Multiple Harmful Algae Bloom (HAB) (K. brevis) data sets were obtained for this data layer, including Harmful Algal BloomS Observing System data (HABSOS) from NCEI (1953-2018) and the NOAA HAB Operational Forecast System Dataset (2007-2018). Data includes samples from Texas Parks and Wildlife Department (TPWD) the Louisiana Department of Health and Hospitals, Florida Fish and Wildlife Conservation Commission, Florida Fish and Wildlife Research Institute (FWRI), and the HAB Monitoring Database. Data were combined and reviewed for erroneous entries. Duplicate data from the resulting database were deleted, leaving a total of 155,307 observations. For the purpose of this K. brevis layer, only data from 2000-2018 were used due to sampling biases. An 8 square kilometer hexagonal grid was created covering the Gulf of Mexico and eastern Florida. Each year between 2000-2018 was evaluated separately. If a grid cell contained an observation that registered a value of over 100,000 cells/L (the lethal toxicity level for finfish), the grid cell received a value of '1‘. All other grid cells received a value of '0’ to represent observations below the lethal level of concern to fish. After all years were evaluated, the values for each year were totaled to obtain a resulting frequency value for that grid cell, representing the number of years that grid cell had values greater than or equal to 100,000 cells/L for the years 2000-2018.