Optical characteristics of two contrasting Case 2 waters and their influence on remote sensing algorithms

This paper describes the results of measurements of inherent and apparent optical properties of two contrasting Case 2 waters (Southern Baltic and off the west coast of Ireland). The experiments were carried out over two seasons, both before the peak of the phytoplankton growth period, and during the bloom events. The first study was made when the concentration of chlorophyll (phytoplankton pigment) was similar in both waters and in the range of 0.6-3.23 mg m(-3). The second study was made when concentrations of chlorophyll were very high, reaching 14 mg m(-3) in the west coast of Ireland and 70 mg m(-3) in Southern Baltic. Optical measurements of surface reflectance were made with a prdfiling spectroradiometer. The spectral shapes of the particulate absorption spectra and the diffuse attenuation coefficient in both regions and seasons are compared. The contribution of detritus and coloured dissolved organic matter absorption to the apparent optical properties of water have impact on the accuracy of remote sensing retrieval algorithm for chlorophyll a. Differences have been found between algorithms based on conventional spectral bands (e.g. SeaWiFS) and proposed new spectral channels. The most accurate ratio for chlorophyll retrieval (490/550) was for the western Irish shelf (R-2=76%, and the standard error of the estimate ranged from 30% to 37%), with poor results for this ratio in the Baltic. The R-rs (550/590) ratio gave better results for the Baltic (R-2=75%, and the standard error of the estimate ranged from 20% to 55%), with poor results for the Irish shelf. The results show that a unique combination of spectral bands needs to be applied in Baltic waters to achieve an acceptable accuracy of the in-water remote sensing algorithm whereas the more commonly accepted band ratios were acceptable for the Irish Shelf. This suggests that for accurate determination of chlorophyll from satellite-borne sensors in different coastal waters a wider choice of spectral bands is needed. (C) 2003 Elsevier Science Ltd. All rights reserved.