Using a Gaussian decomposition approach to model absorption spectra of chromophoric dissolved organic matter

Abstract

The chromophoric dissolved organic matter (CDOM) is a significant water constituent influencing inherent and apparent optical properties of natural waters and plays a key role in ecosystem functioning. The spectral slope (S) describing the approximate exponential decline in CDOM absorption with increasing wavelength is widely used for tracing changes in the chemical composition of CDOM. The currently accepted method of characterizing CDOM absorption (i.e., fitting a simple exponential model) can lead to loss of information and large errors. We propose a better method for modeling CDOM absorption spectra based on a Gaussian decomposition approach that removes the errors associated with the choice of the spectral range used to estimate S. Using artificially generated spectra with known parameters (n=1000), we show that our method provides robust estimates of S closely resembling the original values. On average, the error on S estimations was 0.16% for the proposed method compared to 27% and 11% for the traditional modeling approaches fitted over 300–700nm and 240–700nm respectively. We further demonstrate the ability of the method to decompose and model chromophores present in complex spectra from oceanic water samples from around the world. The proposed method opens avenues for long-term or cross-site comparison studies of the dynamics of the CDOM pool and constitutes a promising supplement to techniques based on CDOM fluorescence.