Properties of humic acids as biomarkers of lake catchment management

Abstract.

The aim of this work was to determine the influence of catchment management on elemental composition, and thermal and spectrometric properties of humic acids isolated from littoral bottom sediments in lakes within agricultural and forest catchments. This study shows that there is significant influence of catchment management on properties of humic acids found in the bottom sediments of the littoral zone. Humic acids (HA) in sediments of the littoral zone in lakes in forested catchments had higher contents of carbon and oxygen and lower contents of hydrogen and nitrogen relative to HA samples in sediments in lakes dominated by agricultural fields in the catchment. Differences in elemental composition are also reflected by calculated values of atomic ratios. Lower values in H:C and N:C ratios and higher values in O:H ratios were found in HAs from sediments in forest lakes. Differential thermal analysis showed that HAs from sediments in lakes within forest catchments had higher values of parameter Z (which is proportional to the amount of substance reacting at a higher temperature range) than HA samples from lakes draining agricultural fields. Significant differences were found in the spectral properties of humic acids. Higher absorbance values at wavelength 280 and 465 nm and higher values of the A_2/6 and A_4/6 ratios were found in HA extracted from sediments in forest lakes. The values of the fourth derivative of the spectra at wavelengths 404 and 660 nm were higher for HA from sediments in lakes draining agricultural fields than HAs from sediments in forest lakes. Differences occurring in elemental composition as well as in spectral and thermal properties indicate that humic acids formed in lakes with a forest catchment contain higher quantity of aromatic structures than HA from lakes with an agricultural catchment.