ARTICLES 464 Chinese Science Bulletin Vol. 51 No. 4 February 2006 Chinese Science Bulletin 2006 Vol. 51 No. 4 464—471 DOI: 10.1007/s11434-006-0464-2 Remote sensing of algal blooms using a turbidity-free function for near-infrared and red signals LI Yan, SHANG Shaoling, ZHANG Caiyun, MA Xiaoxin, HUANG Liwei, WU Jingyu & ZENG Yindong State Key Laboratory of Marine Environmental Science, Xiamen Uni- versity, Xiamen 361005, China Correspondence should be addressed to Li Yan (email: liyan@xmu. edu.cn) Abstract This article presents a method for real- time mapping of algal blooms in turbid coastal waters using the remote sensing reflectance of red band (Channel 1, 580ˉ680 nm) and near-infrared band (Channel 2, 720ˉ1100 nm) of the AVHRR sensor on the NOAA series satellites. A turbidity-free function for near-infrared and red signals, α 0 = (b b (1) /b b (2) )(a (2) / a (1) ) based on the first order b b /(a+b b ) model deduc- ing equation R rs (2)−1 = α 0 R rs (1) −1 + g −1 (1−α 0 ), were se- lected as a chlorophyll-a related index for detecting algal blooms, and the algal blooms with chlorophyll-a concentration of 64ˉ256 mg/L could be defined by window of 1.6 < α 0 < 5.2 and 0.01< R rs (2) /g < 0.2. Such turbidity-free two-band method is supported by both sea-truth data and remote sensing experiment for an algal blooms event on the near-shore water off the Minjiang estuary of southeastern China during early June of 2003. Comparisons of this algorithm with other published algorithms, one-band method (i.e. method of bright water) or two-band methods (i.e. method of ratio, method of NDVI, and method of subtracting) have suggested that the turbidity-free function method could be regarded as a standard algorithm in capabilities of AVHRR imagery or other high resolution but wide near-infrared and red band imagery for detecting algal blooms events in coastal waters. Keywords: remote sensing, marine optics, algal bloom, algorithm, estuary. Harmful algal bloom acts upon important environ- mental problems in coastal waters. A unified and im- plementing method for survey and monitoring algal blooms has been developed since last decade. However there is a data gap of about tens years in the period with rapid increasing of algal blooms events. The 20 years AVHRR imagery on the NOAA series of weather satel- lites seems to be a main source to mining the data of algal blooms within that gap. During last decade, most people engaged in remote sensing paid a more attention to the algorithm to detect algal bloom using signals of red band (Channel 1, 580ˉ680 nm) and near-infrared (NIR) band (Channel 2, 720ˉ1100 nm) of AVHRR data [1ˉ10] . They developed a series of algorithms based on one-band or two-band information, i. e, method of bright water, method of ratio, method of normalized difference vegetation index (NDVI) and method of subtracting. Unfortunately it is difficult to find out a robust method for retrieving algal blooms information from both turbid and less turbid coastal waters. Based on the former studies and our observations for the algal bloom event on the near-shore water off the Minjiang estuary of southeastern China during early June of 2003, we are trying to set up and verify a tur- bidity-free algorithm for algal blooms detecting using AVHRR style wide bands along with the well-known bio-optical understandings. 1 Reflectance spectra for algal bloom waters Additional to a background of suspended particles backscatter, almost all the above water reflectance spectra of algal blooms are characterized by two spec- tral signatures for absortion and one spectral signature for reflection [7,9,11,12] . (1) Spectral signatures for absortion near 440 nm. There is a positive relationship between the absortion coefficient at 440nm and chlorophyll-a concentration. (2) Spectral signatures for absortion near 670 nm. There is also a positive relationship between the absor- tion coefficient at 670 nm and chlorophyll-a concentra- tion. (3) Spectral signatures for a backscatter or fluores- cence in the range from 680 nm to 740 nm. The strong signal near 683 nm referred to the contribution of re- emission of light of chlorophyll-a and was widely used in remote sensing to map chlorophyll-a [11,12] . Such strong signal could be shifted toward the direction of long wave while the algae become denser or concen- trated to the near surface layer. There is a positive rela- tionship between chlorophyll-a concentration and the wave lengthen of peak in reflectance spectrum [13,14] . Visual observations of brownish-red discoloration of