Particulate Organic Carbon Input Research Articles

Leaf litter breakdown phenology in headwater stream networks is modulated by groundwater thermal regimes and litter type

AbstractLeaf litter dominates particulate organic carbon inputs to forest streams. Using data‐informed simulations, we explored how litter type (slow‐ vs. fast‐decomposing species), pulsed autumn litter inputs, groundwater‐mediated temperature regimes, and climate warming affect litter breakdown in a 3rd‐order stream network. We found that the time‐dependent interactions of these variables govern network‐scale litter breakdown phenology, with greater thermal sensitivity of slow‐decomposing litter for both current and future scenarios. Groundwater thermal inputs modified litter breakdown phenology by reducing spring and summer and elevating winter litter breakdown fluxes. Under future warming scenarios, the source depth of contributing groundwater influenced summer detrital resources; shallow groundwater‐fed streams had reduced summer resources compared to deep groundwater‐fed streams. Our results demonstrate that predicting in‐stream carbon cycling requires explicit consideration of the phenology of resource inputs and the seasonal timing of environmental factors, notably stream thermal regimes.

Progress on the Effect of Nitrogen on Transformation of Soil Organic Carbon

Carbon and nitrogen are the essential elements constituting living organisms and are closely coupled during biogeochemical cycles. Due to the atmospheric nitrogen deposition and increased agricultural nitrogen fertilizer input, the effect of nitrogen on the sequestration of soil organic carbon (SOC) is controversial. To facilitate a comprehensive understanding of this issue, the progress of recent studies on the different SOC stabilization mechanisms is reviewed. Based on the differences in the stability and fate mechanisms of particulate organic carbon (POC) and mineral-associated organic carbon (MAOC), nitrogen input can increase POC input and inhibit microbial decomposition of POC by increasing terrestrial biomass, changing the quality of litter and promoting the formation of aggregates. N input reduces the chemical stability of MAOC by altering the chemical bonding of mineral–organic complexes. This study has promising implications for understanding the effect of N on SOC transformation by different stabilization mechanisms to promote soil carbon sequestration.

Transport of particulate organic carbon in the lower Yellow River (Huanghe) as modulated by dam operation

The Yellow River has changed into a highly dam-controlled system over the recent decades. Based on gauging records of water-sediment discharge and particulate organic carbon (POC) datasets, this paper investigates the dam impacts on spatio-temporal variability of hydrology and POC transport. With a large part of suspended sediment and POC normally preserved within the Xiaolangdi (XLD) reservoir, erosion of riverbed downstream the reservoir has now become a primary source of sediment and POC to coastal ocean (172 Mt/yr for sediment and 0.56 Mt/yr for POC, respectively). Sediment accumulation and POC burial within the XLD reservoir are 246 Mt/yr and 0.60 Mt/yr, respectively, globally accounting for ∼10% and ∼1% of the retentions in reservoirs. Downstream the XLD reservoir, the averaged Δ 14 C values of POC was −451 ± 25‰, corresponding to a 14 C age of 4763 ± 366 yr BP, indicating that the POC in the lower reaches of the Yellow River was ∼2400 yr younger than that in the Loess Plateau region where most of the sediment and POC are sourced. Petrogenic POC fluxes upstream and downstream the XLD dam were 0.12 Mt/yr and 0.06 Mt/yr, respectively, clearly indicative of large-scale trapping of petrogenic POC by the XLD dam. At present, the XLD dam regulation intercepts this petrogenic POC from the Loess Plateau and enhances the downstream input of biospheric POC ( Fm = 0.85) to the coast. In contrast to the sediment and POC delivery of the rivers worldwide that are primarily dominated by tectonic, climatic and geographic settings, human activities, especially the dam regulation in the Yellow River, have significantly altered river hydrologic regime and overwhelmed the sediment and POC delivery within the river-coast continuum, which has profound impacts on carbon cycling in the coastal margin. Schematic diagram for POC and sediment budget within the Yellow River-Coast Continuum from 2002 to 2014 based on observed and reconstrcuted datasets. • The XLD reservoir acts as a sink of POC coupled with periodic rapid sediment accumulation. • Downstream riverbed erosion is a dominant source of POC to the coastal margin. • Dam regulation has substantially transformed the POC delivery during the WSRS.

