Phytoplankton response to the environmental and climatic variability in a temperate lake over the last 14,500 years in eastern Latvia

Abstract

Phytoplankton species are the primary producers in lakes and play important roles in food-web structures. Any shift in their diversity and productivity has an impact on other aquatic life forms. We use a range of environmental variables to explore the possible drivers influencing phytoplankton composition over the last 14,500 years in a temperate lake Lielais Svētiņu, eastern Latvia. Using pollen, non-pollen palynomorphs, temperature reconstructions and lithological information as proxies of environmental factors, we statistically test their associations with the fossil phytoplankton community composition. Our results reveal that during the Late Glacial, the climate warming, the decrease in landscape openness, and increase in organic matter were significant environmental variables affecting dynamics of phytoplankton communities , especially in the prevalence of Botryococcus, Tetraedron, Scenedesmus and Pediastrum. According to the Redundancy Analysis and Generalized Least Squares models, Pediastrum, Scenedesmus and Tetraedron were positively associated with waterlogging tolerance that indicates moist soils in surroundings of the lake, during the Early Holocene. The 8.2 ka cold event with a 2–3 °C cooling led to a strong environmental disturbance for nearly 700 years, indicated by an increased chlorophyta accumulation rates and a decrease in the organic matter. Our results indicate that Coelastrum reticulatum and C. polychordum are characteristic for the 8.2 ka cold event. Positive association between cyanobacteria and mean air summer temperature suggests that a warming favoured cyanobacteria over other phytoplankton taxa between 8000 and 4000 cal yr BP. High nutrient loads and water turbidity were more important for the dynamics of cyanobacteria from 4000 to 2000 cal yr BP. Human-driven trophic level change was recorded in the last 2000 years by abundances of fungi Sporormiella and Sordaria, and by the peaks of Gloeotrichia pisum, C. reticulatum and C. polychordum indicating eutrophication.