Effect of Catchment Characteristics on the Biodegradability of Dissolved Natural Organic Matter

Summary

Dissolved natural organic matter (DNOM) is omnipresent in natural freshwaters. It is the main source of food for aquatic heterotrophic respiration, though little is known of the bioavailability of the DNOM when flowing through the aquatic system. Most of the DNOM in surface waters is allochthonous (i.e., derived from the terrestrial environment). Therefore, the lake-water chemistry acts as a sentinel of the biogeochemical processes in the catchment. The increase in concentrations of DNOM (browning) is boosting aquatic respiration and suppressing primary production, causing the lakes to become net sources of greenhouse gases. The biodegradability of DNOM has been measured in a set of lake water samples from southeast Norway using a simple analytical method based on monitoring the rate of oxygen consumption during incubation. The spatial variation in biodegradability of the DNOM was not clearly explained by water chemistry. But there was a significant (p < 0.05) empirical correlation between the biodegradability of DNOM and site and catchment characteristics. Forest cover is a key explanatory factor for the differences in biodegradability of the DNOM. Both high cover of forest and low cover (natural non-forest) had relatively high correlations, at -0.26 and 0.25. The variable that had the highest impact on the biodegradability of DNOM was the lake altitude. Altitude reflects multiple catchment characteristics that affect the DNOM, which again affects the biodegradability of DNOM. Lakes at high altitudes have foremost more low molecular weight (LMW) DNOM, which has an important effect on the biodegradability of DNOM.