Rapid changes in dissolved humic substances in Spirit Lake and South Fork Castle Lake, Washington

One major effect of the eruption of Mount St. Helens, Washington, was a large increase of dissolved organic material in the lakes of the area devastated near the volcano. Much of this material was aquatic fulvic acid derived from plants and soils from the surrounding watershed. During the 3 yr after the eruption, substantial chemical changes occurred in the aquatic fulvic acid: a decrease in total sulfur content and in the content of aliphatic and heterocyclic sulfur groups; an increase in carbohydrate content and in polysaccharide pyrolysis fragments; a decrease in phenolic hydroxyl content; an increase in the heterogeneity of the fulvic acid material; and a decrease in copper‐complexing properties. Natural abundance 13 C‐NMR (nuclear magnetic resonance spectra), also showed a progressive change in the distribution of carbon atoms with aliphatic moieties becoming more abundant than aromatic moieties. These changes were more likely to have been caused by alterations of fulvic acid molecules on exposure to sunlight, by oxygenated conditions, and by microbial transformation in the epilimnion rather than by new sources of fulvic acid. These results demonstrate that rapid changes may occur within the heterogeneous mixture of molecules comprising fulvic acid, which has been generally described as a refractory or stable pool of dissolved organic material.