THE IMPACT OF HYDROCLIMATE VARIABILITY ON DIATOM ASSEMBLAGES FROM CLEAR LAKE, IOWA OVER THE LAST 2,000 YEARS

The Midwest region of the United States, a global agricultural center and home to 70 million people, is projected to experience both increased precipitation and more frequent and severe extreme hydroclimate events in response to global climate change. Already, precipitation in the region has increased every decade since the 1970s. Hydroclimate has varied in the region’s past; however, a lack of high-resolution paleoclimate records for the region limits our understanding of past hydroclimate variability. Lakes are sensitive to changes in climate at the local and regional scale, while their sediments preserve records of environmental change over time. This study focused on diatom assemblages from a 2,000-yr record of Clear Lake, located in north-central Iowa. The goals of this study were to 1) assess shifts in dominant diatom species throughout the record, and relate these shifts to local and regional variation in hydroclimate, 2) determine if the lake showed a response to known periods of recent climate variability, i.e., the Medieval Climate Anomaly (MCA; 950–1250CE) and Little Ice Age (LIA; 1300–1800CE), 3) determine if the lake responded to landscape change as a result of anthropogenic influence, and 4) understand how this record fits into a proposed hydroclimate dipole between the Midwest and the Great Plains/Western U.S. regions. We found that diatoms at Clear Lake clearly responded to hydroclimatic change throughout the 2,000 year history, particularly through the connection of the lake to adjacent wetland areas. In the earliest part of the record, benthic diatom abundances were higher, indicating an increase of shallower water habitats. We interpret this as indicating higher lake levels in response to increased precipitation, which caused the lake to overflow into adjacent wetlands. Lake level likely decreased through time, as benthic abundances decreased throughout the record. Additionally, the lake shows responses to the MCA and the LIA that match observations from other Midwestern lake records. During the MCA, lake level was likely higher as a result of increased precipitation. While during the LIA, lake level was lower, suggesting drought was more common.