction; RMSEP = jacknifed RMSE; r² = jacknifed coefficient of determination.