Optical and molecular-level characterization of fluvial organic matter biodegradation in a highly urbanized river system

Abstract. Rapid urbanization worldwide is changing both the transport of organic matter (OM) and CO₂ emission in urban streams and rivers, yet little is known as to how the altered quality of riverine OM affects biodegradation and CO₂ emission. The relationships between the chemical properties and biodegradation of riverine OM, including dissolved and particulate OM (DOM and POM), were examined against the level of anthropogenic perturbation along the Han River, a river system in South Korea that has been highly modified by dams and urban water pollution. DOM optical properties and biodegradable dissolved organic carbon (BDOC), together with in situ measurements of the partial pressure of CO₂ (pCO₂) using a membrane-enclosed sensor, were compared between the up-, mid-, and downstream reaches of the Han River and three urban tributaries in a basin-scale field campaign combined with a 7 day incubation of both filtered and unfiltered samples. Another 5 day incubation was conducted with unfiltered water samples from a downstream river site and an urban tributary, in isolation and mixed (1 : 1), to measure changes in dissolved CO₂ concentrations at 10-min intervals and BDOC and optical properties at intervals of 1–2 days. Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) was used to detect molecular-level changes in DOM composition between the initial and post-incubation samples. The average BDOC concentration in the urban tributaries was 4–12 times higher than those of three mainstem reaches, while BDOC was highly variable at three downstream sites and tended to be higher at the mainstem sites affected by agricultural runoff or dams than at the forested headwater stream. Longitudinal increases in protein-like and microbial humic-like fluorescence, fluorescence index (FI), and biological index (BIX) reflected increasing inputs of anthropogenic DOM along the downstream reach and urban tributaries. These optical indices, along with pCO₂, were significantly correlated with BDOC concentrations measured at 12 sites. The cumulative CO₂ production measured in the second incubation was greatest in the mixture, followed by the urban tributary and mainstem samples in the descending order. The amount of CO₂ produced in the mixture was greater than the BDOC measured in the same sample or the average of CO₂ produced in the separate samples, indicating a mixing-enhanced biodegradation of DOM including the fraction transformed from soluble components of POM. FT-ICR-MS analysis revealed a much larger number of molecules consumed (3984) than those produced (771) during the incubation of the mainstem sample in contrast to the produced molecules (2789) exceeding the consumed molecules (1479) in the tributary sample, indicating a high rate of OM processing in the urban tributary that might be limited in the availability of immediately consumable organic materials. Overall results suggest that water pollution, along with impoundment effects of dams, can alter the optical properties and biodegradability of both DOM and POM in highly urbanized watersheds to such a degree that can induce a priming effect on OM biodegradation and CO₂ emission.