By means of H:C and 0:C atomic ratios information was obtained on the basic structure, and on structural changes with varying molecular weight and origin, of fulvic and humic acids sampled at various locations of a watershed within the Laurentide provincial park, Quebec. In order to investigate also the effect of humification, humic matter produced by a streptomycete and sampled at regular intervals during a 6-month incubation period was included in the investigation. Also studied was the effect of ultraviolet irradiation on the structure of either recently formed or more humified humic substances. It was shown that fulvic and humic acids are more alike in aquatic than in terrestrial environments. They are also less aromatic in natural waters than in soils and, at least in the case of fulvic acids, are richer in oxygen. Contrary to what is normally observed for humus from natural environments, it would appear that for certain types of humus such as fulvic acids from a microbial culture, increases in molecular weight or in the degree of humification would result in material with a less aromatic character. Humus obtained from natural waters was found to be in a more advanced state of humification at the end of summer than at the beginning. Recently formed fulvic acids appeared to be very sensitive to ultraviolet irradiation.

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SUMMARY. 1. Between February and September 1974, concentrations of fulvic and humic acids in waters from a cryoboreal region of the Precambrian Shield in Quebec varied between 0.5–6.1 mg 1−1 and 0.1–6.5 mg 1−1 respectively. Whereas the fulvic acids usually remained relatively constant during this period, humic acids showed appreciable increases. 2. The number and weight average molecular weights of the fulvic acids (∼5000 and ∼22,000 daltons respectively) were lower than those of the humic acids (∼20,000 and ∼75,000 daltons respectively). The average molecular weight of the aquatic humic matter often decreased during late summer and early autumn. During spring floods the molecular weight of fulvic acids increased and that of humic acids declined. 3. Because the aquatic humic acids were more intensely coloured than fulvic acids, they contributed more than 30% of the colour of the water even though they accounted for only 15% of the concentration of humic matter. The colour intensity of both fulvic and humic acids generally appeared to increase during the latter part of the summer, when there was normally a decrease in the value of the colour quotient (E4/E6 ratio), indicating the accumulation in the waters of more humified material.

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