Elsevier

Geochimica et Cosmochimica Acta

Volume 84, 1 May 2012, Pages 614-627
Geochimica et Cosmochimica Acta

An initial investigation into the organic matter biogeochemistry of the Congo River

https://doi.org/10.1016/j.gca.2012.01.013Get rights and content

Abstract

The Congo River, which drains pristine tropical forest and savannah and is the second largest exporter of terrestrial carbon to the ocean, was sampled in early 2008 to investigate organic matter (OM) dynamics in this historically understudied river basin. We examined the elemental (%OC, %N, C:N), isotopic (δ13C, Δ14C, δ15N) and biochemical composition (lignin phenols) of coarse particulate (>63 μm; CPOM) and fine particulate (0.7–63 μm; FPOM) OM and DOC, δ13C, Δ14C and lignin phenol composition with respect to dissolved OM (<0.7 μm; DOM) from five sites in the Congo River Basin. At all sample locations the organic carbon load was dominated by the dissolved phase (∼82–89% of total organic carbon) and the total suspended sediment load was principally fine particulate material (∼81–91% fine suspended sediment). Distinct compositional and isotopic differences were observed between all fractions. Congo CPOM, FPOM and DOM all originated from vegetation and soil inputs as evidenced by elemental, isotopic and lignin phenol data, however FPOM was derived from much older carbon pools (mean Δ14C = −62.2 ± −13.2‰, n = 5) compared to CPOM and DOM (mean Δ14C = 55.7 ± 30.6‰, n = 4 and 73.4 ± 16.1‰, n = 5 respectively). The modern radiocarbon ages for DOM belie a degraded lignin compositional signature (i.e. elevated acid:aldehyde ratios (Ad:Al) relative to CPOM and FPOM), and indicate that the application of OM degradation patterns derived from particulate phase studies to dissolved samples needs to be reassessed: these elevated ratios are likely attributable to fractionation processes during solubilization of plant material. The relatively low DOM carbon-normalized lignin yields (Λ8; 0.67–1.12 (mg(100 mg OC)−1)) could also reflect fractionation processes, however, they have also been interpreted as an indication of significant microbial or algal sources of DOM. CPOM appears to be well preserved higher vascular plant material as evidenced by its modern radiocarbon age, elevated C:N (17.2–27.1) and Λ8 values (4.56–7.59 (mg(100 mg OC)−1)). In relation to CPOM, the aged FPOM fraction (320–580 ybp 14C ages) was comparatively degraded, as demonstrated by its nitrogen enrichment (C:N 11.4–14.3), lower Λ8 (2.80–4.31 (mg(100 mg OC)−1)) and elevated lignin Ad:Al values similar to soil derived OM. In this study we observed little modification of the OM signature from sample sites near the cities of Brazzaville and Kinshasa to the head of the estuary (∼350 km) highlighting the potential for future studies to assess seasonal and long-term OM dynamics from this logistically feasible location and derive relevant information with respect to OM exported to the Atlantic Ocean. The relative lack of OM data for the Congo River Basin highlights the importance of studies such as this for establishing baselines upon which to gauge future change.

Introduction

Fluvial systems act as conduits between the biogeochemical cycles of terrestrial and marine ecosystems. In addition rivers are increasingly recognized as significant biogeochemical processors of terrestrially derived organic matter (OM) within the global carbon cycle (Mayorga et al., 2005, Cole et al., 2007, Battin et al., 2008, Battin et al., 2009, Aufdenkampe et al., 2011). Due to the multiple sources of OM to rivers, and the different controls with respect to carbon processing and exchange (e.g. tropical to Arctic environments, seasonally inundated areas, organo-mineral complexation, etc.) the reactivity and chemistry of various fractions of dissolved and particulate OM (DOM and POM) vary between and within fluvial systems (Mayorga et al., 2005, Aufdenkampe et al., 2007, Bouillon et al., 2009). One system where these sources and controls have been studied in depth is the Amazon River Basin, where detailed geochemical analyses were conducted on three distinct fractions of riverine OM; coarse particulate OM (CPOM, >63 μm), fine particulate OM (FPOM, 0.7–63 μm) and dissolved OM (DOM, <0.7 μm). These studies demonstrate that Amazon CPOM, FPOM and DOM are compositionally distinct and thus play different roles within the Amazon River Basin (Hedges et al., 1986, Hedges et al., 1994, Hedges et al., 2000). Further studies have delivered a comprehensive assessment of carbon biogeochemical processing in the Amazon River Basin (Richey et al., 2002, Mayorga et al., 2005).

Unfortunately, the number of studies focused on the Amazon is a striking exception with only a few other major river-systems examined in terms of carbon processing, for example the Ganges–Brahmaputra (Galy et al., 2007, Galy et al., 2008), the Fly-Strickland system (Alin et al., 2008) and the Yukon River Basin (Striegl et al., 2007, Spencer et al., 2008). Tropical river-systems have disproportionally high carbon transport and outgassing compared to temperate and Arctic rivers, yet are underrepresented in global estimates (Aitkenhead and McDowell, 2000, Battin et al., 2008, Aufdenkampe et al., 2011). In addition, many tropical river-systems are facing a range of local pressures including land-use change and associated impacts (e.g. deforestation, conversion to agriculture, increases in discharge, incidence of fire) and global pressures (e.g. increasing temperatures, shifts in precipitation patterns and intensity), all of which impact the mobilization, transport, processing and deposition of carbon and minerals to and within river basins (e.g. Soares-Filho et al., 2006, Laporte et al., 2007, Coe et al., 2009, Coe et al., 2011, Aragao and Shimabukuro, 2010). With these points in mind this study provides the first detailed investigation of OM and carbon processing in the Congo River Basin.

