Urbanization and seasonality strengthens the CO₂ capacity of the Red River Delta, Vietnam

Abstract

Tropical rivers are dynamic CO2 sources. Regional patterns in the partial pressure of CO2 (pCO2) and relationships with other a/biotic factors in densely populated and rapidly developing river delta regions of Southeast Asia are still poorly constrained. Over one year, at 21 sites across the river system in the Red River Delta (RRD), Vietnam, we calculated pCO2 levels from temperature, pH, and total alkalinity and inter-linkages between pCO2 and phytoplankton, water chemistry and seasonality were then assessed. The smaller, more urbanized, and polluted Day River had an annual median pCO2 of 5,000 ± 3,300 µatm and the larger Red River of 2,675 ± 2,271 µatm. pCO2 was 1.6 and 3.2 times higher during the dry season in the Day and Red rivers respectively than the rainy season. Elevated pCO2 levels in the Day River during the dry season were also 2.4-fold higher than the median value (2,811 ± 3,577 µatm) of calculated and direct pCO2 measurements in > 20 sub/tropical rivers. By further categorizing the river data into Hanoi City vs. other less urban-populated provinces, we found significantly higher nutrients, organic matter content, and riverine cyanobacteria during the dry season in the Day River across Hanoi City. Forward selection also identified riverine cyanobacteria and river discharge as the main predictors explaining pCO2 variation in the RRD. After accounting for the shared effects (14%), river discharge alone significantly explained 12% of the pCO2 variation, cyanobacteria uniquely a further 21%, while 53% of the pCO2 variance was unexplained by either. We show that the urbanization of rivers deltas could result in increased sources of riverine pCO2, water pollution, and harmful cyanobacterial blooms. Such risks could be mitigated through water management to increase water flows in problem areas during the dry season.

Identifying the controls on nitrate and metabolic state within the Red River delta (Vietnam) with the use of stable isotopes

“…Urbanization contributes a large fraction of total NO3 -; sometimes higher than 50 % of total load, as seen in the middle section of the Day River during dry season. High inputs of domestic waste result in heterotrophic conditions (Salgado, et al, 2022); low dissolved oxygen, as clearly indicated by enriched  18 O-NO3 -.…”

“…With these differential isotopic effects on DO, we can conclude that the oxygen demand in the Day River group is high relative to the Red River group; as dissolved oxygen concentration is lower in the Day River group than in Red River group, while isotopic composition is the same between the two groups. The explanation for the lower DO level (higher oxygen demand) in the Day than the Red River groups is apparently anthropogenic and exacerbated by stagnant waters (Salgado, et al, 2022). Urbanization and industrialization in Day River catchment have caused environmental disaster in the ecosystem (Trinh, et al, 2007;Trinh, et al, 2012).…”

“…Many of the world's largest river systems are becoming significantly impacted by anthropogenic activities, including, importantly, the pollution of these systems by key nutrients (Strokal, et al, 2016;Steffen, et al, 2015). Nutrients discharged from human activities such as industry, agriculture and urban settlements (often wastewater) can overload these once pristine systems, leading to environmental degradation, eutrophication and ecosystem collapse (Trinh, et al, 2007;Salgado, et al, 2022). Alongside the environmental issues such pollutants cause, these waters become unsafe for human consumption, or for further use downstream of the pollutant source (Zeng, et al, 2023).…”

Dammed deltas: Sinking Asian deltas in a warming world

“…11 Both ''rice bowl'' deltas of Vietnam in the Red River and Mekong Deltas are locally polluted by nitrogen from fertilizer overapplication and there are pollution hotspots for nutrients and heavy metals within the Red River Delta close to the city of Hanoi. 12 Due to intensive nutrient cycling and additions within deltas it is hard to predict the downstream implications of damming for coastal environments where nutrients are important for supporting marine food webs. Despite the restorative capacity of deltas to sequester excess loads of nutrients, heavy metals, and other pollutants, it may be that system thresholds are being rapidly approached, which may not become evident until tipping points are reached, making adaptive management a harder task, especially given the risks of river downcutting and remobilization of legacy sediments.…”

“…Nutrient monitoring can highlight local water quality issues, while basinscale nutrient balances can support decision-making on agriculture, fisheries, and inform carbon policy under different scenarios of dam operation. 12 Unfortunately, despite numerous methodological advances to determine environmental flows and optimize dam releases, implementation has not followed suit. 16 Integrating environmental flow requirements in the design of new impoundment schemes is relatively easier than changing operations of existing dams, but the international cooperation necessary to allow either is not in place in these deltas.…”