Labile Dissolved Organic Carbon Research Articles

A comparison of extraction procedures for water‐extractable organic matter in soils

SummaryThe characteristics of dissolved organic matter (DOM) in soils are often determined through laboratory experiments. Many different protocols can be used to extract organic matter from soil. In this study, we used five air‐dried soils to compare three extraction methods for water‐extractable organic matter (WEOM) as follows: (i) pressurised hot‐water‐extractable organic carbon (PH‐WEOC), a percolation at high pressure and temperature; (ii) water‐extractable organic carbon (WEOC), a 1‐hour end‐over shaking; and (iii) leaching‐extractable organic carbon (LEOC), a leaching of soil columns at ambient conditions. We quantified the extraction yield of organic carbon; the quality of WEOM was characterized by UV absorbance, potential biodegradability (48‐day incubation) and parallel factor analysis (PARAFAC) modelling of fluorescence excitation emission matrices (FEEMs). Biodegradation of dissolved organic carbon (DOC) was described by two pools of organic C. The proportions of labile and stable DOC pools differed only slightly between the WEOC and LEOC methods, while PH‐WEOC contains more stable DOC. The mineralization rate constants of both labile and stable DOC pools were similar for the three methods. The FEEMs were decomposed into three components: two humic‐like fluorophores and a tryptophan‐like fluorophore. The effect of extraction method was poorly discriminant and the most similar procedures were PH‐WEOC and LEOC while WEOC extracts were depleted in humic‐like fluorophores. This study demonstrates that WEOM quality is primarily determined by soil characteristics and that the extraction method has a smaller, but still significant, impact on WEOM quality. Furthermore, we observed considerable interaction between extraction procedure and soil type, showing that method‐induced differences in WEOM quality vary with soil characteristics.

Longitudinal patterns in carbon and nitrogen fluxes and stream metabolism along an urban watershed continuum

An urban watershed continuum framework hypothesizes that there are coupled changes in (1) carbon and nitrogen cycling, (2) groundwater-surface water interactions, and (3) ecosystem metabolism along broader hydrologic flowpaths. It expands our understanding of urban streams beyond a reach scale. We evaluated this framework by analyzing longitudinal patterns in: C and N concentrations and mass balances, groundwater-surface interactions, and stream metabolism and carbon quality from headwaters to larger order streams. 52 monitoring sites were sampled seasonally and monthly along the Gwynns Falls watershed, which drains 170 km2 of the Baltimore Long-Term Ecological Research site. Regarding our first hypothesis of coupled C and N cycles, there were significant inverse linear relationships between nitrate and dissolved organic carbon (DOC) and nitrogen longitudinally (P < 0.05). Regarding our second hypothesis of coupled groundwater-surface water interactions, groundwater seepage and leaky piped infrastructure contributed significant inputs of water and N to stream reaches based on mass balance and chloride/fluoride tracer data. Regarding our third hypothesis of coupled ecosystem metabolism and carbon quality, stream metabolism increased downstream and showed potential to enhance DOC lability (e.g., ~4 times higher mean monthly primary production in urban streams than forest streams). DOC lability also increased with distance downstream and watershed urbanization based on protein and humic-like fractions, with major implications for ecosystem metabolism, biological oxygen demand, and CO2 production and alkalinity. Overall, our results showed significant in-stream retention and release (0–100 %) of watershed C and N loads over the scale of kilometers, seldom considered when evaluating monitoring, management, and restoration effectiveness. Given dynamic transport and retention across evolving spatial scales, there is a strong need to longitudinally and synoptically expand studies of hydrologic and biogeochemical processes beyond a stream reach scale along the urban watershed continuum.

The effect of long-term water table manipulations on dissolved organic carbon dynamics in a poor fen peatland

Dissolved organic carbon (DOC) production, consumption, and quality displayed differences after long-term (~55 years) hydrological alterations in a poor fen peatland in northern Michigan. The construction of an earthen levee resulted in areas of a raised and lowered water table (WT) relative to an unaltered intermediate WT site. The lowered WT site had greater peat aeration and larger seasonal vertical WT fluctuations that likely elevated peat decomposition and subsidence with subsequent increases in bulk density, vertical hydraulic gradient, decreased hydraulic conductivity (Ksat), and a greater pore water residence time relative to the unaltered site. The raised WT site displayed a decreased Ksat combined with seasonal upwelling events that contributed to a longer residence time in comparison to the unaltered site. These differences are potentially contributing to elevated DOC concentrations at the lowered and raised WT site relative to the unaltered site. Additionally, spectrophotometric indices and chemical constituent assays indicated that the lowered site DOC was more aromatic and contained elevated concentrations of phenolics compared to the intermediate site. The raised site DOC was less aromatic, less humified, and also had a greater phenolic content than the intermediate site. Furthermore, an incubation experiment showed that DOC in the raised site contained the greatest labile carbon source. Based on our results, long-term WT alterations will likely impose significant effects on DOC dynamics in these peatlands; however, WT position alone was not a good predictor of DOC concentrations, though impoundment appears to produce a more labile DOC whereas drainage increases DOC aromaticity.

