The term ‘Blue Carbon’ was coined about a decade ago to highlight the important carbon sequestration capacity of coastal vegetated ecosystems. The term has paved the way for the development of ...programs and policies that preserve and restore these threatened coastal ecosystems for climate change mitigation. Blue carbon research has focused on quantifying carbon stocks and burial rates in sediments or accumulating as biomass. This focus on habitat-bound carbon led us to losing sight of the mobile blue carbon fraction. Oceans, the largest active reservoir of carbon, have become somewhat of a blind spot. Multiple recent investigations have revealed high outwelling (i.e., lateral fluxes or horizontal exports) of dissolved inorganic (DIC) and organic (DOC) carbon, as well as particulate organic carbon (POC) from blue carbon habitats. In this paper, we conceptualize outwelling in mangrove, saltmarsh, seagrass and macroalgae ecosystems, diagnose key challenges preventing robust quantification, and pave the way for future work integrating mobile carbon in the blue carbon framework. Outwelling in mangroves and saltmarshes is usually dominated by DIC (mostly as bicarbonate), while POC seems to be the major carbon species exported from seagrass meadows and macroalgae forests. Carbon outwelling science is still in its infancy, and estimates remain limited spatially and temporally. Nevertheless, the existing datasets imply that carbon outwelling followed by ocean storage is relevant and may exceed local sediment burial as a long-term (>centuries) blue carbon sequestration mechanism. If this proves correct as more data emerge, ignoring carbon outwelling may underestimate the perceived sequestration capacity of blue carbon ecosystems.
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•Odum's outwelling hypothesis is framed in a marine carbon sequestration context.•Mangrove, saltmarsh, seagrass and macroalgae ecosystems effectively sequester carbon.•Outwelling may exceed sediment burial as a long-term carbon sequestration mechanism.•Carbon outwelling estimates remain spatially and temporally limited.•It is time to focus not only on local carbon burial, but also exports to the sea.
Our current knowledge on production, composition, transport, pathways and transformations of organic carbon in tropical mangrove environments is reviewed and discussed. Organic carbon entering ...mangrove foodwebs is either produced autochthonously or imported by tides and/or rivers. Mangrove litter and benthic microalgae are usually the most important autochthonous carbon sources. Depending on local conditions, phytoplankton and seagrass detritus imported with tides may represent a significant supplementary carbon input. Litter handling by the fauna not only affects microbial carbon transformations, but also the amount of organic carbon available for export. Most mangrove detritus that enters the sediment is degraded by microorganisms. Aerobic respiration and anaerobic sulfate reduction are usually considered the most important microbial respiration processes, but recent evidence suggests that iron respiration may be important in mangrove sediments as well. Organic carbon that escapes microbial degradation is stored in sediments and in some mangrove ecosystems, organic-rich sediments may extend to several meters depth. Many mangrove forests also lose a significant fraction of their net primary production to coastal waters. Large differences occur between mangrove forests with respect to litter production and export. Mangrove-derived DOC is also released into the water column and can add to the total organic carbon export. Numerous compounds have been characterized from mangrove tissues, including carbohydrates, amino acids, lignin-derived phenols, tannins, fatty acids, triterpenoids and
n-alkanes. Many of these may, together with stable isotopes, exhibit a strong source signature and are potentially useful tracers of mangrove-derived organic matter. Our knowledge on mangrove carbon dynamics has improved considerably in recent years, but there are still significant gaps and shortcomings. These are emphasized and relevant research directions are suggested.
Both climate change and land-use changes represent serious environmental issues throughout Africa, and are expected to lead to significant changes in river hydrology and geochemistry. Freshwater ...bivalve shell geochemistry may provide useful information about past river characteristics (discharge, geochemistry) providing a baseline to help understand modern environmental changes. Shells of two species of freshwater mussels (Chambardia wissmanni and Aspatharia dahomeyensis) were collected from two large African rivers (Oubangui and Niger Rivers) and were analyzed for Sr/Ca, Ba/Ca, Mg/Ca and Mn/Ca ratios along their maximal growth axis. Shell data were compared with geochemical data collected in these rivers every two weeks over a two-year period. To verify whether element ratios in shells were related to the host water, the elemental concentrations measured in the shells were set in a temporal context based on δ18Oshell cycles. Shells from the Oubangui River to some extent recorded water element ratios, with variations in water Sr/Ca ratios being reflected in all three bivalve specimens analyzed, while only the youngest specimen exhibited complete coverage. The two older specimens recorded only segments of the water data, which suggests an influence of growth rate. This relationship was not observed in A. dahomeyensis shells from the Niger, suggesting a species-specific relationship between freshwater shells and water. The Ba/Ca and Mg/Ca record showed no similarity between the water elements and analyzed shells, but an ontogenetic decrease was observed in older specimens. Shell Mn/Ca ratios tracked pH in shells from the Oubangui, with a time-lag, which might be due to bivalves taking up the increased Mn via phytoplankton which are expected to show a similar time-lag. This relationship was not observed in shells from the Niger, where due to the high turbidity throughout the year we expect very low primary production rates. In conclusion, no consistent relationships between water chemistry and shell chemistry were observed. While Sr/Ca and Mn/Ca ratios in shells of C. wissmanni from the Oubangui were most promising as proxies of water chemistry, the utility of element ratios as tracers is compromised by species-specific effects or by site-specific phenomena.
