Bioindicator for the Sources and Drivers of Carbon Dioxide and Dissolved Methane Concentrations in Urban Rivers

Rivers draining urban areas are reported to be a noteworthy source of atmospheric greenhouse gases (GHGs), yet the drivers of carbon dioxide partial pressure (pCO2) and dissolved methane (dCH4) remain unclear in urban rivers. This study identified key controls of biotic and abiotic drivers across urbanization gradients. pCO2 and dCH4 increased with urbanization intensity, alongside sewage-associated biotic variables (Bacteroidales abundance, RAB; Methylocystis abundance) and abiotic factors (electrical conductivity, dissolved organic carbon, total dissolved nitrogen, total dissolved phosphorus, chlorophyll a). In highly urban segments, abiotic factors explained 87% of pCO2 variation, while RAB explained 95% of dCH4 variation, pointing to sewage inputs as a major methane source. Seasonally, pCO2 showed comparable averages but a wider range in the wet season. Dissolved oxygen was negatively correlated with pCO2, explaining 65% (wet) and 58% (dry) of the variation, indicating biological metabolic control. The negative relation between dCH4 and methane monooxygenase (MMO) abundance suggests biological oxidation regulation. A significantly lower dCH4 in the wet season was linked to higher MMO and faster flow. Overall, including biological factors and bioindicators improves understanding of pCO2 and dCH4 sources and drivers in urban rivers.