Transport of organic carbon in the world’s rivers

Microbial community structure in the river sediments from upstream of Guanting Reservoir: Potential impacts of reclaimed water recharge

This work systematically investigated the microbial community structure in the river sediments from upstream of Guanting Reservoir, Beijing, China. A total of 6 wastewater treatment plants (WWTPs) locate along the main rivers connected to the reservoir. Water and sediment samples were collected at sites near the effluents of WWTPs (regarded as W groups) or at the upstream/downstream rivers (R groups) to reveal the roles of the reclaimed water recharge. Multivariate techniques including typical statistical analysis, redundancy analysis (RDA), nonmetric multidimensional scaling analysis, and molecular ecological network analysis were used to evaluate the results and their relationships. The representative C/N/P water parameters and concentrations of target organic contaminants kept stable for W and R sites, while the microbial community parameters varied greatly for two groups. The microbial population at W sites were higher but with a lower biological diversity (with a lower Shannon index) than that at R sites, indicating WWTPs greatly altered the microbial community structure at the local reach. RDA results revealed that total organic carbon (TOC) and organophosphorus pesticides (OPPs) were two dominant factors affecting the function and composition of microbial communities at the phylum level. The network analysis revealed that the microbes with the most interactions mainly from R sites and they had closer relationships with each other.

Chromophoric dissolved organic matter composition and load from a coastal river system under variable flow regimes

Chromophoric dissolved organic matter (CDOM) exported from riverine catchments can influence biogeochemical processes in coastal environments with implications for water quality and carbon budget. Despite recent efforts to quantify C fluxes during high flow events, knowledge gaps exist regarding the fluxes and yield of terrestrial, reactive vs. recalcitrant CDOM under episodic to base-flow conditions from uplands to downstream estuaries. We used stream dissolved organic carbon (DOC) concentrations and CDOM optical properties using parallel factor analysis to characterize composition and fluxes under variable flow conditions for a coastal river basin in the SE USA. Our findings showed that episodic flows (>75th percentile) were marked by the elevated flux of humic acid-like CDOM and lower in-stream autochthonous production, or microbial degradation. Further, 70% of the terrestrial CDOM was exported during high flows, with a 3-fold increase in CDOM flux during episodic events, including Hurricane Irene in 2011. While, low flows (<25th percentile) were marked by an increased abundance of microbial, humic CDOM that can be easily processed within the estuary. Due to greater wetland coverage in the Neuse, the annual CDOM yield was 5-6 times higher than the larger rivers, such as the Mississippi, USA, and Changjiang, China. We suggest that similar coastal watersheds in SE USA or elsewhere may contribute substantial amounts of reactive CDOM to the estuaries during high flow conditions and can have negative water quality implications for the coastal C dynamics. These findings can help predict the evolution of coastal C cycling under projected climate change and inform the development of appropriate management strategies.

Small streams dominate US tidal reaches and will be disproportionately impacted by sea-level rise

Rivers and streams represent <0.6% of the Earth's land surface but play a disproportionately large role in global biogeochemical cycles and provide locally relevant ecosystem services. However, knowledge of how rivers influence material budgets and ecosystem services has major gaps due to the lack of explicit consideration of tidally-influenced reaches. Focusing on the conterminous US, we provide a foundation for understanding the role of tidal streams. We find that 66% of tidal stream length is contributed from low order streams (< 4th order), and that terrestrial ecosystem production in low-lying coastal zones is 30% greater than in adjacent terrestrial ecosystems. This prevalence of small streams indicates that small coastal watersheds dominate tidally influenced spatial domains. Furthermore, we find that relative sea-level rise (RSLR) will have a disproportionate impact on low order tidal streams and their terrestrial interfaces - 1 m RSLR will decrease the tidal stream land-water interface by 17% and the total surface area of US tidal streams by 31%. Upstream reaches of tidal zones will be extended in response to RSLR, but gains will be more than offset by coastal losses because topographic gradients become steeper moving inland, and accretion rates may not keep pace with RSLR. These results highlight previously unrecognized dominance, high productivity, and disproportionate future loss of low-order coastal ecosystems. This indicates a critical need to focus research on small tidal stream systems under contemporary and future conditions.

