Insights from pollen, non-pollen palynomorphs and testate amoebae into the evolution of Lake Simcoe

Abstract

Comparing the distribution of little-known non-pollen palynomorphs (NPP) with pollen and testate amoebae (thecamoebians) in a long piston core from the main basin of Lake Simcoe illustrates their potential as paleolimnological indicators. Sediments rich in spruce and pine pollen in core LS07 PC5 contain sparse, low-diversity microfossil assemblages recording cold, oligotrophic waters (mainly the testate amoeba Difflugia oblonga, the desmid genus Cosmarium, and Pediastrum integrum). Marked climate warming reconstructed from pollen data using transfer functions is associated with a sharp increase in algal NPP abundance and diversity, including Botryococcus spp., the desmid genus Staurastrum, and Pediastrum simplex. A Centropyxis-dominated testate amoeba fauna, and sparse, low-diversity NPP record slightly brackish conditions attributed to the early Holocene drought. Diverse benthic desmid and Pediastrum spp. (including P. brevicorne) floras, together with abundant Botryococcus, record warm, mesotrophic waters associated with a hemlock-maple-beech pollen assemblage from which January temperatures several degrees warmer than today were reconstructed. A decline in NPP abundance and diversity and the re-establishment of a Centropyxis-dominated testate amoeba fauna supports drought as a cause for the regional “hemlock decline”. The most marked change in aquatic microfossil assemblages coincides with anthropogenic impact. The appearance of dinoflagellate cysts and a marked increase in difflugiid testate amoebae (notably Cucurbitella tricuspis) records eutrophication below the ragweed rise that is attributed to the agricultural activities of the Wendat Nation. Two phases of anthropogenic impact are distinguished within the ragweed zone: (1) an NPP assemblage nearly devoid of desmids but rich in Peridinium wisconsinense and Pediastrum boryanum, attributed to Euro-Canadian land clearing and agriculture, and (2) an assemblage rich in Botryococcus, Peridinium willei/P. volzii, Codonella cratera, and meso-eutrophic desmids with planktonic life habits, is attributed to urbanization and industrialization.