In 2014, a harmful algae bloom shut off drinking water for nearly half a million people in Toledo, Ohio. The city spent 65 million dollars responding. They've invested half a billion dollars in upgrades since. And every summer, the blooms come back.

Harmful algal blooms are getting worse. And the reason why is more complicated than most people realize, and more alarming than most officials or lake managers are admitting.

In this episode, environmental scientist and host Ally breaks down the full science of toxic algae blooms to help you think like a scientist, including the ancient biology behind them, how phosphorus fuels toxic growth, how lake stratification and turnover distribute nutrients through the water column, and why new peer reviewed research suggests that even lakes with reduced pollution are still experiencing blooms.

This is the science behind the headlines, explained.

References:

• Nürnberg GK. (2025). Importance of considering internal phosphorus loading during climate change. Lake and Reservoir Management, 41:3, 165–179.
• Jane SF et al. (2023). Longer duration of seasonal stratification contributes to widespread increases in lake hypoxia and anoxia. Global Change Biology, 29(4), 1009–1023.
• Harrison JW et al. (2025). Hypolimnetic photosynthesis precedes a Microcystis bloom in a temperate, oligo-mesotrophic reservoir. Lake and Reservoir Management, 41:194–209.
• Meyers K et al. (2025). National forecasting of cyanobacterial harmful algal bloom events: a 3-year model evaluation. Lake and Reservoir Management.
• Chaffin JD et al. (2023). Microcystin congeners in Lake Erie follow the seasonal pattern of nitrogen availability. Harmful Algae, 127, 102466.
• Taranu ZE et al. (2014). Nitrogen forms influence microcystin concentration and composition via changes in cyanobacterial community structure. PLOS ONE.
• NOAA/NCCOS ongoing research: Evaluating the Effects of Nitrogen Form and Concentration on Toxin Phenotypes of Microcystis. coastalscience.noaa.gov
• CDC MMWR report: McCarty CL et al. (2016). Community Needs Assessment After Microcystin Toxin Contamination of a Municipal Water Supply — Lucas County, Ohio. MMWR, 65(35):925–929.
• Schopf JW. The Fossil Record of Cyanobacteria. In: Whitton BA, editor. Ecology of Cyanobacteria II. Springer; 2012. — 3.5 billion year stromatolite evidence
• Gueneli N et al. (2024). Oldest thylakoids in fossil cells directly evidence oxygenic photosynthesis. Nature. — 2.1 billion year undisputed confirmed fossil
• CNN. (August 3, 2014). 400,000 in Toledo, Ohio, water scare await test results. CNN.com
• Circle of Blue. (2015). The Toledo Water Crisis, One Year Later. circleofblue.org
• The Statehouse News Bureau. (August 2, 2024). A decade ago, Toledo lost access to its water. statenews.org
• University of Toledo. (July 15, 2024). 10 Years After Water Crisis, UToledo Researchers Remain Committed to Protecting Region's Drinking Water.
• Chaffin JD, Westrick JA, Reitz LA, Bridgeman TB. (2023). Microcystin congeners in Lake Erie follow the seasonal pattern of nitrogen availability. Harmful Algae, 127, 102466. DOI: 10.1016/j.hal.2023.102466. — nitrogen availability influences microcystin congener composition and therefore bloom toxicity; more toxic forms dominate late season as nitrogen depletes
• Lisboa, MS, RL Schneider, LG Rudstam, MT Walter. 2025. Groundwater phosphorus contributions comparable to tributaries in a large, mesotrophic, polymictic lake. Science of the Total Environment. 1008:180978. doi.org/10.1016/j.scitotenv.2025.180978