Terrigenous organic carbon drives methane dynamics in cascade reservoirs in the upper Yangtze China

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 source 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 transport 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 cascade reservoirs located in the main channel of the upper Yangtze River. Based on the δ13C-POC and N/C mole ratio of particulate organic matter, the results of multi-endmember stable isotope mixing models by a Bayesian model showed that terrigenous POC and autochthonous POC accounted for approximately 55% ± 18% and 43% ± 19% (SD, n = 179) of POC, respectively. Together with other hydrological and environmental parameters, we found that the input of terrigenous POC was dominantly influenced by water level variations and flow regulation due to reservoir operation. The cumulative effect of POC caused by cascade dams was not apparent. Terrigenous POC were more likely to drive CH4 accumulation in our study. Evident low level of CH4 in both reservoirs were likely affected by low sedimentation of POC and microbial CH4 oxidation. We hope our study could provide a conceptual framework for further modeling of CH4 dynamics in cascade reservoirs.

The Spatiotemporal Evolution of Storm Pulse Particulate Organic Carbon in a Low Gradient, Agriculturally Dominated Watershed

Streams and rivers integrate and transport particulate organic carbon (POC) from an array of aquatic and terrestrial sources. Storm events greatly accelerate the transport of POC. The sequences by which individual POC inputs are mobilized and transported are not well-documented but are predicted to be temporally transient and spatially dependent because of changes in forcing functions, such as precipitation, discharge, and watershed morphology. In this study, the 3rd−4th order agricultural stream network, Clear Creek in Iowa, U.S.A., was sampled at a nested series of stations through storm events to determine how suspended POC changes over time and with distance downstream. Carbon and nitrogen stable isotope ratios were used to identify changes in POC. A temporal sequence of inputs was identified: in-channel algal production prior to heavy precipitation, row crop surface soils mobilized during peak precipitation, and material associated with the peak hydrograph that is hypothesized to be an integrated product from upstream. Tile drains delivered relatively 13C- and 15N-depleted particulate organic carbon that is a small contribution to the total POC inventory in the return to baseflow. The storm POC signal evolved with passage downstream, the principal transformation being the diminution of the early flush surface soil peak in response to a loss of connectivity between the hillslope and channel. Bank erosion is hypothesized to become increasingly important as the signal propagates downstream. The longitudinal evolution of the POC signal has implications for C-budgets associated with soil erosion and for interpreting the organic geochemical sedimentary record.

Shelf and deep-water benthic macrofauna assemblages from the western Gulf of Mexico: Temporal dynamics and environmental drivers

Shelf and deep-water soft-bottom macrofauna were explored in the western Gulf of Mexico in terms of species and functional trait assemblages. Their variation was analysed as functions of depth and time, and the relationship with sea-bottom environmental conditions was examined to disentangle their association with potential environmental drivers. Four consecutive cruises (two per year, at the end of the dry and rainy seasons) were performed during 2016–2017 at 27 fixed stations distributed from 42 to 3565 m depth. Changes in macrofauna composition were tested considering species and functional trait assemblages. Environmental variables associated with sediment features (i.e., grain structure, organic matter, pH, redox), oceanographic conditions (i.e., temperature, dissolved oxygen, particulate organic carbon flux) and potential contaminants (i.e., hydrocarbons and metals) were analysed to identify potential drivers that would shape the structure of macrofauna assemblages. The results suggest that the structures of both species and functional trait assemblages change according to depth and show temporal variation in composition at seasonal and interannual scales. The effect of temporal variation represented about 12% of total variation in the assemblages (11.4 for species and 12.5% for functional-traits). Different patterns of spatial and temporal variation between shelf and deep benthic communities were observed. Variation in species assemblages on the shelf were related to the variation in lead, polycyclic aromatic hydrocarbons and the fine sand ratio. In the deep benthos, particulate carbon flux showed high correlation with the spatial and temporal variation in species assemblage. In the deep benthos the changes in the species assemblage between the dry and the rainy seasons and the interannual variation were highly correlated with particulate organic carbon input in the area.

