Linking phosphorus dynamics with hypereutrophic conditions on the millennial scale: The paleolimnology of shallow and subtropical Lake Wauberg, Florida, USA

Eutrophication and harmful algal blooms pose a global challenge to water quality and ecosystem services. Whereas eutrophication has been linked to nutrient additions in conjunction with human activities, much less is known about water quality trends when nutrient additions persist for centuries or millennia. Here, we used paleolimnological techniques to reconstruct eutrophication and cyanobacteria dynamics in Lake Wauberg, FL, USA, a lake that has experienced millennial-scale nutrient additions from natural phosphate geology. We measured photosynthetic pigments, cyanotoxins, and nutrient concentrations on a sediment core spanning the last ~6900 years. Our primary hypothesis is that the long-term total phosphorus (TP) additions caused constant cyanobacteria dominance throughout the entire history of the lake. Focusing on the last 5 ka BP with constant lake conditions, photosynthetic pigments and cyanotoxins demonstrated a strong positive relationship with TP over other nutrients. By dividing TP inputs into three levels, primary producers positively increased with low TP inputs but showed no change under moderate levels. Under high (2.2–3 mg g⁻¹) and extreme (>3 mg g⁻¹) TP sedimentary concentrations over the last 0.3 ka BP, substantial increases in cyanobacteria abundance, rapid production of microcystins (MCs), and a possible shift to N-fixation occurred. These data show that chronic and additive TP inputs can produce asynchronous responses in the primary producer community and MC concentrations with substantial increases occurring at higher TP thresholds. Linking the historic ecological response to TP periods with current limnological conditions could provide new directions in forecasting and managing aquatic ecosystems that experience chronic TP inputs.