Human impacts on riverine carbon fluxes in the Mekong-Tonle Sap river system
Streams and rivers draining urban population centers transport not only inorganic contaminants but also organic carbon (C) of anthropogenic origin. This human-induced alteration of water quality has rarely been linked to carbon fluxes along large rivers draining rapidly growing population centers in developing countries. To examine the effects of water pollution on natural riverine C fluxes, chemical characterizations of dissolved and particulate organic carbon (DOC and POC) were combined with in-situ measurements of partial pressure of CO2 (pCO2) in the Mekong-Tonle Sap river system receiving ever-increasing amounts of urban runoff from the metropolitan Phnom Penh. In-stream measurements of pCO2 using a non-dispersive infrared sensor wrapped in a gas-permeable membrane, along with grab water sampling, were conducted in four urban tributary streams and three main-stem reaches of the Tonle Sap and three main-stem reaches of the Mekong during the early monsoon period when the reversed flow from the Mekong to the Tonle Sap significantly alters water quality downstream up to the Tonle Sap Lake. Water samples were analyzed for DOC, POC, stable and radioactive isotopes of DOC and dissolved inorganic C, UV absorbance, fluorescence excitation emission matrices (EEMs), alkalinity, and dissolved nutrients.
Urban streams exhibited extremely low levels of dissolved oxygen but very high levels of pCO2, dissolved nutrients, DOC, and POC, as compared with the generally homogenized measurement results along two rivers as a consequence of the monsoonal reversed flow. Very high intensities of protein-like fluorescence EEM peaks compared to similar humic-like fluorescence intensities across all sampling locations pointed to a potentially labile characteristic of DOC in urban streams. The inflow of urban streams significantly increased pCO2 in downstream reaches of the reversely flowing Tonle Sap, the upstream of which had a relatively low value of pCO2 similar to that for the Mekong. Slight, but noticeable downstream changes in the radio carbon age of DOC in the Tonle Sap indicated a potential effect of aged C compounds released through relatively small volumes of urban runoff on both organic C characteristics and riverine pCO2 in receiving river waters of large flow. The potential effects of urban water pollution on riverine C fluxes shown in this exploratory study suggest that water pollution should be explicitly taken into consideration to better understand organic C transformations and CO2 evasion in large river systems under increasing pressures of anthropogenic disruptions.