Karenia brevis on the West Florida Shelf: MODIS Satellite Tracking of Bloom Dynamics Following Hurricanes Helene and Milton

Karenia brevis, a harmful algal bloom (HAB) organism behind Florida’s red tide, persists in the Gulf of Mexico year-round at low concentrations. It can rapidly multiply and cause toxic outbreaks that harm marine life and human respiratory health. Red tide has been observed to suppress or intensify following tropical cyclones. Hurricanes Helene and Milton made landfall on Florida’s Gulf Coast 13 days apart in fall 2024, whereafter K. brevis proliferated along southwest Florida, with cell concentrations reaching millions of cells per liter. Remote sensing (MODIS Aqua ocean-color and sea surface temperature, 2024–2025 hurricane seasons), in situ, and hurricane best-track/reanalysis data were examined with time series analysis, statistical analysis, and spatial analysis. We found a hurricane-associated cascade within 18 days of Milton’s landfall: wind forcing, then river discharge, then chlorophyll a and K. brevis peaks, followed ~6 weeks later by another K. brevis peak within ~2% of the near-term peak in magnitude. These findings indicate that elevated wind speed and river discharge, used as a proxy for potential nutrient delivery, during and after hurricane passage are associated with a long-term K. brevis response.