Geological Sciences‪.‬

Paleoenvironmental Changes Recorded in Diatom Assemblages from a Small Coastal Dune Lake Near Holland, Michigan. Trevor Daly, Geological and Environmental Sciences Department, Hope College, Holland MI Twenty nine diatom genera have been identified in 260 cm of core representing 5600 years of deposition in Gilligan Lake: a small inland lake at the edge of the Lake Michigan coastal dune complex southwest of Holland. Fragilaria and the predominantly epiphytic Cocconeis are abundant in the lowermost core suggesting the formation of a relatively shallow, nutrient rich lake towards the end of the Nipissing transgression. A change in lake chemistry is indicated by a decrease in Fragilaria and an increase in the acidophilic group Eunotia at 200cm. For the next 5,000, years several changes in water chemistry arc marked by shifts in the relative importance of the two dominant benthic genera Eunotia and Navicula. An increase in abundance of the predominantly planktonic centric genera (Cyclotella, Melosira and Coscinodiscus) above 200 cm probably reflects an increase in depth. Smaller shifts in the relative abundances of planktonic to benthic + epiphytic diatoms may represent smaller changes in water depth or clarity. The upper 10 cm is marked by a decrease in total centric diatoms, and an increase in the relative importance of Eunotia and Melosira, potentially indicating a recent increase in nutrient levels and turbidity and a decrease in pH.