Harmful Algal Bloom Visual Reports and Algal Toxin Data in the Illinois River Basin, 2005 - 2025

Harmful algal blooms (HABs) are characterized by excessive growth of algae. In freshwater environments, algal blooms, including those caused by cyanobacteria, can produce toxins that may impact human health, wildlife, and the environment. HABs are a growing public health concern but they remain difficult to predict due to the complex conditions driving their formation. This data release contains harmonized data for two types of information: visual bloom reports and algal toxin data. Both types of information are used to support model development and better understanding of HABs in the Illinois River Basin (ILRB). Visual HAB reports are contained in IEPAHABsReportCompilation.csv. These are event response data from credible HABs report forms submitted by the public or Illinois Environmental Protection Agency (IEPA) staff to investigate potential cyanobacteria blooms. In 2023, a copy of the HAB report forms from 2013-2022 were received after a Freedom of Information Act (FOIA) was submitted to the IEPA. In 2026, a second FOIA request was submitted to the IEPA requesting reports after 2022. The data from these reports were harmonized into common columns and compiled into a shared format. This dataset covers the state of Illinois and contains location data for all suspected and confirmed algal blooms, as well as identifies when events occurred within the ILRB. Algal toxin data for Anatoxin-a, Cylindrospermopsin, Saxitoxin, and Microcystin are contained in algaltoxins.csv and includes data from multiple sources. Data received from the IEPA via the aforementioned FOIAs also included algal toxin data from event response and routine monitoring of public water supply intakes and lake beaches in Illinois. These data were supplemented with additional algal toxin data collected in the Indiana and Wisconsin counties that overlap with the ILRB. Supplemental data were sourced from the Water Quality Portal (WQP; NWQMC, 2026), United States Geological Survey (USGS) Water Data for the Nation (WDFN; USGS, 2026), and a USGS Open-File Report on cyanotoxin data in Illinois (Terrio and others, 2013). Data from the WQP and WDFN were downloaded programmatically. All data were harmonized into a shared format. Lastly, a site information file (algaltoxins_site_info.csv) is included that contains site identification information and location data for all sites present in the algal toxin data.

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