Spectral absorption properties of dissolved and particulate matter in Lake Erie

Spectral absorption properties of particulate and dissolved matter were determined for Lake Erie waters in order to investigate the natural variability of the absorption coefficients required as inputs to optical models for converting satellite observations of water colour into water quality information. Particulate absorption measured using the quantitative filter technique yielded absorption spectra containing a fraction that could not be attributed to phytoplankton pigments, living heterotrophs, mineral sediments, or organic detritus but were indicative of additional absorption by a fraction of dissolved organic matter present in colloidal and/or particle-bound form. Erroneously high phytoplankton absorption coefficients measured at short wavelengths using the filter technique suggested that this particle-bound DOM is removed along with phytoplankton pigments during bleaching by sodium hypochlorite and as such is mistakenly incorporated into the phytoplankton absorption signal. Observations suggest that the selective sorption of fractions of DOM onto suspended particles may be responsible for significant variability in the absorption coefficients of particulate and dissolved matter and may be an important contributor to the total spectral absorption signals in Lake Erie waters. This reservoir of coloured organic matter, which to date has not been seriously considered in the optical properties of coastal and inland waters, may produce significant uncertainties in the parameterization of optical models and the interpretation of in situ and remotely sensed aquatic colour signals.