A study on the response of carbon cycle system in the Pearl River Estuary to riverine input variations

Abstract Human activities associated with estuaries have already affected estuarine carbon cycles and flows. Especially with rapid development and urbanization, widespread wastewater treatment plants in watersheds have altered the amounts and proportions of riverine nutrients and organic carbon, which has strongly impacted carbon dynamics and budgets in coastal oceans. In this study, a well-validated carbon cycle model was used to reveal the response of carbon dynamics to reasonable estimated disturbance to riverine inputs (i.e., nutrients, dissolved organic carbon (DOC) and particulate organic carbon (POC)) in the Pearl River Estuary (PRE). Scenario results demonstrate the distinct response patterns and spatial variability of carbon dynamics. Specifically, with phytoplankton blooms and biological production closely linked to riverine nutrients, riverine nutrient alterations mainly impact photosynthesis in the PRE, especially in the lower part of the PRE. As DOC fuels bacterial respiration in the water column, heterotrophic respiration is particularly sensitive to riverine DOC disturbances. POC not only relates to light penetration in turbid estuarine waters, which impacts photosynthesis, but also undergoes deposition, which affects the biochemical reactions in the sediment. Thus, perturbations to riverine POC input led to significant changes in both biological production and sediment dissolved inorganic carbon (DIC) release. Multiple linear regression results reveal that both net community production and sediment DIC release flux modulate the variations of air-sea CO2 flux in the PRE, which highlights the roles of terrestrial POC input and deposition in a shallow estuary. When delivered into the PRE, a high proportion of POC tends to be deposited and contribute to sediment DIC release or carbon sequestration, while a significant fraction of riverine DOC is transported offshore to shelf regions. This study underscores the impacts of intensive human activities on carbon cycle system in coastal oceans and the importance of terrestrial organic carbon reduction and nutrient control in upstream basins.

Organic carbon fractions in temperate mangrove and saltmarsh soils

Coastal wetlands, such as mangrove and saltmarsh environments, can store significant amounts of soil organic carbon (SOC); however, most studies focus on tropical and subtropical environments. We assessed SOC stocks and fractions in temperate mangrove (two sites) and saltmarsh (sites SM1, SM2 and SM3) environments in southern Australia. The SOC fractions were separated according to particulate organic carbon (POC), humic carbon (HC) and recalcitrant carbon (RC) by size fractionation. Saltmarsh sites generally had the highest SOC content (up to 12.4% SOC). The POC fraction was the highest at the surface in the saltmarsh site and decreased relative to the HC and RC fractions with depth. Conversely, the proportion of POC at the mangrove sites did not decrease with depth, forming up to 76% of the SOC. The vertical displacement of soil of up to 5.8 mm year–1 at the saltmarsh sites, measured using root ingrowth bags, suggest significant contributions of POC via root materials. Retention of these POC inputs are likely to be related to waterlogging, which decreases decomposition rates – with much lower soil moisture content at SM1, where the lowest POC content occurred below the surface, compared with SM2 and SM3.

Dominance of in situ produced particulate organic carbon in a subtropical reservoir inferred from carbon stable isotopes

Sources of particulate organic carbon (POC) play important roles in aqueous carbon cycling because internal production can provide labile material that can easily be turned into CO2. On the other hand, more recalcitrant external POC inputs can cause increased loads to sedimentary organic matter that may ultimately cause CH4 release. In order to differentiate sources, stable isotopes offer a useful tool. We present a study on the Itupararanga Reservoir (Brazil) where origins of POC were explored by comparing its isotope ratios (δ13CPOC) to those of dissolved inorganic carbon (δ13CDIC). The δ13CPOC averaged around − 25.1‰ in near-surface waters, which indicates higher primary production inferred from a fractionation model that takes into account carbon transfer with a combined evaluation of δ13CPOC, δ13CDIC and aqueous CO2. However, δ13CPOC values for water depths from 3 to 15 m decreased to − 35.6‰ and indicated different carbon sources. Accordingly, the δ13CDIC values of the reservoir averaged around + 0.6‰ in the top 3 m of the water column. This indicates CO2 degassing and photosynthesis. Below this depth, DIC isotope values of as low as − 10.1‰ showed stronger influences of respiration. A fractionation model with both isotope parameters revealed that 24% of the POC in the reservoir originated from detritus outside the reservoir and 76% of it was produced internally by aqueous CO2 fixation.

