THE NON-POLLEN PALYNOMORPH RECORD OF MEROMICTIC LAKES: VALUE IN STUDIES OF ANTHROPOGENIC IMPACT

Meromictic lakes, in which perennial stratification leads to bottom water anoxia that eliminates bioturbation, provide an ideal natural laboratory to study the response of non-pollen palynomorphs to cultural eutrophication. Accumulations of mineral and organic sediments that form annual countable layers in meromictic lakes provide a high-resolution chronology of lacustrine processes. A variety of algal palynomorphs, such as desmids (chrysophyte algae), dinoflagellates, and chlorophyte algae (e.g. Pediastrum and Botryococcus) were identified in varved sediments from Crawford Lake, Ontario (McCarthy and Krueger, 2013; Krueger, 2011) and Sluice Pond, Massachusetts (Drljepan et al. 2014; Hubeny et al., 2015). Exceptional preservation, including the preservation of cellulosic dinoflagellate thecae and diverse dinocysts at times of increasingly depressed bottom water dissolved oxygen concentration (DO) due to biochemical oxygen demand (BOD), provides unprecedented insights into the response of phytoplankton to documented phases of human settlement in the watersheds of both lakes. High concentrations of eutrophic, planktonic desmids (e.g. Staurastrum) in varves that developed in the deep basin of Sluice Pond following a sharp rise in Ambrosia (ragweed pollen) record increased nutrient loading. A reduction in benthic desmids over time suggests decreasing DO and clarity of the water column resulting from cultural eutrophication and siltation. Similarly, desmid concentrations increased rapidly in Crawford Lake with the onset of both periods of habitation in the region (Iroquoian village ca. AD 1268–1486 and Euro-Canadian settlers since AD 1822), but desmid species rapidly declined as nutrient loading continued. Further phycological and palynological work on these largely ignored non-pollen palynomorphs is encouraged.