Seasonal Occurrence of Red Tide (SORT) on the West Florida Shelf: from Observations to Dynamics

Abstract:

The West Florida Shelf (WFS) is known for a regularly occurring red tide, which contains toxic dinoflagellate Karenia brevis. These harmful algal blooms produce hazardous brevetoxins which adversely affect marine and human life, as well as the fishing industry. Decades of research has been focused on seasonal occurrence of the red tide, suggesting a viable mechanism of an offshore bloom initiation followed by transport to the coast. K. brevis migrate vertically in the water column and are adapted for low-light, low-nutrient conditions, so in order for Karenia to out compete other fast-growing species in oligotrophic water, it must interact with and take advantage of shelf physics. The WFS features dynamical circulation influenced by seasonal winds and the Loop Current. What remains unaddressed are (1) whether physical conditions allow for the formation of such an offshore niche, and (2) whether the time scale needed for bloom formation matches the physical transport. This study offers an integrated investigation of the winds and tide gauge observations, the satellite altimetry and derived currents, and long-term red tide monitoring dataset over the last two decades. The data-based analysis resolves a key period of each year on the WFS where a bottom convergence zone can form in waters between 50 and 100 m isobaths near 28^oN. This zone results from the coexistence of bottom Ekman upwelling in the outer shelf and the wind-driven downwelling in the inner shelf, and usually appears 1-2 months before the red tide appears near the coast. As a result, an interannual pattern emerges: the presence (absence) of this zone is in line with years of strong (weak/no) fall red tide along the WFS. The result confirms the existence of an offshore niche for seed populations of K. brevis, and the dynamical explanation has important implications for red tide prediction.