Variations in organic carbon sourcing along a trans-Himalayan river determined by a Bayesian mixing approach

Rivers transfer particulate organic carbon (POC) from eroding mountains into geological sinks. Organic carbon source composition and selective mobilization have been shown to affect the type and quantity of POC export, but their combined effects across complex mountain ranges remain underexplored. Here, we examine the variation in organic carbon sourcing and transport in the trans-Himalayan Kali Gandaki River catchment, along strong gradients in precipitation, rock type and vegetation. Combining bulk stable nitrogen, and stable and radioactive organic carbon isotopic composition of bedrock, litter, soil and river sediment samples with a Bayesian end-member mixing approach, we differentiate POC sources along the river and quantify their export. Our analysis shows that POC export from the Tibetan segment of the catchment, where carbon bearing shales are partially covered by aged and modern soils, is dominated by petrogenic POC. Based on our data we re-assess the presence of aged biospheric OC in this part of the catchment, and its contribution to the river load. In the High Himalayan segment, we observed low inputs of petrogenic and biospheric POC, likely due to very low organic carbon concentrations in the metamorphic bedrock, combined with erosion dominated by deep-seated landslides. Our findings show that along the Kali Gandaki River, the sourcing of sediment and organic carbon are decoupled, due to differences in rock organic carbon content, soil and above ground carbon stocks, and geomorphic process activity. While the fast eroding High Himalayas are the principal source of river sediment, the Tibetan headwaters, where erosion rates are lower, are the principal source of organic carbon. To robustly estimate organic carbon export from the Himalayas, the mountain range should be divided into tectono-physiographic zones with distinct organic carbon yields due to differences in substrate and erosion processes and rates.

Modern and late Pleistocene particulate organic carbon transport by the Amazon River: Insights from long-chain alkyl diols

The relative abundance of the C32 1,15 long-chain alkyl diol (LCD) is an emerging proxy for the input of riverine aquatic particulate organic carbon (POC) into coastal oceans. This compound has the potential to complement other established proxies reflecting riverine terrestrial POC input and allows for a more nuanced assessment of riverine POC export to coastal seas. The current understanding of this proxy is, however, limited. In this study, we compare different indices for riverine sediment input to coastal marine waters (i.e. C32 1,15-LCD, BIT index and Fe/Ca ratio) in a source-to-sink assessment in the Amazon River drainage system and the northeast South American continental margin, and we test their down-core applicability in a marine gravity core containing late Pleistocene fluvial Amazonian sediments. We show that the relative abundance of the C32 1,15-LCD is highest in water bodies with low flow velocity and low turbidity such as the downstream portion of lowland tributaries and floodplain lakes. Relative C32 1,15-LCD abundance is lowest in Andean white water tributaries where autotrophic productivity is hindered by high turbidity and high flow velocity. We also find that suspended particulate matter from all major tributaries during the extreme 2015 dry season has a similar LCD distribution to that of floodplain lakes. This indicates that the chemical composition of the tributaries is less relevant for the LCD distribution than their physical properties such as flow velocity and turbidity. Results from marine surface sediments offshore the Amazon River estuary show significant positive correlations between all three studied proxies. In contrast, we find that the relative C32 1,15-LCD abundance in the down-core record is anti-correlated to the BIT index and Fe/Ca ratio. While BIT index and Fe/Ca ratio show high (low) values during Heinrich stadials (Dansgaard-Oeschger interstadials), the C32 1,15-LCD proxy shows the opposite signal. BIT values are also higher during Marine Isotope Stage (MIS) 2 than during MIS 3, in contrast to trends in the C32 1,15-LCD proxy. We posit that this pattern arises from a reduction in relative C32 1,15-LCD abundance and total LCD productivity in the Amazon River during MIS 2 when less-humid conditions and lower sea level led to reduced area of floodplains. During Heinrich stadials, Andean sediment input increased and led to higher turbidity that resulted in lower C32 1,15-LCD production. Our study shows that major changes in water discharge, sediment transport and river morphology can lead to discrepancies between the BIT index and the relative abundance of the C32 1,15-LCD. Thus, we suggest that Amazonian aquatic and terrestrial POC pools had contrasting responses to changes related to both climate (e.g. increased Andean precipitation) and river morphology (e.g. steeper along-channel slope due to falling and low stand sea level).

