Cyanotoxin Production in Subtropical Lakes over the Last 150 Years with Implications for Human and Ecosystem Health

Abstract

Lakes are experiencing a modern period of increased eutrophication due to cultural impacts and climate change. One common characteristic of lake eutrophication is the increase in cyanobacteria, which possess a competitive advantage over other phytoplankters. This competitive advantage often leads to cyanobacteria dominance and the formation of harmful algal blooms (HABs). HABs can confound ecosystem services through anoxia, biological stress, and the production of toxins, called cyanotoxins, that can be hazardous to ecosystem and human health. Cyanotoxin production within the water column has been widely monitored, however, less work has been done to ascertain the storage and fate of these cyanotoxins within the sediment. Here, I measured a suite of paleolimnological proxies on four sub-tropical lakes in central Florida, USA, to achieve 3 primary research objectives: 1) create a better understanding of cyanotoxin sediment storage over the past ~150 years, 2) determine the probability and ecosystem impacts from cyanotoxin resuspension, and 3) use the sediment record from Lake Carlton as a case study to identify how anthropogenic alterations to connected lakes affected phytoplankton community structure and cyanotoxin production. Sediment cores were collected from Lakes Bonny, Carlton, Marian, and Thonotosassa, and nutrients, photosynthetic pigments, and cyanotoxins were measured throughout each sediment record. Results show microcystins deposited in the greatest concentrations in the sediment over the last ~20 years, but were measurable throughout the sediment record and are not a novel phenomenon of the past few decades. Cyanotoxin storage amounts within each lake suggest that resuspension events from storms are capable of increasing water column toxin values exceeding recommended monitoring thresholds. Finally, Lake Carlton cyanobacteria and toxin dynamics demonstrated the large-scale change shallow lakes can experience in response to multiple human-related stressors. These results collectively show the importance of understanding sedimentary toxin dynamics in shallow lake systems.