RISE OF THE GREEN ALGAE: ALGAL PALYNOMORPHS RECORD THE HISTORY OF WALDEN POND

Walden Pond is a kettle lake that formed after the retreat of the Laurentide Ice Sheet. This small (0.25km²) lake is the deepest in Massachusetts (Zmax = 30.5 m) and it experiences stratification leading to bottom water anoxia, key for organic preservation. As well as being of historical significance (popularized by Thoreau) it has research potential given its unique limnological characteristics, protected status and lack of surface inlet or outlet streams. Concord was first settled in the early 17^th century and Walden Pond became the site of recreational activity after the Fitchburg railroad was built in the mid 1800’s until the mid 1970’s when the Massachusetts Department for Conservation and Recreation took control of the site. Pollen analysed from sediment cores records climate change and provides a chronology by comparison with regional pollen zones, while algal palynomorphs in the same slides record variations in primary productivity. The Ambrosia rise ~28cm in core WAL-15 GC1, a ~65cm gravity core from the deep western basin (~30.5m water depth), records European land clearing. This correlates with a shift in dominance from cysts of dinophytes (mainly Peridinium willei, P. volzii, and P. cinctum) and cyanophytes (Microcystis spp. and Woronichinia spp.) to abundant and diverse green algal palynomorphs. Charophytes (e.g., Staurastrum and Cosmarium) and chlorophytes (e.g., Coelastrum reticulatum, Pediastrum, and Acanthosphaera spp.) record cultural eutrophication peaking during the early 20^th century. Abundant spruce, pine and herb pollen near the base of the ~7.96m long WAL-17 LC1 core from the deep basin confirms that sediments span the postglacial. Abundant cysts of Parvodinium spp., particularly P. umbonatum, record oligotrophic conditions during the early Holocene, while the middle to late Holocene is characterised by a meso-eutrophic assemblage rich in the Peridinium cysts and cyanophyte palynomorphs found in the lower part of the WAL-15 GC1 core. These limnological changes are recorded by sediment geochemistry and physical properties, confirming that European colonists were the greatest perturbation to the Walden Pond ecosystem in its history, but an increase in P. umbonatum near the top of the WAL-15 GC1 core suggests that recent conservation efforts have produced an improvement in water quality.