Natural Diet of Coral-Excavating Sponges Consists Mainly of Dissolved Organic Carbon (DOC)

Coral-excavating sponges are the most important bioeroders on Caribbean reefs and increase in abundance throughout the region. This increase is commonly attributed to a concomitant increase in food availability due to eutrophication and pollution. We therefore investigated the uptake of organic matter by the two coral-excavating sponges Siphonodictyon sp. and Cliona delitrix and tested whether they are capable of consuming dissolved organic carbon (DOC) as part of their diet. A device for simultaneous sampling of water inhaled and exhaled by the sponges was used to directly measure the removal of DOC and bacteria in situ. During a single passage through their filtration system 14% and 13% respectively of the total organic carbon (TOC) in the inhaled water was removed by the sponges. 82% (Siphonodictyon sp.; mean±SD; 13±17 μmol L−1) and 76% (C. delitrix; 10±12 μmol L−1) of the carbon removed was taken up in form of DOC, whereas the remainder was taken up in the form of particulate organic carbon (POC; bacteria and phytoplankton) despite high bacteria retention efficiency (72±15% and 87±10%). Siphonodictyon sp. and C. delitrix removed DOC at a rate of 461±773 and 354±562 μmol C h−1 respectively. Bacteria removal was 1.8±0.9×1010 and 1.7±0.6×1010 cells h−1, which equals a carbon uptake of 46.0±21.2 and 42.5±14.0 μmol C h−1 respectively. Therefore, DOC represents 83 and 81% of the TOC taken up by Siphonodictyon sp. and C. delitrix per hour. These findings suggest that similar to various reef sponges coral-excavating sponges also mainly rely on DOC to meet their carbon demand. We hypothesize that excavating sponges may also benefit from an increasing production of more labile algal-derived DOC (as compared to coral-derived DOC) on reefs as a result of the ongoing coral-algal phase shift.

Peat porewater dissolved organic carbon concentration and lability increase with warming: a field temperature manipulation experiment in a poor-fen

Studies conducted across northern Europe and North America have shown increases in dissolved organic carbon (DOC) in aquatic systems in recent decades. While there is little consensus as to the exact mechanisms for the increases in DOC, hypotheses converge on such climate change factors as warming, increased precipitation variability, and changes in atmospheric deposition. In this study, we tested the effects of warming on peat porewater composition by actively warming a peatland with infrared lamps mounted 1.24 m above the peat surface for 3 years. Mean growing season peat temperatures in the warmed plots (n = 5) were 1.9 ± 0.4 °C warmer than the control plots at 5 cm depth (t statistic = 5.03, p = 0.007). Mean porewater DOC concentrations measured throughout the growing season were 15 % higher in the warmed plots (73.4 ± 3.2 mg L−1) than in the control plots (63.7 ± 2.1 mg L−1) at 25 cm (t = 4.69, p < 0.001). Furthermore, DOC from the warmed plots decayed nearly twice as fast as control plot DOC in laboratory incubations, and exhibited lower aromaticity than control plot porewater (reduction in SUVA254 in heated plots compared with control plots). Dissolved organic nitrogen (DON) concentrations tracked DOC patterns as expected, but the amount of dissolved N per unit C decreased with warming. Previous work has shown that warming increased net primary production at this site, and together with measured increases in the activities of chitinases and glucosidases we suggest that the increased DOC concentrations observed with warming were derived in part from microbial-plant interactions in the rhizosphere. We also detected more nitrogen containing compounds with higher double bond equivalents (DBE) unique to the warmed plots, within the pool of biomolecules able to deprotonate (16 % of all compounds identified using ultrahigh resolution ion electrospray mass spectrometry); we suggest these compounds could be the products of increased plant, microbial, and enzyme activity occurring with warming. With continued warming in peatlands, an increase in relatively labile DOC concentrations could contribute to dissolved exports of DOC in runoff, and would likely contribute to the pool of efficient electron donors (and acceptors) in the production of CO2 and CH4 in terrestrial and aquatic environments.

