The variations in optical properties of CDOM throughout an algal bloom event

For studies in bio-optical oceanography, visible light properties by classes of dissolved organic matter should be characterized. The regional adjustment of model parameters is one approach which is being widely used to refine bio-optical models. In the present study, buoy and laboratory data were acquired, and during the 15-day observation period an algal bloom event occurred. The absorption coefficient of CDOM at 443 nm, ag(443), changed in the range of 0.09–0.35 m−1 and 0.1–0.34 m−1 for two depths, 0.32 m and 2.3 m., respectively, throughout the entire period. CDOM absorption was larger for bloom conditions than for non-bloom conditions. In addition, the fraction of CDOM in total absorption was higher during the bloom event than that of non-bloom conditions. The spectral slope of CDOM absorption, Sg, regressed over 400–500 nm, ranged from 0.015 to 0.0185 nm−1 with an average of 0.0166 nm−1. CDOM fluorescence intensity (Fcdom) was obtained using an internal Raman standard and varied over the range of 467.44–1538.23 in relative units. Variations in Fcdom showed a similar pattern to that of CDOM absorption. A robust non-linear relationship between Fcdom and CDOM absorption was found, with a correlation coefficient of 0.893, throughout the whole observation. An attempt to describe Sg with absorption showed a promising consequence that can be described with a negative correlation during the bloom, however, without any distinguishable tendency for non-bloom time span. A two-band ratio algorithm was also carried out for retrieving CDOM absorption. The variations in optical properties of CDOM may be related to the complicated environment in the estuarine waters, which may result from different sources of CDOM.