High Resolution Satellite Data reveals Massive Export of Carbon and Nitrogen-Rich Seagrass Wrack from Greater Florida Bay to the Open Ocean after Hurricane Irma

Episodic storms are known to be important drivers of ocean ecosystem processes, but the impacts are notoriously difficult to quantify with traditional sampling techniques. Here, we use stunning high spatial resolution satellite imagery from Sentinel 2A collected 13 September 2017, only days after Hurricane Irma passed directly over the Florida Keys, to quantify massive amounts of floating vegetative material. This Category 4 storm passed directly over the Florida Keys, bringing wind gusts over 35 m s-1 and creating turbulence in the water column that scoured the seafloor. The imagery reveals as initial estimate of 40 km2 of surface drifting material. Although the identity of the brown material cannot be fully determined without a hyperspectral sensor, the accumulations are consistent with our past research showing large aggregations of seagrass leaves or "wrack" advected under high winds from dense beds of Syringodium filiforme within Greater Florida Bay to the oceanic waters of the Atlantic. Using measurements of wrack collected from this area, we estimate that this single event corresponds to a total export of 9.7 x 1010 gC and 2.7 x 109 gN from the seagrass beds. This high amount of export is not considered typical for many types of tropical seagrass meadows that are thought to highly recycle nutrients within the beds. Elemental analysis of seagrass leaves from Greater Florida Bay is consistent with nitrogen-fixation in the beds, which could provide the means to sustain a large export of nitrogen from the meadows. As the wrack travels at the sea surface, some of these nutrients are exuded into the surrounding waters providing a nutrient subsidy of dissolved and particulate carbon and nitrogen and making the wrack an ecological hot spot for organisms. Although wrack can potentially remain floating for months, the ultimate fate of the wrack is to either wash ashore, providing connectivity between marine and terrestrial ecosystems, or sink to the seafloor. If most of the wrack sinks, this single localized event represents 0.3-0.7% of the average daily carbon export of phytoplankton to the seafloor for the entire ocean (5-12 Pg C yr-1). New earth-observing satellite technology is allowing for more detailed analyses of the important role of episodic events in shaping aquatic ecology and influencing global biogeochemistry.