Observations of abnormally cool temperatures along the sea floor are an indication of why Karenia brevis (K. brevis), the organism that causes red tide, increased and remains a threat along Florida’s west coast, according to preliminary results from a three-week mission led by the University of South Florida College of Marine Science.
Marine scientists from USF and Florida Fish and Wildlife Conservation Commission (FWC) recently deployed an autonomous robotic glider to the northwest of Clearwater Beach and closely monitored it as it traversed along the middle of the continental shelf between Pasco County and Sarasota County. Traveling along a zig-zag path between 25 and 50 miles offshore, the glider covered the region where red tide is thought to originate.
The unusually cool bottom water and its increased salt content are both indicative of water being upwelled from the deeper Gulf of Mexico and transported toward the shore along the near seafloor. This comes after the entire west Florida continental shelf was thrown into an upwelling circulation in July. That’s when the Gulf of Mexico Loop Current shed an eddy and retreated to the southeast, coming in contact with the shelf slope near the Dry Tortugas.
“The persistence of the upwelling circulation through the present time also explains whyK. breviscells were eventually transported around the Florida Keys to the east coast of Florida, thereby causing the present 2018 red tide outbreak to cover three regions, the west coast of Florida, the Panhandle and the east coast of Florida,” said Robert Weisberg, PhD, Distinguished University Professor of Physical Oceanography.
Within these cooler, saltier near bottom waters, the glider’s sensors also detected elevated levels of chlorophyll and reduced levels of oxygen. While the glider is incapable of directly measuringK. brevis, the data collected does indicate its presence, and more recent water samples tend to confirm this.
By tracking the ocean circulation on the west Florida continental shelf, as driven both by winds and the Loop Current, Weisberg and his colleagues have been able to account for the occurrence or lack of occurrence of major red tide blooms for 20 out of the past 25 years. With this experience, they predicted in June that 2018 would be a particularly bad year for red tide. In addition to the conditions being conducive for a spring 2018 bloom formation, the bloom from 2017 remained present south of Venice, Florida. The upwelling circulation that began toward the end of July then brought the new bloom to the near shore, adding to what was already there.
Red tide is a naturally occurring phenomenon. Some years are worse than others, depending on the water properties of the coastal ocean in which are determined mostly by its circulation. Regularly sampling the water properties of the west Florida shelf, using gliders and other means are essential for a better understanding of Florida’s coastal ocean ecology and our ability to predict the consequences of either human-induced or natural changes.