Spatial variability of macrobenthic production in the Bering Sea

Despite being located at higher latitudes with seasonal ice-cover, the Bering shelves and slope are still one of the most productive regions of the world. Existing reports regarding marine production of the Bering Sea are mainly confined to its high water column production and high biomass of macrobenthos. Compared with biomass, secondary production estimates are more functionally based and have assumed a fundamental role in the quantification of ecosystem dynamics. Based on Brey’s empirical model (in: Brey, Population dynamics in benthic invertebrates. A virtual handbook, Alfred Wegener Institute for Polar and Marine Research, Germany, 2001), macrobenthic production across the majority of the Bering Sea was quantified during the 4th, 5th and 6th Chinese Arctic Scientific Expeditions. Mean total production (TP) and community P/B for the entire survey area were 220.6 ± 341.5 kJ m−2 year−1 and 0.4 ± 0.2 year−1, n = 46, respectively. Higher TP occurred in the shallower shelves and slope with values exceeding 131.0 kJ m−2 year−1, whereas lower TP occurred in the deep basin with values of 1.0 kJ m−2 year−1. Comprehensive analysis revealed that the sediment, water flow, temperature and depth were the major factors affecting the macrobenthic production. Owing to the relatively high biological production in the upper water column and the tight pelagic–benthic coupling, higher production occurred in the shallow shelves and slope, particularly in the northern Bering Sea. In contrast, the high-nutrient, low-chlorophyll characteristic and deficient particulate organic carbon input led to low production in the deep basin. Similar to the conditions in the Arctic and Antarctic, the energy conversion rate of macrozoobenthos was relatively low for the Bering Sea.

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

Rivers link land and ocean ecosystems, the two largest active carbon reservoirs in the world, and transport tremendous amounts of particulate organic carbon (POC) into marginal seas annually. Due to high spatiotemporal variations, estimations of riverine POC flux into the sea can be relatively inexact when based on field measurement only. In this study, remote sensing algorithms of riverine POC flux through the Xuliujing hydrological station in the Changjiang River Estuary (CRE) were developed using satellite data with hourly resolution from the Geostationary Ocean Color Imager (GOCI). Based on data collected during four seasonal cruises in the CRE from 2014 to 2016, POC concentration showed a significant linear relationship to total suspended matter (TSM) (N = 426, R2 = 0.97, p < 0.01). Thus, surface POC concentration was calculated by satellite-derived TSM. From the in-situ data, the vertical POC concentration profile at Xuliujing showed an exponentially increasing curve (p < 0.01), but a linear decrease (p < 0.01) for the water flow profile from the surface to the bottom. Combining the GOCI-derived surface POC concentration and vertical profiles of POC and water flow, we estimated monthly riverine POC fluxes to the CRE. Results showed that monthly POC flux at Xuliujing (0.071 ± 0.022 Tg C) was 22.64% higher, on average, than the commonly used value at the non-tidal Datong hydrological station. Moreover, under human activity pressures, the influences of POC input from the Changjiang River on POC concentration at Xuliujing have weakened in recent years. Monthly and diurnal POC variations in the CRE were mainly impacted by wind speed and tidal processes, respectively. Thus, satellite monitoring with high spatiotemporal resolution has great significance for accurately estimating riverine POC flux to estuary.

Petrogenic organic carbon retention in terrestrial basins: A case study from perialpine Lake Constance

Inland waters play a major role in the global carbon cycle, with particulate organic carbon (POC) burial in terrestrial wetlands surpassing that in ocean sediments. Lake Constance, the second largest lake at the periphery of the European Alps, receives POC sourced from both aquatic and terrestrial productivity as well as petrogenic OC (OCpetro) from bedrock erosion. Distinguishing POC inputs to lake sediments is key to assessing carbon flux and fate as reworked OCpetro represents neither a net sink of atmospheric CO2 nor source of O2. New stable and radiocarbon isotopic data indicate that 11 (9–12) Gg/yr of OCpetro is buried in Lake Constance with underlying sediments on average containing 0.3 (0.25–0.33) wt% OCpetro. Extrapolation of these results suggests that 27 TgOCpetro/yr (12–54 TgOC/yr) could be subject to temporary geological storage in lakes globally, which is comparable to estimates of 43−25+61 TgOCpetro/yr delivered to the ocean by rivers (Galy et al., 2015). More studies are needed to quantify OCpetro burial in inland sedimentary reservoirs in order to accurately account for atmospheric carbon sequestration in terrestrial basins.

