THE RISE OF THE GREEN ALGAE: USING ALGAL PALYNOMORPHS AS PROXIES OF NATURAL EVENTS AND ANTHROPOGENIC ACTIVITIES
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Anthropogenic activities can speed up eutrophication by increasing the rate of nutrient influx into lakes. Climate change threatens the health of freshwater ecosystems, increasing the frequency and intensity of harmful algal blooms and associated anoxia. The acid-resistant remains of freshwater algae and cyanobacteria commonly found in palynological macerations remain underutilized, even though their fossil record extends back to the Precambrian. Comparing algal palynomorph assemblages in sediments deposited in two meromictic lakes in northeastern Massachusetts illustrated their response to different stressors over the last few centuries. The abundance and diversity of algal palynomorphs increased as nutrient influx to both Sluice and Walden Pond increased with land clearing by European colonists, recorded by an increase in Ambrosia and other non-arboreal pollen in the same slides. As the City of Lynn became heavily industrialized, Sluice Pond received a lot of effluent, and algal palynomorph diversity declined sharply. Walden Pond remained recreational and primarily impacted by summer day-visitors, so a diverse dinoflagellate cyst assemblage together with planktonic colonial cyanophytes and chlorophytes persisted until major fires occurred in Walden Woods in the early 20th century. The different types of land-use between the two sites were hypothesized to be the driving factor that determines algal palynomorph assemblages. Sediments deposited since the mid-20th century at both sites contain abundant green algal palynomorphs, an observation noted in several other studies world-wide. The observed rise of the green algae at two different sites is likely attributed to increasing atmospheric CO2 and the Great Acceleration. This might be a useful auxiliary proxy marking the proposed Anthropocene Epoch, but further research is needed to better understand the taxonomy and taphonomy of algal palynomorphs, with a focus on green algae and cyanobacteria, to improve their utility as biomonitors and paleolimnological proxies.