Origins, seasonality, and fluxes of organic matter in the Congo River

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 À1 of total suspended sediment (TSS), 1.96 Tg yr À1 of particulate organic carbon (POC), and 12.48 Tg yr À1 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.

Flux and Seasonality of Dissolved Organic Matter From the Northern Dvina (Severnaya Dvina) River, Russia

“…Interestingly, the high (Ad/Al) v ratios observed during the spring freshet period (Figure c) suggest that at this time of maximum DOM export the DOM may be highly degraded (Opsahl & Benner, ). Alternatively, the elevated (Ad/Al) v signature may be a reflection of the physical processes mobilizing the lignin into the dissolved phase, especially extensive leaching at this time of year (Hernes et al, ; Spencer et al, ).…”

“…Estimation of lignin biomarker fluxes to the Arctic Ocean has been previously used to calculate Arctic Ocean residence times for terrestrially derived DOM (Mann et al, ; Opsahl et al, ; Spencer et al, ). The diagnostic ratios derived from lignin phenols also provide source information, indicate processes such as phase changes and degradation, and have the ability to distinguish between woody and nonwoody, and angiosperm and gymnosperm sources (Hedges & Mann, ; Hernes et al, ; Spencer et al, ). In conjunction with lignin biomarker analyses, chromophoric DOM (CDOM)‐derived parameters have been utilized by a number of studies to assess DOM source and composition exported by Arctic rivers (Spencer et al, ; Stedmon et al, ; Walker et al, ).…”

The Genesis and Exodus of Vascular Plant DOM from an Oak Woodland Landscape

“…Characterizing fractionation effects in lignin biomarkers has significantly improved interpretation of DOM and lignin parameters in various environments, explaining apparent degradation trends that previously were conundrums (Hernes et al, , 2013aSpencer et al, 2008Spencer et al, , 2016. Lignin fractionation studies to date have primarily consisted of snapshot measurements, in which dissolved and particulate lignin is allowed to reach equilibrium, then a single measurement of each is compared to assess the magnitude of fractionation (Hernes et al, , 2013a.…”

“…A recent emphasis on time-series sampling in the past decade, for example, has led to significant modifications in measurements of river fluxes and seasonally evolving DOM and POM compositions Spencer et al, 2010bSpencer et al, , 2016Ellis et al, 2012). However, there is a great need for parsing out in situ sources and processing of riverine OM vs. external inputs, and one potential approach could be to pair studies of concrete-lined aquaducts with nearby rivers to try to constrain the relative role of external vs. internal.…”

“…Biomarkers such as lignin offer the specificity to trace DOM derived from angiosperm vs. gymnosperms, and woody vs. non-woody tissues (Hedges and Mann, 1979;Spencer et al, 2016). Optical parameters derived from UV-visible absorbance can trace the bulk properties of chromophoric DOM (CDOM) such as aromaticity, microbial vs. terrigenous sources, and high-vs. lowmolecular weight properties (Traina et al, 1990;Helms et al, 2008;Spencer et al, 2013).…”