Impacts of anthropogenic inputs on hypoxia and oxygen dynamics in the Pearl River estuary

Abstract. In summer, the Pearl River estuary (PRE) experiences hypoxia, largely driven by the high input of fresh water with low dissolved oxygen (DO), abundant nutrients and particulate organic carbon from the Pearl River network. In this study, we used a well-validated physical–biogeochemical model together with a DO species-tracing method to study the responses of hypoxia and oxygen dynamics to the anthropogenic perturbations of riverine inputs (i.e. DO, nutrients, and particulate organic carbon) in July–August 2006. Model results showed that hypoxia in the PRE was most sensitive to riverine inputs of particulate organic carbon, followed by DO concentrations and nutrients. Specifically, a 50 % decrease (increase) in riverine input of particulate organic carbon led to a 47 % decrease (64 % increase) in hypoxic area, with the sediment oxygen demand and water column production being the two most important processes contributing to changes in DO concentration. Changes in the riverine inputs of DO and nutrients had little impact on the simulated hypoxia because of the buffering effects of re-aeration (DO fluxes across the air–sea interface); i.e. the re-aeration responded to the changes in surface apparent oxygen utilization (AOU) associated with river-induced variations of oxygen source and sink processes. The PRE features shallow waters (with averaged depth of 10 m) in which oxygen provided by the re-aeration could penetrate to bottom waters via vertical diffusion and largely offset the changes in DO contributed by other oxygen source and sink processes. This study highlights the importance of re-aeration in reducing hypoxia variability in shallow estuaries.

Impacts of Accumulated Particulate Organic Matter on Oxygen Consumption and Organic Micro-Pollutant Elimination in Bank Filtration and Soil Aquifer Treatment

Bank filtration (BF) and soil aquifer treatment (SAT) are efficient natural technologies in potable water reuse systems. The removal of many organic micro-pollutants (OMPs) depends on redox-conditions in the subsoil, especially on the availability of molecular oxygen. Due to microbial transformation of particulate and dissolved organic constituents, oxygen can be consumed within short flow distances and induce anoxic and anaerobic conditions. The effect of accumulated particulate organic carbon (POC) on the fate of OMPs in BF and SAT systems is not fully understood. Long-term column experiments with natural sediment cores from the bank of Lake Tegel and from a SAT basin were conducted to investigate the impact of accumulated POC on dissolved organic carbon (DOC) release, on oxygen consumption, on mobilization of iron and manganese, and on the elimination of the organic indicator OMPs. The cores were fed with aerated tap water spiked with OMPs to exclude external POC inputs. Complete oxygen consumption within the first infiltration decimeter in lake sediments caused mobilization of iron, manganese, and DOC. Redox-sensitive OMPs like diclofenac, sulfamethoxazole, formylaminoantipyrine, and gabapentin were eliminated by more than 50% in all sediment cores, but slightly higher residual concentrations were measured in effluents from lake sediments, indicating a negative impact of a high oxygen consumption on OMP removal.

Carbon cycling in the deep eastern North Pacific benthic food web: Investigating the effect of organic carbon input

