Comment on bg-2021-234

Methane (CH4) emissions from freshwaters to the atmosphere have a profound impact on global atmospheric greenhouse gas (GHG) concentrations. Anthropogenic footprints such as dam construction and reservoir operation significantly changed the fate and transport of CH4 in freshwaters. The type of particulate organic carbon (POC) in reservoirs is a critical factor controlling CH4 production and emissions. However, little is known of how reservoir operation mediates the distribution of POC and regulates CH4 accumulation in cascade hydroelectric reservoirs. Here, spatial and temporal variations in POC and CH4 were explored in the Xiluodu (XLD) and Xiangjiaba (XJB) reservoirs which are deep valley dammed cascade reservoirs located in the main channel of the upper Yangtze River. Based on the δ13C-POC and N / C mole ratios of particulate organic matter, the results of multi-endmember stable isotope mixing models by a Bayesian model show that terrestrial POC and autochthonous POC accounted for approximately 56 ± 19 % and 42 ± 19 % (SD, n = 181) of POC, respectively. CH4 concentrations and δ13C-CH4 in the cascade reservoirs were potentially influenced by CH4 oxidation. Together with other physicochemical parameters and structural equation model, these results suggested that the input of terrestrial POC was dominantly influenced by water level variations and flow regulation due to reservoir operation. The cumulative effect of POC caused by cascade reservoirs was not apparent at a bimonthly scale. Terrestrial POC was more likely to dominate CH4 accumulation in cascade reservoirs under reservoir operation.

Combined use of radiocarbon and stable carbon isotope to constrain the sources and cycling of particulate organic carbon in a large freshwater lake, China

Ecosystems are generally linked via fluxes of nutrients and energy across their boundaries. For example, freshwater ecosystems in temperate regions may receive significant inputs of terrestrially derived carbon via autumnal leaf litter. This terrestrial particulate organic carbon (POC) is hypothesized to subsidize animal production in lakes, but direct evidence is still lacking. We divided two small eutrophic lakes each into two sections and added isotopically distinct maize litter to the treatment sections to simulate increased terrestrial POC inputs via leaf litter in autumn. We quantified the reliance of aquatic consumers on terrestrial resources (allochthony) in the year subsequent to POC additions by applying mixing models of stable isotopes. We also estimated lake-wide carbon (C) balances to calculate the C flow to the production of the major aquatic consumer groups: benthic macroinvertebrates, crustacean zooplankton, and fish. The sum of secondary production of crustaceans and benthic macroinvertebrates supported by terrestrial POC was higher in the treatment sections of both lakes. In contrast, total secondary and tertiary production (supported by both autochthonous and allochthonous C) was higher in the reference than in the treatment sections of both lakes. Average aquatic consumer allochthony per lake section was 27–40%, although terrestrial POC contributed less than about 10% to total organic C supply to the lakes. The production of aquatic consumers incorporated less than 5% of the total organic C supply in both lakes, indicating a low ecological efficiency. We suggest that the consumption of terrestrial POC by aquatic consumers facilitates a strong coupling with the terrestrial environment. However, the high autochthonous production and the large pool of autochthonous detritus in these nutrient-rich lakes make terrestrial POC quantitatively unimportant for the C flows within food webs.

Differential transport and degradation of bulk organic carbon and specific terrestrial biomarkers in the surface waters of a sub-arctic brackish bay mixing zone

Sources of particulate organic carbon (POC) and trophic–dynamic relationships were studied in a reservoir with low sportfish populations. Fish community structure and POC input from tributaries, phytoplankton primary production, and vascular vegetation on mudflats were estimated. Gizzard shad (Dorosoma cepedianum) averaged 39.8% of the total fish biomass in 1988, and as much as 94% of the total forage fish biomass was too large to serve as prey for most predators. Phytoplankton primary productivity averaged 1182 mg C∙m−2∙d−1in 1987 and 1988, contributed 33.57 Gg POC∙yr−1to the reservoir, and apparently was phosphate limited. POC inflow from tributaries contributed 60.00 Gg∙yr−1with 79% of POC <75 μm in size. Winter and spring stormflow was responsible for 92% of the total POC transported. Considering POC size fractions available to gizzard shad, POC input from tributaries, phytoplankton, and mudflats contributed 21, 54, and 25% of the total POC input, respectively. The trophic–dynamic analysis indicated that phytoplankton POC was adequate to support the reservoir fish community. A more efficient transfer of carbon in the food web might be accomplished by stocking with a smaller forage fish, such as threadfin shad (Dorosoma petenense), which are not common in the reservoir.

