Tropical cyclones drive oxygen minimum zone shoaling and simultaneously alter organic matter production.

Abstract

Tropical cyclones regularly form above the ocean's largest subsurface oxygen minimum zone (OMZ) in the eastern tropical North Pacific Ocean (ETNP), yet how these powerful storms affect this biogeochemically important region remains unknown. We captured multiple direct and potentially interactive oceanographic effects of a Category 4 hurricane (Bud) during a 2018 research cruise in the ETNP. Profiles and samples collected directly beneath Bud's wake revealed rapid OMZ shoaling of 29 to 50 meters, reaching depths as shallow as 41 meters. Untargeted mass spectrometry-based characterization of organic matter, along with elevated particulate organic carbon and chlorophyll concentrations, demonstrated production and accumulation of distinct organic compounds-including phytoplankton biomarkers-within a hurricane-generated phytoplankton bloom. 16<i>S</i> rRNA transcripts from active microbes were dominated by degraders of phytoplankton-derived organic matter near the surface and by anaerobic bacteria (including sulfate-reducing bacteria) within the shoaled OMZ-indicating rapid microbial responses. Tropical cyclones therefore severely disrupt OMZ biogeochemistry through vertical OMZ expansion and altered carbon cycling.