Leaching of dissolved organic carbon from mineral soils plays a significant role in the terrestrial carbon balance

The leaching of dissolved organic carbon (DOC) from soils to the river network is an overlooked component of the terrestrial soil C budget. Measurements of DOC concentrations in soil, runoff and drainage are scarce and their spatial distribution highly skewed towards industrialized countries. The contribution of terrestrial DOC leaching to the global-scale C balance of terrestrial ecosystems thus remains poorly constrained. Here, using a process based, integrative, modelling approach to upscale from existing observations, we estimate a global terrestrial DOC leaching flux of 0.28 ± 0.07 Gt C year-1 which is conservative, as it only includes the contribution of mineral soils. Our results suggest that globally about 15% of the terrestrial Net Ecosystem Productivity (NEP, calculated as the difference between Net Primary Production and soil respiration) is exported to aquatic systems as leached DOC. In the tropical rainforest, the leached fraction of terrestrial NEP even reaches 22%. Furthermore, we simulated spatial-temporal trends in DOC leaching from soil to the river networks from 1860 to 2010. We estimated a global increase in terrestrial DOC inputs to river network of 35 Tg C year-1 (14%) from 1860 to 2010. Despite their low global contribution to the DOC leaching flux, boreal regions have the highest relative increase (28%) while tropics have the lowest relative increase (9%) over the historical period (1860s compared to 2000s). The results from our observationally constrained model approach demonstrate that DOC leaching is a significant flux in the terrestrial C budget at regional and global scales.

Plant source and soil interact to determine characteristics of dissolved organic matter leached into waterways from riparian leaf litter

Wetting of leaf litter accumulated in riparian zones during rainfall events provides pulses of dissolved organic matter (DOM) to rivers. Restoring riparian vegetation aims to reduce sediment and nutrient transport into rivers, however DOM from leaf litter can stimulate phytoplankton growth and interfere with water treatment processes. Improved understanding of the loads and chemical composition of DOM leached from leaf litter of different plant species, and how subsequent leaching through soils affects DOM retention or transformation, is needed to predict the outcomes of riparian revegetation. To investigate this, we simulated rapid leaching of rainfall through the leaf litter of two riparian tree species with and without subsequent leaching through soil, comparing dissolved organic carbon (DOC) and nitrogen (DON) loads, and DOM chemical composition (via spectroscopic and novel NMR-fingerprinting techniques). Plant source affected the load and composition of DOM leaching, with Eucalyptus tereticornis leaching more DOC than Casuarina cunninghamiana. Additionally, E. tereticornis DOM had a higher sugar, myo-inositol, benzoic acid, flavonoid and oxygenated aromatic content. More than 90% of leaf litter DOM was retained in the soil under simulated repeated heavy rainfall. The DOM chemistry of these species determined the total loads and changes in DOM composition leaching through soil. Less E. tereticornis DOM was retained by the soil than C. cunninghamiana DOM, with sugars, myo-inositol and amino acids being poorly retained compared to fatty acids and aromatic compounds. It also appears that DOM from E. tereticornis litter primed the soil, resulting in more DON being leached compared with bare soil. In comparison, C.cunninghamiana litter resulted in greater retention of DON, oxygenated aromatic compounds and the amino acid tryptophan. This study provides new information on how a range of DOM sources and transformations affect the DOM ultimately leached into waterways, key to developing improved models of DOM transformations in catchments.

Changes in riverine organic carbon input to the ocean from mainland China over the past 60 years

Compared to rivers in Europe and North America, Chinese rivers that discharge into oceans have different organic carbon (OC) transport characteristics. Out of the top 25 largest rivers worldwide, three (Changjiang, Huanghe, and Zhujiang rivers) are located in China, along with numerous small rivers. Thus, synthesized estimates of total riverine OC flux from Chinese rivers into marginal seas are critical but remain deficient. In this study, we developed relationships between riverine OC (dissolved OC, or DOC, and particulate OC, or POC) and basin characteristic variables (basin population density, precipitation, and riverine suspended sediment concentration) to estimate annual riverine DOC and POC fluxes during 1953-2016. The results showed that rivers in mainland China transported 9.63 Tg C of OC to the marginal seas in 2008, with 4.61 Tg C of DOC and 5.02 Tg C of POC. Of this transported OC, 14.28% DOC and 17.49% POC were transported by small southeastern rivers, whose drainage areas covered only 6.68% of the total. Because of intensifying human activities, DOC export increased but POC export decreased during 1953-2016. Additionally, basin population growth and reservoir water capacity were the major factors for increasing DOC flux and decreasing POC flux, respectively. Overall, the DOC/POC ratio increased for OC transport in Chinese rivers. Therefore, this study is important for understanding human-induced impacts on environmental change and the carbon cycle in marginal seas.

Enhanced particulate Hg export at the permafrost boundary, western Siberia

Arctic permafrost soils contain large amounts of organic carbon and the pollutant mercury (Hg). Arctic warming and associated changes in hydrology, biogeochemistry and ecology risk mobilizing soil Hg to rivers and to the Arctic Ocean, yet little is known about the quantity, timing and mechanisms involved. Here we investigate seasonal particulate Hg (PHg) and organic carbon (POC) export in 32 small and medium rivers across a 1700 km latitudinal permafrost transect of the western Siberian Lowland. The PHg concentrations in suspended matter increased with decreasing watershed size. This underlines the significance of POC-rich small streams and wetlands in PHg export from watersheds. Maximum PHg concentrations and export fluxes were located in rivers at the beginning of permafrost zone (sporadic permafrost). We suggest this reflects enhanced Hg mobilization at the permafrost boundary, due to maximal depth of the thawed peat layer. Both the thickness of the active (unfrozen) peat layer and PHg run-off progressively move to the north during the summer and fall seasons, thus leading to maximal PHg export at the sporadic to discontinuous permafrost zone. The discharge-weighed PHg:POC ratio in western Siberian rivers (2.7 ± 0.5 μg Hg: g C) extrapolated to the whole Ob River basin yields a PHg flux of 1.5 ± 0.3 Mg y^-1, consistent with previous estimates. For current climate warming and permafrost thaw scenarios in western Siberia, we predict that a northward shift of permafrost boundaries and increase of active layer depth may enhance the PHg export by small rivers to the Arctic Ocean by a factor of two over the next 10-50 years.