The deep ocean benthic environment plays a role in long-term carbon sequestration. Understanding carbon cycling in the deep ocean floor is critical to evaluate the impact of changing climate on the oceanic systems. Linear inverse modeling was used to quantify carbon transfer between compartments in the benthic food web at a long time-series study site in the abyssal northeastern Pacific (Station M). Linear inverse food web models were constructed for three separate years in the time-series when particulate organic carbon (POC) flux was relatively high (1990: 0.63 mean mmol C m?2 d?1), intermediate (1995: 0.24) and low (1996: 0.12). Carbon cycling in all years was dominated by the flows involved in the microbial loop; dissolved organic carbon uptake by microbes (0.80–0.95 mean mmol C m?2 d?1), microbial respiration (0.52–0.61), microbial biomass dissolution (0.09–0.18) and the dissolution of refractory detritus (0.46–0.65). Moreover, the magnitude of carbon flows involved in the microbial loop changed in relation to POC input, with a decline in contribution during the high POC influxes, such as those recently experienced at Station M. Results indicate that during high POC episodic pulses the role of faunal mediated carbon cycling would increase. Semi-labile detritus dominates benthic faunal diets and the role of labile detritus declined with increased total POC input. Linear inverse modeling represents an effective framework to analyze high-resolution time-series data and demonstrate the impact of climate change on the deep ocean carbon cycle in a coastal upwelling system.

Modeling total particulate organic carbon (POC) flows in the Baltic Sea catchment

The largest input source of carbon to the Baltic Sea catchment is river discharge. A tool for modeling riverine particulate organic carbon (POC) loads on a catchment scale is currently lacking. The present study describes a novel dynamic model for simulating flows of POC in all major rivers draining the Baltic Sea catchment. The processes governing POC input and transport in rivers described in the model are soil erosion, in-stream primary production and litter input. The Baltic Sea drainage basin is divided into 82 sub-basins, each comprising several land classes (e.g. forest, cultivated land, urban areas) and parameterized using GIS data on soil characteristics and topography. Driving forces are temperature, precipitation, and total phosphorous concentrations. The model evaluation shows that the model can predict annual average POC concentrations within a factor of about 2, but generally fails to capture the timing of monthly peak loads. The total annual POC load to the Baltic Sea is estimated to be 0.34 Tg POC, which constitutes circa 7–10 % of the annual total organic carbon (TOC) load. The current lack of field measurements of POC in rivers hampers more accurate predictions of seasonality in POC loads to the Baltic Sea. This study, however, identifies important knowledge gaps and provides a starting point for further explorations of large scale POC mass flows.

Weak Response of Animal Allochthony and Production to Enhanced Supply of Terrestrial Leaf Litter in Nutrient-Rich Lakes

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.

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

In order to reveal particulate organic carbon (POC) source and mode of transport in the largest river basin on Earth, we sampled the main sediment-laden tributaries of the Amazon system (Solimões, Madeira and Amazon) during two sampling campaigns, following vertical depth-profiles. This sampling technique takes advantage of hydrodynamic sorting to access the full range of solid erosion products transported by the river. Using the Al/Si ratio of the river sediments as a proxy for grain size, we find a general increase in POC content with Al/Si, as sediments become finer. However, the sample set shows marked variability in the POC content for a given Al/Si ratio, with the Madeira River having lower POC content across the measured range in Al/Si. The POC content is not strongly related to the specific surface area (SSA) of the suspended load, and bed sediments have a much lower POC/SSA ratio. These data suggest that SSA exerts a significant, yet partial, control on POC transport in Amazon River suspended sediment. We suggest that the role of clay mineralogy, discrete POC particles and rock-derived POC warrant further attention in order to fully understand POC transport in large rivers.To examine the source of POC in the Amazon Basin in more detail, we use radiocarbon (14C) content and the stable isotope composition (δ13C) of POC. 14C activity demonstrates that rock-derived POC is a significant component of river bed sediments and contributes to POC across the river depth-profiles of suspended sediments. We estimate that the flux of rock-derived POC may reach ∼10% of the total POC export by the Amazon River. After correcting for rock-derived POC input, we find that POC from the terrestrial biosphere (biospheric POC) is mostly sourced from C3-plants. Higher biospheric POC δ13C values in the Madeira River (by ∼0.5–1‰) are best explained by a small (<5%) contribution of C4-grasses from Bolivian savannas. Finally, we use 14C to estimate the mean age of biospheric POC exported from the Amazon Basin. The data show that biospheric POC is younger in the Solimões River (1120years) than in the Madeira River (2850 years). These ages are much younger than the corresponding estimates of sediment residence time in the basin, suggesting that lowland areas and/or young POC from above-ground biomass may contribute disproportionately to the biomarker signals in terrigenous sediments of the Amazon delta.