Reconstructions of Aquatic Primary Producer Dynamics, Cyanobacteria Dominance and Cyanotoxin Concentrations in Subtropical Lake Ecosystems Throughout the Holocene and Late Pleistocene

Abstract

Harmful algal blooms (HABs) are intensifying globally, threatening freshwater ecosystems and public health. Although modern HABs have been extensively studied in the context of anthropogenic nutrient enrichment, the long-term ecological dynamics driving cyanobacteria dominance and cyanotoxin production remain underexplored. Lake sediments serve as natural archives of past environmental change, yet paleolimnological approaches have rarely been applied over millennial timescales in subtropical lake systems. This dissertation reconstructs the dynamics of primary producers, cyanobacteria dominance, and cyanotoxin concentrations throughout the Holocene and Late Pleistocene in four shallow lakes in the southeastern United States. By utilizing sedimentary photosynthetic pigments and cyanotoxins (total microcystins, MCs), it investigates the ecological patterns underlying long-term eutrophication and HAB development. Chapter 1 introduces the background and rationale for this research. Chapter 2 examines a ~6,900-year sediment record from Lake Wauberg, Florida, where natural phosphate geology led to millennial-scale enrichment of total phosphorus (TP) that sustained cyanobacteria. However, sharp increases in cyanobacteria and MCs only occurred once TP exceeded ~2.2 mg g⁻¹ in the past ~300 years. Chapter 3 investigates the deposition of MCs in two hypereutrophic Florida lakes, Dora and Marian, with sediment records spanning approximately 7,000 years. The results indicate that MC deposition has occurred throughout the history of these lakes, with its concentration varying over time and being primarily driven by nutrient levels, particularly TP, and primary producer composition, rather than climate. Chapter 4 analyzes a ~27,500-year record from Lake Waccamaw, North Carolina, demonstrating persistent cyanobacteria dominance and MC production under natural climate variability during both glacial and interglacial periods. Abrupt shifts in aquatic and terrestrial ecosystems during Interstadial 3 (~27.8-26.4 ka BP) and the early Holocene (~11.4-7 ka BP) underscore the roles of hydroclimatic forcing and nutrient stoichiometry in shaping the dynamics of aquatic primary producers. Together, these studies demonstrate the value of millennial-scale paleolimnological records in identifying ecological thresholds, understanding the natural baseline conditions of HABs, and contextualizing modern changes in lake systems.