Detection of Red Tides Using the Red Shifts from Meris 681 Nm Peak to 709 Nm Peak: a Case Study in the East China Sea

Detection of red tides (harmful algal blooms or HABs) from space has been problematic in optically complex waters, mainly due to interference from non-pigment constituents to the satellite signal. Using in situ and MERIS (Medium Resolution Imaging Spectrometer) measurements, the spectral characteristics of red tide and non-red tide waters were studied in the East China Sea coastal area. Two surveys were carried out in the spring of 2004 and 2005 respectively, when red tides often occur in the East China Sea. The in situ data showed a 700 nm peak in the surface reflectance spectrum when red tides occurred. Combined with this information, MERIS 681 nm and 709nm bands are used to detect a red tide event in June 2005. Using red tide information from the State Oceanic Administration (SOA) of China as the ground truth, we found that when red tide occurred, values of Maximum Chlorophyll Index (MCI, based on the 709-nm band) were high. Therefore it can be used as an effective tool for red tides detection. Notably, it is found that values of Fluorescence Line Height (FLH, based on the 681nm band) are low while values of MCI are high and ‘red shifts’ from 681 nm fluorescence peak to 709 nm ‘red tide’ peak for highly productive waters. These results provide valuable information that may be used to monitor red tides