article · Global Biogeochemical Cycles
The Congo River is a significant contributor of organic matter to the Atlantic Ocean. An analysis of the elemental, stable isotopic, and biomarker composition of its particulate and dissolved organic matter reveals clear seasonal variations despite a stable annual water discharge. Dissolved organic matter comes largely from vascular plants, showing the greatest inputs during peak river discharge in November and December. Particulate organic matter is predominantly composed of soil-derived mineral-associated materials, though inputs of fresher plant matter increase alongside higher river flows. Over a fourteen-month observation period, the river exported 29.21 million tonnes of total suspended sediment, 1.96 million tonnes of particulate organic carbon, and 12.48 million tonnes of dissolved organic carbon per year. Furthermore, the Congo is 2.5 times more efficient at exporting dissolved lignin per unit volume than the Amazon, suggesting ocean lignin is more reactive than previously thought.
Understanding how major tropical river basins deliver organic carbon to oceans is crucial for accurately modelling global carbon cycles. Even with steady river flow, the chemical nature of this runoff changes with the seasons. Revealing that ocean-bound organic matter like lignin degrades faster than prior estimates helps improve predictions of global climate dynamics and marine nutrient balances.
The abstract does not indicate an application pathway, as this research represents early-stage basic environmental science focused on biogeochemical cycling and ocean carbon fluxes.
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Abstract The Congo River in central Africa represents a major source of organic matter (OM) to the Atlantic Ocean. This study examined elemental (%OC, %N, and C:N), stable isotopic ( δ 13 C and δ 15 N), and biomarker composition (lignin phenols) of particulate OM (POM) and dissolved OM (DOM) across the seasonal hydrograph. Even though the Congo exhibits an extremely stable intra‐annual discharge regime, seasonal variability in OM composition was evident. DOM appears predominantly derived from vascular plant inputs with greater relative contribution during the rising limb and peak in discharge associated with the major November–December discharge maximum. Generally, POM appears to be sourced from soil‐derived mineral‐associated OM (low C:N, low Λ 8 , and higher (Ad:Al) v ) but the relative proportion of fresh vascular plant material (higher C:N, higher Λ 8 , and lower (Ad:Al) v ) increases with higher discharge. During the study period (September 2009 to November 2010) the Congo exported 29.21 Tg yr −1 of total suspended sediment (TSS), 1.96 Tg yr −1 of particulate organic carbon (POC), and 12.48 Tg yr −1 of dissolved organic carbon. The Congo exports an order of magnitude lower TSS load in comparison to other major riverine sources of TSS (e.g., Ganges and Brahmaputra), but due to its OM‐rich character it actually exports a comparable amount of POC. The Congo is also 2.5 times more efficient at exporting dissolved lignin per unit volume compared to the Amazon. Including Congo dissolved lignin data in residence time calculations for lignin in the Atlantic Ocean results in an approximately 10% reduction from the existing estimate, suggesting that this material is more reactive than previously thought.
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DOI: 10.1002/2016gb005427
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