•The relationship between water and shell chemistry is not consistent.•Element incorporation mechanisms in freshwater bivalve shells appear to be species-specific.•Ba/Ca and Mg/Ca ratios were found to be insensitive to changes in environmental conditions.•Mn/Ca and Sr/Ca ratios have potential as recorders of environmental processes.•Sr/Ca record in freshwater bivalves is complicated by species-specific effects.
The impact of human activities on the concentrations and composition of dissolved organic matter (DOM) and particulate organic matter (POM) was investigated in the Walloon Region of the Meuse River ...basin (Belgium). Water samples were collected at different hydrological periods along a gradient of human disturbance (50 sampling sites ranging from 8.0 to 20,407 km²) and during a 1.5 year monitoring of the Meuse River at the city of Liège. This dataset was completed by the characterization of the DOM pool in groundwaters. The composition of DOM and POM was investigated through elemental (C:N ratios), isotopic (δ¹³C) and optical measurements including excitation emission matrix fluorescence with parallel factor analysis (EEM–PARAFAC). Land use was a major driver on fluvial OM composition at the regional scale of the Meuse Basin, the composition of both fluvial DOM and POM pools showing a shift toward a more microbial/algal and less plant/soil-derived character as human disturbance increased. The comparison of DOM composition between surface and groundwaters demonstrated that this pattern can be attributed in part to the transformation of terrestrial sources by agricultural practices that promote the decomposition of soil organic matter in agricultural lands and subsequent microbial inputs in terrestrial sources. In parallel, human land had contrasting effects on the autochthonous production of DOM and POM. While the in-stream generation of fresh DOM through biological activity was promoted in urban areas, summer autochthonous POM production was not influenced by land use. Finally, soil erosion by agricultural management practices favored the transfer of terrestrial organic matter via the particulate phase. Stable isotope data suggest that the hydrological transfer of terrestrial DOM and POM in humanimpacted catchment are not subject to the same controls, and that physical exchange between these two pools of organic matter is limited.
The role played by river networks in regional and global carbon (C) budgets is receiving increasing attention. Despite the potential of radiocarbon measurements (Δ14C) to elucidate sources and ...cycling of different riverine C pools, there remain large regions for which no data are available and no comprehensive attempts to synthesize the available information and examine global patterns in the 14C content of different riverine C pools. Here we present new 14C data on particulate and dissolved organic C (POC and DOC) from six river basins in tropical and subtropical Africa and compiled >1400 literature Δ14C data and ancillary parameters from rivers globally. Our analysis reveals a consistent pattern whereby POC is progressively older in systems carrying higher sediment loads, coinciding with a lower organic carbon content. At the global scale, this pattern leads to a proposed global median Δ14C signature of −203‰, corresponding to an age of ~1800 years B.P. For DOC exported to the coastal zone, we predict a modern (decadal) age (Δ14C = +22 to +46‰), and paired data sets confirm that riverine DOC is generally more recent in origin than POC—in contrast to the situation in ocean environments. Weathering regimes complicate the interpretation of 14C ages of dissolved inorganic carbon, but the available data favor the hypothesis that in most cases, more recent organic C is preferentially mineralized.
Key Points
Δ14CPOC is well correlated with %POC consistent with a two‐source mixing model
Global flux‐weighted %POC suggests a median Δ14CPOC age of ~1800 years B.P.
