The Coastal Ocean Circulation Influence on the 2018 West Florida Shelf K. brevis Red Tide Bloom

Blooms of the harmful alga, Karenia brevis on the west Florida continental shelf are thought to initiate offshore before manifesting as a nuisance along the coastline. Contributing to such blooms are a complex sequence of events occurring within oligotrophic waters, which in any given year may or may not be facilitated by the ocean circulation. Once initiation occurs, the delivery from the region of offshore origination to the region of coastline manifestation requires an upwelling circulation, whereby K. brevis cells are advected shoreward along the bottom. The 2018 K. brevis bloom was particularly intense owing to cells from the preceding 2017 bloom being reinforced by a newly formed bloom in 2018, a year when the offshore conditions in spring through early summer were again favorable for bloom development. As an event response to determine the potential for new cells to be delivered to the shore, a glider was deployed from 24 August 2018 to 17 September 2018 with a track line designed to map water properties over the hypothesized initiation region. The coastal ocean circulation during the deployment interval was generally upwelling favorable, but the passage of Tropical Storm Gordon temporarily disrupted this flow, after which K. brevis appeared along the Florida Panhandle coast. Strong upwelling then reestablished and K. brevis was subsequently observed along Florida's east coast. We describe the glider deployment, the K. brevis observations, and we use a numerical circulation model to account for the K. brevis manifestation as occurred along Florida's west, Panhandle, and east coasts. Plain Language Summary We account for the intensity and location of the 2018 Karenia brevis red tide outbreak on the west Florida continental shelf by a combination of water property observations and numerical circulation model simulations. These confirm the initiation region being offshore, the manifestation region being along the shoreline and the delivery mechanism (from initiation to manifestation) being an upwelling favorable coastal ocean circulation. The intensity is attributed to the cells remaining in the manifestation region from the prior 2017 bloom being reinforced by cells newly formed offshore in 2018. Key Points The results support the hypothesis that K. brevis red tides originate offshore Manifestation along the shoreline as a toxic bloom occurs via an upwelling circulation The intensity of the 2018 west Florida shelf red tide was due to a new 2018 event