PALEOLIMNOLOGY OF HYPEREUTROPHIC SHELBURNE POND, VERMONT

Eutrophication not only causes changes in productivity and water chemistry, but also impacts aesthetic and recreational values of a lake. It is usually the latter reasons that spur attempts to restore bodies of water to their original, pre-settlement conditions. However, some lakes are naturally eutrophic, and it may be impractical to attempt to change their trophic status. Paleolimnological analyses may be used to determine whether anthropogenic forcing has caused a lake to become eutrophic, and therefore may be a candidate for remediation.

This study utilizes sediments from hypereutrophic Shelburne Pond, in Shelburne, VT. The lake's watershed was occupied by European settlers in 1725 A.D, was heavily grazed by sheep during the 1800s and early 1900s, and has been logged, farmed, and fertilized to the present. Since several anthropogenic activities may have contributed to the eutrophication of Shelburne Pond, it is a good candidate for determining if humans have caused the lake's productivity to increase.

In 2006, we collected a 3.8 m long piston core and a 79 cm long freeze core from the deepest part of the lake. Core samples were analyzed for a large array of indicators of trophic state, including organic matter content, C stable isotopes, C/N, total and available P concentrations, biogenic Si, diatom composition, and paleopigments. All paleoproductivity proxies indicate that Shelburne Pond was oligo-mesotrophic before European settlement, and became increasingly productive since the mid 19th century (~1850). Eutrophication rates intensified after ~1900, and reached peak levels during the past two decades (post-1990). Comparison of the sedimentary record with historical data suggests a causal relationship between deteriorating water quality in the lake and human activities in its watershed. Forest clearing since 1810, a switch to mechanized agriculture around 1850, and intensive dairy farming during most of the 20th century, all resulted in progressive nutrient enrichment.

Despite these significant recent trends, data extending past the post-settlement record suggest that Shelburne Pond's productivity levels were at times quite significant during the past few thousand years. The causes of these, apparently natural, fluctuations remain to be investigated.