Red tides - harmful algal blooms - occur when microscopic algae multiply to higher-than-normal concentrations.
Red tides occur naturally every year from off shores, then propagates to shores which can bloom quickly due to rain water runoffs from land, bringing nutrients to the algae.
In some years, the bloom did not occur such as in 2019 and 2020. Some other years, the bloom was terrible. The worst blooms occurred in the summer/fall of 2021.
March 19, 2023: Good news: red tide is disappearing.
Map of current red-tide:
https://myfwc.maps.arcgis.com/apps/View/index.html?appid=87162eec3eb846218cec711d16462a72
Red tides may kill fish and cause respiratory irritation to persons with preexisting respiratory problems (asthma, etc.)