The Congo is the largest river in Africa and the second largest river in the World after the Amazon in terms of drainage basin size (∼3.7 × 106 km2) and water discharge (Runge, 2007, Laraque et al., 2009). In contrast to the well studied Amazon, the Congo has a very low total suspended sediment (TSS) yield (8.5 Mg km−2 yr−1) and despite its discharge is only ranked twelfth in the World with respect to annual TSS load. However, Congo TSS has been reported to have high particulate organic carbon (POC) content and therefore ranks fifth in terms of annual POC flux to the oceans at 2 Tg yr−1 (Mariotti et al., 1991, Coynel et al., 2005). With respect to dissolved organic carbon (DOC) the Congo is estimated to export 12.4 Tg yr−1, which for comparison is equivalent to the DOC loads of the three largest Arctic Rivers (Yenisey, Lena and Ob) combined, or greater than six times that of the Mississippi (Coynel et al., 2005, Raymond et al., 2007). Combining DOC and POC loads, the Congo exports 14.4 Tg yr−1of organic carbon to the Atlantic Ocean and is the second major exporter of terrestrial carbon to the oceans after the Amazon (Coynel et al., 2005). The Congo also drains the second largest area of tropical rainforest in the World (∼18% of the total global tropical rainforest) and much of the rainforest within the Congo Basin is still in a pristine state, especially within the borders of the Democratic Republic of Congo (DR Congo), with currently just 1% forest disturbance estimated (Laporte et al., 2007).

Despite the apparent importance of the Congo Basin in terms of global carbon budgets the ability to conduct research in the region has been affected by political instability, civil unrest, war and limited infrastructure (Baccini et al., 2008, Prunier, 2008). However, as peace and stability return to the region enabling trade and investment, industrial logging, conversion of pristine forest to agricultural lands and settlement expansion are growing threats to the Congo Basin’s forest (Laporte et al., 2007, Koenig, 2008). Therefore, a primary goal of this study was to improve the understanding of carbon dynamics within the Congo River Basin and to determine a baseline against which future studies can gauge impacts. Previous studies on the Congo River-estuary system have estimated TSS, POC and DOC fluxes (Coynel et al., 2005, Laraque et al., 2009) and examined estuarine dynamics (Eisma and Van Bennekom, 1978, Cadee, 1984, Pak et al., 1984). However, to date only limited biochemical composition or isotope data exists for Congo River OM (Mariotti et al., 1991, Spencer et al., 2009, Spencer et al., 2010a, Stubbins et al., 2010). Firstly, this study provides an initial assessment of the sources of OM within the Congo River via elemental (%OC, %N, C:N), isotopic (δ13C, Δ14C, δ15N), and biochemical compositional (lignin phenols) data for CPOM, FPOM and DOM with a focus on the last ∼350 km of the river to the head of its estuary. As well as four sites on the river mainstem an additional sample site was examined (small savannah stream). Although we recognize the limitations of a single sample site, this site was chosen to assess OM composition in a small order tributary with strong connectivity between terrestrial and aquatic environments, and thus little time for degradative processes in-river in comparison to the mainstem sites. Secondly, we examine the impact of the Malebo Pool (a major widening of the river) and downstream rapids on the quantity and composition of OM exported to the Atlantic. Understanding the influence of river morphology on OM in this region is especially important, as existing budgets have been determined upstream of the rapids. Logistical constraints mean that future studies will likely also focus above the rapids with respect to time series sampling to assess seasonal OM dynamics as well as to set baselines for examination of ongoing perturbation within the Basin. Thus we evaluate whether sampling at Brazzaville–Kinshasa provides relevant information with respect to OM exported to the Atlantic Ocean.

Section snippets

Study site

The Congo River rises from headwaters located in the south-eastern part of the DR Congo, near the border with Zambia (Runge, 2007). From its headwaters it flows in a generally northern direction, receiving numerous tributaries that enter predominantly from the east, and drain the western slopes of the Central African Rift Valley. Near Kisangani at the Wagenia Falls (Boyoma Falls, formerly Stanley Falls) the river officially becomes known as the Congo. As the Congo flows on toward Mbandaka it

Physicochemical characteristics, OM concentrations and elemental compositions

Water temperature and pH showed little variation between mainstem sites 1–4 (28.5–28.8 °C and 6.45–6.54 respectively) and site 5 (small savannah tributary) was slightly cooler and had higher pH (27.9 °C and 6.64 respectively; Table 1). Congo River mainstem (sites 1–4; Table 2) samples ranged in TSS concentration from 18.0–20.3 mg L−1. The lowest TSS concentration (13.2 mg L−1) was found in the Luilu River (site 5; small savannah tributary) (Table 2). Fine suspended sediments (FSS) dominated the TSS

Organic matter distribution

The TSS concentrations measured at the Congo River mainstem sites (1–4; Table 2) are similar to previous studies focused on this region of the river at the same point in the annual hydrograph (∼24–26 mg L−1; Mariotti et al., 1991, Coynel et al., 2005). In comparison to other major global rivers the Congo has a relatively low TSS concentration at all sites within the basin (1–5), particularly in relation to other tropical rivers such as the Amazon, Fly and Strickland (Richey et al., 1986, Ludwig

Acknowledgments

We thank the Regie de Voies Fluviales in Kinshasa (D.R. Congo) for providing hydrology data and Kevin Burkhill at the University of Birmingham for producing Fig. 1. This study was supported by a NERC radiocarbon grant (#1244.1007) to RGMS and AB and discretionary funds awarded to JS by the Department of Plant Sciences, UC Davis. RGMS and JS are very grateful to Richard N. Tshibambe for facilitating the required visas and permits and Lodi Jean Paul Lama for their assistance in the D.R. Congo.

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