Detection of red tides in the Southwestern Florida coastal region using ocean color data

Near real-time ocean color data from the Seaviewing Wide Field-of-view Sensor (SeaWiFS) was used to detect and trace harmful algal bloom (HAB) (also termed as red tides) in the Southwest Florida coastal water, which was treated as Case-2 water, i.e., its optical characteristics are influenced not only by phytoplankton and related particles, but also by other substances, that vary independently of phytoplankton, notably inorganic particles in suspension and yellow substances. Similar to Ahn et al. (2006), a red tide index was constructed from in-situ radiometric measurements of the three SeaWiFS bands centered at 411 nm, 510 nm, and 555 nm to achieve derivation of indices that are then related to absorbing characteristics of harmful algae (i.e., Lw at 443 nm) from which a best fit with a cubic polynomial function is obtained providing indices of higher ranges for HABs and lower and slightly reduced ranges for turbid and non-bloom water. In order to quantify the HABs in terms of chlorophyll (Chl), an empirical relationship is established between the RI and in-situ Chl in surface water which yields a Red tide index Chlorophyll Algorithm (RCA). In contrast, the band-ratio chlorophyll product of SeaWiFS in this complex coastal environment provided false information. The red tide that formed from November to December 2004 off SW Florida was revealed by RCA imagery, and was confirmed by field sampling to contain medium (10⁴ to 10⁵ cells L^-1) to high (>10⁵ cells L^-1) concentrations of the toxic Karenia brevis. The RCA imagery also showed that the bloom started in mid October south of Charlotte Harbor, and that it developed and moved to the south and southwest in the subsequent weeks. Our results show that the SeaWiFS data provides an unprecedented tool for research and managers to study and monitor algal blooms in coastal environments.