It seems likely that the occurrence of a discrete mass of water, with a salinity lower than that of normal Gulf of Mexico surface water, is a necessary prerequisite for the occurrence of red tide off the Florida Coast.
Slobodkin, L. B.. 1953. "A possible initial condition for red tides on the coast of Florida." Journal of Marine Research 12, (1). https://elischolar.library.yale.edu/journal_of_marine_research/789