River-ocean continuums are rich environments key to the transformations of organic matter and nutrients from many sources. Human impacts on these ecosystems can be local, upstream, or global. Particulate organic matter provides one tool for exploring these processes; inventories and carbon and nitrogen stable isotopes are useful indicators. This study conducted comprehensive field investigations from the upper reaches of the Yangtze River to the coastal waters to explore the distribution and spatial-temporal variation of particulate organic carbon (POC) and nitrogen (PON), and dissolved organic carbon (DOC) in surface waters from April–May (Spring) and October–November (Autumn) 2014. Carbon and nitrogen isotopes (δ13C and δ15 N) were used to assess organic matter sources. Numerous environmental factors were also measured. PON and POC exhibited similar spatial and temporal variations along the river mainstream. POC increased from the upper to lower reaches, similarly in both seasons; POC declined seaward from the river mouth. In contrast, PON showed greater seasonal and spatial variations along the continuum, with greater primary production in offshore areas. DOC along the continuum was higher in autumn, likely related to greater river discharge and warmer waters. The DOC/POC ratios by weight were >1 in most samples except for the ones collected from the turbidity maximum zone, suggesting that DOC contributed to a larger fraction of the total organic matter inventories. End-member mixing models revealed that the proportion of organic matter from upstream (allochthonous) sources was >70% in the mainstream during both seasons. In coastal waters, the proportion of allochthonous sources was only >57%. Compared with other major world rivers, DOC and allochthonous POC are especially high in the Yangtze River-ocean continuum. Human activities causing variations in terrestrial inputs are likely an important driving factor. In addition, the contribution of soil erosion caused by urbanization to riverine organic carbon cannot be ignored.

Source, transport and fluxes of Amazon River particulate organic carbon: Insights from river sediment depth-profiles

Efficient transport of terrestrial particulate carbon in a tectonically-active marginal sea off southwestern Taiwan

Small rivers draining mountain islands are important in the transfer of terrestrial particulate organic carbon (POC) to the oceans. This input has implications for the geochemical stratigraphic record. We have investigated the stable isotopic composition of POC ([delta]13Corg) in rivers draining the mountains of Taiwan. In 15 rivers, the suspended load has a mean [delta]13Corg that ranges from -28.1 ±0.8[per mille sign] to -22.0 ±0.2[per mille sign] (on average 37 samples per river) over the interval of our study. To investigate this variability we have supplemented suspended load data with measurements of POC in bedrock and river bed materials, and constraints on the composition of the terrestrial biomass. Fossil POC in bedrock has a range in [delta]13Corg from -25.4 ±1.5[per mille sign] to -19.7 ±2.3[per mille sign] between the major geological formations. Using coupled [delta]13Corg and N/C we have found evidence in the suspended load for mixing of fossil POC with non-fossil POC from the biosphere. In two rivers outside the Taiwan Central Range anthropogenic land use appears to influence [delta]13Corg, resulting in more variable and lower values than elsewhere. In all other catchments, we have found that 5[per mille sign] variability in [delta]13Corg is not controlled by the variable composition of the biomass, but instead by heterogeneous fossil POC.
\nIn order to quantify the fraction of suspended load POC derived from non-fossil sources (Fnf) as well as the isotopic composition of fossil POC ([delta]13Cfossil) carried by rivers, we adapt an end-member mixing model. River suspended sediments and bed sediments indicate that mixing of fossil POC results in a negative trend between N/C and [delta]13Corg that is distinct from the addition of non-fossil POC, collapsing multiple fossil POC end-members onto a single mixing trend. As an independent test of the model, Fnf reproduces the fraction modern (Fmod) in our samples, determined from 14C measurements, to within 0.09 at the 95% confidence level. Over the sampling period, the mean Fnf of suspended load POC was low (0.29 ±0.02, n=459), in agreement with observations from other mountain rivers where physical erosion rates are high and fossil POC enters river channels. The mean [delta]13Cfossil in suspended POC varied between -25.2 ±0.5[per mille sign] and -20.2 ±0.6[per mille sign] from catchment to catchment. This variability is primarily controlled by the distribution of the major geological formations. It also covers entirely the range of [delta]13Corg found in marine sediments which is commonly thought to derive from mixing between marine and terrigenous POC. If land-sourced POC is preserved in marine sediments, then changes in the bulk [delta]13Corg observed offshore Taiwan could instead be explained by changes in the onshore provenance of sediment. The range in [delta]13Corg of fossil organic matter in sedimentary rocks exposed at the surface is large and given the importance of these rocks as a source of clastic sediment to the oceans, care should be taken in accounting for fossil POC in marine deposits supplied by active mountain belts.

