Characteristics and Bioavailability of Dissolved Natural Organic Matter in a Boreal Stream during Storm Flow

Abstract

DNOM (Dissolved Natural Organic Matter) is characterized as a heterogeneous mixture of organic macromolecules. DNOM is naturally formed in soils, sediments and natural waters by decay and alteration through biodegradation of plant and microbial remains in a process called humification. The material has a profound influence on water chemistry and thus plays an important role in the natural environment. A main characteristic of DNOM is its weak organic acids which allow it to regulate pH and act as a transport medium for pollutants and nutrients from the terrestrial to the aquatic environment and as an important source for food for aquatic organisms. DNOM`s ability to absorb light darkens the colour of water resulting in a yellow-brownish colour. In natural waters receiving high concentrations of DNOM light is inhibited from reaching the deeper parts of the water column. Such conditions may severely affects the organisms living there by preventing photosynthesis, but also by affecting the waters temperature causing severe changes in both production and species diversity. During the past 30 years there has been a widespread increase in colour of surface waters in Europe and North-America. In Southern Norway the concentration of DNOM has more than doubled in just a few years, while the amount of colour has increased even more. This indicates that DNOM is not only increasing in concentration, but is also changing its physical and chemical character. The increase and change in DNOM is hypothesized to be driven by a number of factors; reduction in the amount of acid rain, changes in anthropogenic land use and climate change. The objective of this thesis was to test a hypothesis that an increase in precipitation will not only increase the transport of DNOM to surface waters, but also supply the waters with DNOM of larger molecular weight and greater aromaticity, which could account for the change in relative colour. Both objectives were achieved by conducting an episode study where samples of water were collected from a 1st order stream in a virgin boreal forest area. The samples were collected sequentially during a rainstorm and subjected to analysis to see how changes in hydrology from dry base flow through a runoff peak affected the amount and chemical character of DNOM. The measurement of total organic carbon (TOC) is used as a proxy for the DNOM. Another objective was to improve a method for measuring biodegradation using novel instrument to monitor oxygen consumption. The results confirmed that concentrations of DNOM increase during episodes. On the other hand, the UV-ViS absorption and fluorescence excitation- emission matrix (EEM) spectra, contradicted the original hypothesis by showing a decrease in molecular weight and aromaticity. This is likely due to large peat coverage in the watershed. DNOM from mineral soils is more aliphatic and of lower molecular weight than DNOM from peat soils. As the water levels increases the change in flow patterns through the catchment affects the overall contribution of DNOM from the different soil sources, with more water originating from the mineral soils. The biodegradation experiments did not revealed any significant differences in the DNOM; however by using glucose as a reference the results from extensive method development showed a clear improvement in signal output when comparing the original method to the improved method. However, more future experiments are required in order to verify if the improved method for biodegradation can produce reliable results.