Chemical and microbial diagenesis of humic matter in freshwaters

The importance of microbial activity in changing the chemical properties of humic material in brown waters was measured as a function of pH, acid neutralization capacity (ANC), and pK_a affinity spectrum from brownwater samples kept in aquaria and sterile flasks. Changes to the humic matter in non-sterile microcosms caused increases in pH and ANC and loss of strong organic acid pK_a sites. Microbial activity was shown to be the main contributor to a decrease in acidity of humic material as samples kept under sterile conditions showed less change than non-sterile ones. Microbial processes were shown to be mostly due to sessile organisms on the walls of the aquaria and in sediments. Our data suggest that acid-base properties of brownwaters can be modified based on residence times, as well as by temperature in their basins. Assuming similar starting humic materials, microbial degradation may account for the lower acidity measured in lakes and large rivers, compared to headwaters.