Riverine inputs of total organic carbon and suspended particulate matter from the Pearl River Delta to the coastal ocean off South China

Global riverine discharge of organic matter represents a substantial source of terrestrial dissolved and particulate organic carbon to the oceans. This input from rivers is, by itself, more than large enough to account for the apparent steady-state replacement times of 4,00-6,000 yr for oceanic dissolved organic carbon. But paradoxically, terrestrial organic matter, derived from land plants, is not detected in seawater and sediments in quantities that correspond to its inputs. Here we present natural 14C and 13C data from four rivers that discharge to the western North Atlantic Ocean and find that these rivers are sources of old (14C-depleted) and young (14C-enriched) terrestrial dissolved organic carbon, and of predominantly old terrestrial particulate organic carbon. These findings contrast with limited earlier data that suggested terrestrial organic matter transported by rivers might be generally enriched in 14C from nuclear testing, and hence newly produced. We also find that much of the young dissolved organic carbon can be selectively degraded over the residence times of river and coastal waters, leaving an even older and more refractory component for oceanic export. Thus, pre-ageing and degradation may alter significantly the structure, distributions and quantities of terrestrial organic matter before its delivery to the oceans.

Satellite estimation of particulate organic carbon flux from Changjiang River to the estuary

Methane accumulation and its potential precursor compounds in the oxic surface water layer of two contrasting stratified lakes

The decomposition of a single macrophytes species may not represent entirely the carbon cycling in aquatic ecosystems, as in the freshwater macrophyte-dominant environments several species grow and decay concomitantly. To assess the interaction of the two species in the decomposition process, the temporal variation of particulate organic carbon (POC) of Hedychium coronarium J. Konig, Typha domingensis Persoon and the mixed sample (50% of each species) was mathematically modeled. Kinetic models were used to verify the temperature and the availability of dissolved oxygen, as regulating factors in decomposition. The aerobic processes favored a faster decay when compared to anaerobic processes. The occurrence of two phases in decomposition was observed: (1) with a rapid mass loss (POCLS) and (2) with a slow degradation (POCR). During the aerobic decomposition, independently of variation in temperature, the effect was always antagonistic. However, under anaerobic conditions, the three types were observed (antagonistic, additive and synergic). The mixed detritus always displayed the highest Q10 coefficient. Modeling mixed detritus decomposition was a reliable predictive framework of litter decomposition at a ecosystemic scale, improving ecosystem response of carbon cycling feedback under an increasing temperature.

The supply of particulate organic carbon (POC) associated with terrigenous solids transported to the ocean by rivers plays a significant role in the global carbon cycle. To advance our understanding of the source, transport, and fate of fluvial POC from regional to global scales, databases of riverine POC are needed, including elemental and isotope composition data from contrasted river basins in terms of geomorphology, lithology, climate, and anthropogenic pressure. Here, we present a new, open-access, georeferenced, global database called Modern River Archives of Particulate Organic Carbon (MOREPOC) version 1.0, featuring data on POC in suspended particulate matter (SPM) collected at 231 locations across 118 major river systems. This database includes 3,424 SPM data entries, among which 2,943 with POC content, 3,260 with stable carbon isotope (δ13C) values, 2,018 with radiocarbon activity (Δ14C) values, 1,838 with total nitrogen content, and 309 with aluminum-to-silicon mass ratios (Al / Si). The MOREPOC database aims at being used by the Earth System community to build comprehensive and quantitative models for the mobilization, alteration, and fate of terrestrial POC. The database is made available on the Zenodo repository in machine-readable formats as data table and GIS shapefile at https://doi.org/10.5281/zenodo.6541925 (Ke et al., 2022).