Experimental calibration of lake-sediment spectral reflectance to chlorophyll a concentrations:: methodology and paleolimnological validation

Chlorophyll a preserved in lake sediments reflects, in part, past primary production. This study assesses the spectral properties of sedimentary chlorophyll a using visible-near infrared reflectance (VNIR) spectroscopy, with the objective of establishing a new, non-destructive paleolimnological proxy. Reflectance spectra were determined from a dilution series (n = 10) involving incremental additions of pulverized modern algae to a lake sediment matrix of low organic content. This enabled an assessment of the development of sediment reflectance spectra in relation to different sediment chlorophyll a concentrations, and subsequent regression of spectral features against measured concentrations of chlorophyll a and derivatives obtained by high performance liquid chromatography ( HPLC). The experiment demonstrates that ubiquitous troughs in sediment reflectance near 675 nm are attributable to chlorophyll a and derivative compounds. A significant correlation (r(2) = 0.98, P < 0.01) was obtained between the area of the reflectance trough in the 650-700 nm interval and summed concentrations of chlorophyll a, all derivative isomers, and degradational pheopigments. A simple linear inference model derived from this experiment was applied to a downcore sequence of VNIR spectra from a productive prairie lake (Alberta, Canada), where it produced inferred sediment chlorophyll a concentrations in concordance with HPLC measurements. Although a larger training set is desirable to further refine the inference model, the analyses reported here demonstrate that reflectance spectroscopy provides a rapid, semi-quantitative method for assessing the chlorophyll a content of lake sediments.