On August 30, 2023, Hurricane Idalia made landfall as a major Hurricane in Florida’s Big Bend after meandering through the northwestern Caribbean and intensifying over the warm waters of the Gulf of America. In its aftermath, damage on land was immediately visible, but observations from satellites and ocean-going robots revealed that the ocean was also profoundly altered–from its surface waters to its deeper layers.
A new study led by Jennifer McWhorter, a University of Miami Cooperative Institute for Marine and Atmospheric Studies Assistant Scientist, bridges scientific expertise across NOAA’s Atlantic Oceanographic and Meteorological Laboratory (AOML) to characterize the biogeochemical impacts of Idalia on the eastern Gulf’s layers and the growth of phytoplankton, commonly called primary production. While satellites can show that phytoplankton and chlorophyll bloom after hurricanes pass, our understanding of the subsurface biogeochemical landscape is limited. The findings offer a three-dimensional view of how a powerful storm alters ocean productivity and biogeochemistry.
To capture Idalia’s impact, the research team paired satellite observations with a saildrone at the sea surface and a Biogeochemical Argo float (BGC-Argo) in deeper waters. Each platform revealed how an interplay between features like the Mississippi River plume, the Loop Current, and upwellings contributed to the ocean’s biogeochemical responses in the wake of Idalia.
The Gulf is shaped by the Loop Current, a flow of warm Caribbean water that travels northward past the Yucatan Peninsula, curves through the eastern Gulf, and exits through the Straits of Florida. Feeding into the northern extension of this system is the Mississippi River plume, a vast outflow of nutrient-rich freshwater. Because freshwater is less dense than seawater, the plume tends to spread across the surface, forming a stratified ocean that resists vertical mixing.
At the same time, the Gulf is dotted with cyclonic eddies which are spinning masses of water with cooler centers that naturally pull nutrient-rich water upward from the depths–a phenomenon known as upwelling. Where nutrients rise, microscopic marine plants called phytoplankton thrive, forming the base of the ocean food web.
During the passage of a hurricane, strong vertical ocean mixing is common, but in analyzing the satellite data and ocean observations post-Idalia, the team found that the Mississippi River plume, rotational eddies, and Loop Current all played significant roles in how the Gulf responded to the storm.
The Mississippi River plume helped create a surface algae bloom by spreading chlorophyll sideways and preventing significant vertical seawater mixing (due to density differences). Meanwhile, Idalia’s powerful winds interacted with a nearby cyclonic eddy, intensifying its natural upwelling. This process seeded nitrate, a key nutrient for phytoplankton growth, into layers of the ocean roughly 20 to 50 meters below the surface. This upwelling of nutrients ultimately fueled a secondary, subsurface phytoplankton bloom, invisible to satellites, but detectable by the BGC-Argo.
“The deviation from the 10-day BGC-Argo mission to an 18-hour profiling frequency enabled our understanding of the biogeochemical changes under these unique conditions and highlights the importance of coupled ocean observations from the surface, subsurface, and from satellites,” said McWhorter.
Primary production underpins marine food webs, supports fisheries, and plays a crucial role in how the ocean absorbs carbon dioxide from the atmosphere. By showing how hurricanes can stimulate productivity at different depths depending on pre-existing ocean conditions, this study highlights why surface observations alone are not enough. Studies like this demonstrate the power of combining satellites with autonomous ocean robots to reveal how storms can rapidly reshape the ocean’s biological and chemical landscape, from the surface all the way to the deep.