DOC is generally younger than POC and largely dominated by modern C
Living in close association with other organisms has proven to be a widespread and successful strategy in nature. Some communities are completely driven by symbiotic associations and therefore, ...intimate relationships among the partners can be expected. Here, we analyzed in-depth the food web of a particularly rich community of arthropods found in strict association with European red wood ants (Formica rufa group). We studied the trophic links between different ant-associated myrmecophiles and food sources associated with the host ant, but also tested predator–prey links among myrmecophiles themselves. Our approach combined direct feeding tests and stable carbon and nitrogen isotope analyses for a large number of myrmecophiles. The results of the direct feeding tests reveal a complex food web. Most myrmecophiles were found to parasitize on ant brood. Moreover, we encountered multiple trophic predator–prey links among the myrmecophiles. The results of the stable isotope analyses complement these findings and indicate the existence of multiple trophic levels and trophic isotopic niche compartmentalization. δ15N values were strongly correlated with the trophic levels based on the direct tests, reflecting that δ15N values of myrmecophiles increased with higher trophic levels. This strong correlation underlines the strength of stable isotopes as a powerful tool to assess trophic levels. In addition, the stable isotope data suggest that most species only facultatively prey on ant brood. The presence of numerous trophic interactions among symbionts clearly contrasts with the traditional view of social insects nests as offering an enemy-free space for symbionts. Interestingly, the ant host can indirectly benefit from these interactions because brood predators are also preyed upon by other myrmecophiles. Overall, this study provides unique insights into the complex interactions in a small symbiont microcosm system and suggests that the interactions between host and symbiont might be mediated by other symbionts in the same community.
Carbon emissions to the atmosphere from inland waters are globally significant and mainly occur at tropical latitudes. However, processes controlling the intensity of CO2 and CH4 emissions from ...tropical inland waters remain poorly understood. Here, we report a data-set of concurrent measurements of the partial pressure of CO2 (pCO2) and dissolved CH4 concentrations in the Amazon (n = 136) and the Congo (n = 280) Rivers. The pCO2 values in the Amazon mainstem were significantly higher than in the Congo, contrasting with CH4 concentrations that were higher in the Congo than in the Amazon. Large-scale patterns in pCO2 across different lowland tropical basins can be apprehended with a relatively simple statistical model related to the extent of wetlands within the basin, showing that, in addition to non-flooded vegetation, wetlands also contribute to CO2 in river channels. On the other hand, dynamics of dissolved CH4 in river channels are less straightforward to predict, and are related to the way hydrology modulates the connectivity between wetlands and river channels.
Nitrogen stable isotope ratios (δ15N) of organic material have successfully been used to track food-web dynamics, nitrogen baselines, pollution, and nitrogen cycling. Extending the δ15N record back ...in time has not been straightforward due to a lack of suitable substrates in which δ15N records are faithfully preserved, thus sparking interest in utilizing skeletal carbonate-bound organic matter (CBOM) in mollusks, corals, and foraminifera. Here we test if calcite Pecten maximus shells from the Bay of Brest and the French continental shelf can be used as an archive of δ15N values over a large environmental gradient and at a high temporal resolution (approximately weekly). Bulk CBOM δ15N values from the growing tip of shells collected over a large nitrogen isotope gradient were strongly correlated with adductor muscle tissue δ15N values (R2=0.99, n=6, p<0.0001). We were able to achieve weekly resolution (on average) over the growing season from sclerochronological profiles of three shells, which showed large seasonal variations up to 3.4‰. However, there were also large inter-specimen differences (up to 2.5‰) between shells growing at the same time and location. Generally, high-resolution shell δ15N values follow soft-tissue δ15N values, but soft-tissues integrate more time, hence soft-tissue data are more time-averaged and smoothed. Museum-archived shells from the 1950s, 1965, and 1970s do not show a large difference in δ15N values through time despite expected increasing N loading to the Bay over this time, which could be due to anthropogenic N sources with contrasting values. Compiling shell CBOM δ15N data from several studies suggests that the offset between soft-tissue and shell δ15N values (Δtissue-shell) differs between calcite and aragonite shells. We hypothesize that this difference is caused by differences in amino acids used in constructing the different minerals, which should be specific to the CaCO3 polymorph being constructed. Future work should use compound specific isotope analyses (CSIA) to test this hypothesis, and to determine whether certain amino acids could specifically track N sources or possibly identify amino acids that are more resistant to diagenesis in fossil shells. In conclusion, bivalve shell CBOM δ15N values can be used in a similar manner to soft-tissue δ15N values, and can track various biogeochemical events at a very high-resolution.
Inland waters (rivers, lakes and ponds) are important conduits for the emission of terrestrial carbon in Arctic permafrost landscapes. These emissions are driven by turnover of contemporary ...terrestrial carbon and additional pre-aged (Holocene and late-Pleistocene) carbon released from thawing permafrost soils, but the magnitude of these source contributions to total inland water carbon fluxes remains unknown. Here we present unique simultaneous radiocarbon age measurements of inland water CO
, CH
and dissolved and particulate organic carbon in northeast Siberia during summer. We show that >80% of total inland water carbon was contemporary in age, but pre-aged carbon contributed >50% at sites strongly affected by permafrost thaw. CO
and CH
were younger than dissolved and particulate organic carbon, suggesting emissions were primarily fuelled by contemporary carbon decomposition. Our findings reveal that inland water carbon emissions from permafrost landscapes may be more sensitive to changes in contemporary carbon turnover than the release of pre-aged carbon from thawing permafrost.
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