A TALE OF TWO LAKES

Walden Pond is a popular recreation area, best known from the writings of Henry David Thoreau. Well-preserved organic-walled microfossils (palynomorphs) and charcoal in sediments record changes in water quality related to recreational use, especially before it was taken over by the Massachusetts Department for Conservation and Recreation in the 1970s, but this hydrologically closed kettle lake remains mesotrophic. Nearby Sluice Pond is another small, meromictic lake, but it is an open lake with a much larger catchment and is thus naturally more nutrient-rich. Its location in a heavily industrialized urban region has produced hypereutrophic conditions in Sluice Pond. While the impact of European settlers to northeastern Massachusetts is recorded by an increase in abundance and change in assemblage of primary producers in both lakes, pollen and non-pollen palynomorphs record possible anthropogenic impact well below the ‘ragweed rise’ that is tentatively attributed to Indigenous land clearing in the small catchment of Walden Pond. The divergent histories of these small two lakes in response to different degrees and types of anthropogenic impact are reflected by palynomorphs, particularly the acid-resistant components of green algae and cyanobacteria which are seldom used as proxies of cultural eutrophication. In addition to eutrophication and siltation/ turbidity resulting from permanent human settlements, sudden-impact events, such as earthquakes and forest fires, are clearly recorded in sediments from these lakes. Surprisingly, however, statistical analysis of the algal palynomorph assemblage identifies the greatest change of the last millennium in both lakes in the early to mid-20th century, when green algae (colonial chlorophytes in Sluice Pond and desmid charophytes in Walden Pond) became the dominant primary producers. The rise of green algae at the expense of cyanobacteria reflects major regime shifts accompanying the Great Acceleration worldwide, primarily the increase in atmospheric CO₂, and thus could be a useful microfossil marker of the Anthropocene Epoch.