Summary and Conclusions

Isotopic composition of carbon and nitrogen and C/N ratios were used to assess the potential contribution of various sources of organic matter to bed sediment in the Tualatin River. Samples of bed sediment, suspended sediment, and seston (suspended particulate material, including plankton and organic detritus) in the Tualatin River were collected during five sampling events in 1998–2000. During the same time period, potential source materials such as soil, plant litter, duckweed, and wastewater treatment facility effluent particulate were collected for comparison of their isotopic composition.

Bed sediment δ¹⁵N was slightly lower in tributaries and in the upper reaches of the river and may indicate that the organic matter in these areas is fresher, less decomposed, or more labile. Higher SOD rates have been measured in the tributaries compared to the main stem Tualatin River, supporting the hypothesis that this material may be more labile. No general trends in δ¹³C or C/N for bed sediment with river location were observed. Bed sediment in the lower river, which is associated with the most phytoplankton growth, was not usually different from bed sediment farther upstream where less algae is present. Bed sediment collected from the lower river in late summer (1999), however, did show a small difference, a likely influence of phytoplankton.

Phytoplankton was evident in seston and suspended sediment samples and characterized by low δ¹³C (<-32 per mil), high δ¹⁵N (>8 per mil), and low C/N (<12). The increasing amount of phytoplankton in the more downstream river samples was reflected in distinct trends in the isotopic data and C/N ratios with river mile. The isotopic data for samples that contained the most phytoplankton did not resemble bed sediment. Decomposition of organic matter tends to decrease the C/N ratio and increase the δ¹⁵N value, further increasing the differences compared to bed sediment. Therefore, phytoplankton likely is not a major source of organic matter to bed sediment. Reductions in the size of the algal community, therefore, may have little effect on the rate of sediment oxygen demand.

Compared to bed sediment, the nitrogen content was heavier (higher δ¹⁵N ) and/or the C/N ratio was lower for several types of samples, including duckweed, periphyton and wastewater treatment facility effluent particulate. Further decomposition of these materials would tend to increase the δ¹⁵N and decrease the C/N, increasing their differences compared to bed sediment. It is, therefore, unlikely that duckweed, periphyton, or wastewater treatment facility effluent are significant sources of organic matter to bed sediment. Periphyton samples exhibited δ¹³C values that spanned the widest range of any of the types of samples collected for this study; the inconsistency of the δ¹³C of periphyton limits the use of δ¹³C as a tracer for this class of sample.

The δ¹³C, δ¹⁵N, and C/N for samples of soil and decomposed terrestrial organic matter closely resembled results for bed sediments. Although soil may be a direct source of organic matter to bed sediment, terrestrial plant material likely is a source of organic matter to both bed sediments and soils. Deciduous litter, woody material, and coniferous litter have δ¹³C values that match soils and bed sediments well. Although the δ¹⁵N and C/N ratios for fresh leaf litter and twigs do not match the δ¹⁵N and C/N ratios for soils and bed sediments, decomposition of those materials is expected to shift the δ¹⁵N and C/N toward those of bed sediment, resulting in a good match. Furthermore, the basin produces a considerable quantity of terrestrial plant litter which easily is transported to the stream network. It is consistent to conclude, therefore, that soil and leaf litter probably are the primary sources of organic matter to Tualatin River bed sediment. Monitoring and management plans that aim to improve oxygen conditions in the river would be most successful by concentrating on sources of soil, leaf litter, and other terrestrially derived organic materials to the river rather than on the instream growth of algae.

First posted August 17, 2010