Quality and reactivity of dissolved organic matter in a Mediterranean river across hydrological and spatial gradients

Understanding DOM transport and reactivity in rivers is essential to having a complete picture of the global carbon cycle. In this study, we explore the effects of hydrological variability and downstream transport on dissolved organic matter (DOM) dynamics in a Mediterranean river. We sampled the main stem of the river Tordera from the source to the sea, over a range of fifteen hydrological conditions including extreme events (flood and drought). By exploring spatial and temporal gradients of DOM fluorescence properties, river hydrology was found to be a significant predictor of DOM spatial heterogeneity. An additional space-resolved mass balance analysis performed on four contrasting hydrological conditions revealed that this was due to a shift in the biogeochemical function of the river. Flood conditions caused a conservative transport of DOM, generating a homogeneous, humic-like spatial profile of DOM quality. Lower flows induced a non-conservative, reactive transport of DOM, which enhanced the spatial heterogeneity of DOM properties. Moreover, the downstream evolution of DOM chemostatic behaviour revealed that the role of hydrology in regulating DOM properties increased gradually downstream, indicating an organised inter-dependency between the spatial and the temporal dimensions. Overall, our findings reveal that riverine DOM dynamics is in constant change owing to varying hydrological conditions, and emphasize that in order to fully understand the role of rivers in the global carbon cycle, it is necessary to take into account the full range of hydrological variability, from floods to droughts.

Suspended solid concentration reduces feeding in freshwater mussels

We examined the effect of TSS concentration on the clearance rates (CR) of newly transformed juvenile and adult Lampsilis fasciola, L. siliquoidea, Ligumia nasuta, and Villosa iris, as increased total suspended solids (TSS) are thought to interfere with feeding processes. Mussel CR were measured in aerated (or swirled for juveniles) chambers at TSS concentrations up to 15mgL^-1 for laboratory-transformed juveniles, and up to 100mgL^-1 for adult mussels. The CR of one-week old animals increased with TSS concentration, likely due to ontological differences in feeding (pedal vs. suspension feeding) and gill development, but CR decreased monotonically with TSS concentration in older animals (two-, three- and four-week old juveniles). The CR of adult mussels were significantly lower at TSS concentrations ≥8mgL^-1, which represented a threshold in CR. Although this threshold occurred at similar concentrations across the four species, the decline in CR was largest in L. fasciola (46% compared to no-TSS control), and smallest in V. iris (21%). Differences among species are likely related to differences in the TSS and substrate found in their source rivers given that greater decline occurred for species in rivers with relatively lower TSS. The decrease in CR as TSS increased is consistent across marine and freshwater bivalves, at both juvenile and adult stages. The decrease in feeding was five times greater in juvenile compared to adult bivalves, which indicates how the vulnerability to environmental stressors differ across life stages. These results demonstrate that TSS reduces suspension feeding rates in freshwater unionids, therefore TSS should be managed to ensure their survival.

A significant net sink for CO2 in Tokyo Bay

Most estuaries and inland waters are significant source for atmospheric CO₂ because of input of terrestrial inorganic carbon and mineralization of terrestrially supplied organic carbon. In contrast to most coastal waters, some estuaries with small freshwater discharge are weak source or sometimes sink for CO₂. Extensive surveys of pCO₂ in Tokyo Bay showed that the overall bay acts as a strong net sink for atmospheric CO₂. Although small area was a consistent source for CO₂, active photosynthesis driven by nutrient loading from the land overwhelmed the CO₂ budget in the bay. Here we show a comprehensive scheme with a border where air-sea CO₂ flux was ±0 between nearshore waters emitting CO₂ and offshore waters absorbing CO₂. The border in Tokyo Bay was extremely shifted toward the land-side. The shift is characteristic of highly urbanized coastal waters with an extensive sewage treatment system in the catchment area. Because highly urbanized coastal areas worldwide are expected to quadruple by 2050, coastal waters such as Tokyo Bay are expected to increase as well. Through extrapolation of Tokyo Bay data, CO₂ emission from global estuaries would be expected to decrease roughly from the current 0.074 PgC year⁻¹ to 0.014 PgC year⁻¹ in 2050.