Regulation of bacterial metabolic activity by dissolved organic carbon and viruses

AbstractThe regulation of bacterial metabolic activity by viruses and dissolved organic carbon (DOC) was examined using natural microbial communities in three treatments (active viruses, inactive viruses, and virus free) at two contrasting coastal sites (pristine vs. eutrophic) with substantial differences in environmental conditions during the wet and dry seasons. Our results showed that net growth rates and production of bacterioplankton were reduced primarily by viruses via repressing metabolically active bacteria with high nucleic acid (HNA) content which had a high capacity for incorporating carbon, while bacterial respiration was primarily regulated by DOC lability. The quality of organic matter played a more important role in regulating bacterial growth efficiency (BGE) than the supply of organic matter in eutrophic coastal waters. The lack of HMW‐DOC and high carbon demand in the virus‐free treatment resulted in a significant increase in cell‐specific bacterial respiration, which was responsible for the lowest bacterial growth efficiency among the three treatments. The presence of viruses did not necessarily lower bacterial growth efficiency since virus‐induced mortality alleviated bacterial carbon demand and enhanced carbon cycling. Virus‐induced mortality was greater in relatively pristine waters than eutrophic waters, likely since the high supply of substrates alleviated the pressure of viral infection, through extracellular proteases produced by bacteria, which might result in the hydrolytic destruction or modification of viral capsids. An important implication of our results was that the input of riverine DOC and nutrients improved bacterial metabolic activity by alleviating virus‐induced mortality of bacteria in estuarine and coastal waters.

In situ rapid measures of total respiration rate capture the super labile DOC bacterial substrates of freshwater

Super labile dissolved organic carbon (DOC) in freshwater supports high rates of bacterial respiration. However, this DOC fraction has been overlooked because of methodological and instrument limitations, until now. Here, a simple inexpensive respiration chamber‐valve system is described that can be attached to an optical dissolved oxygen probe to measure in situ bacterial respiration. The dissolved oxygen (DO) probe and chamber capture a 2 L water sample at depth and immediately begin measuring the DO loss over time at in situ pressure and temperature. DO measures over 5 min provided enough DO data to determine the DOC decay rate, and subsequently, the rate of bacterial respiration and carbon demand. The method was validated for a mixed bacterial community in a batch reactor and applied to the freshwater of boreal, sub‐tropical, and tropical climatic zones. The surface water mean bacterial respiration rate was 0.58 ± 0.4 mol C m−3 d−1, whereas below 6 m, it was 4.8 ± 1.8 mol C m−3 d−1. Globally these are amongst the highest respiration rates measured in freshwater. This in situ method captured the freshwater respiration of the super labile DOC and shows how most often this DOC fraction is missed because water samples are removed from the water body to measure respiration. Terrestrial DOC inputs into freshwater ecosystems and current global estimates of pCO2 outgassing into the atmosphere from freshwater are being underestimated using existing methods because of the time lost with sample manipulation and long incubation times.

Tracing carbon sources in small urbanising streams: catchment‐scale stormwater drainage overwhelms the effects of reach‐scale riparian vegetation

Summary Organic matter provides energy and nutrients to aquatic systems. Alterations to its sources and processing have repercussions for water quality and food‐web stability and structure. Despite worldwide recognition of the impacts of urbanisation, there is limited understanding of the relative importance of catchment‐scale urban stormwater drainage connection and reach‐scale riparian vegetation on organic matter sources. We investigated the effects of catchment‐scale urban stormwater drainage connection and reach‐scale riparian vegetation cover on organic matter sources in small streams. Using stable isotopes and elemental ratios (i.e. δ13C, δ15N and C : N), we traced the origin of microbially respired carbon and standing stocks of dissolved organic carbon (DOC), suspended particulate organic matter (POM) and benthic coarse particulate organic matter (CPOM). Catchment‐scale urban stormwater drainage connection significantly increased the contribution of labile organic matter to POM and DOC standing stocks. Greater POM lability was a product of increased inputs of autochthonous organic matter in more heavily urbanised streams, although the origin of labile DOC was less clear. While reach‐scale riparian vegetation was the most likely source of the terrestrially dominated CPOM observed across most sites, increasing cover had no significant effect on the origin of POM or DOC standing stocks. We conclude that catchment‐scale stormwater drainage impacts overwhelm the effects of reach‐scale riparian vegetation on the sources and lability of POM and DOC in small streams. Our results suggest that the protection or restoration of riparian vegetation, in the absence of modifications to catchment‐scale stormwater drainage connection, is insufficient to mitigate the effects of urbanisation on organic matter sources, lability and processing in these small streams.

Differentiating the degradation dynamics of algal and terrestrial carbon within complex natural dissolved organic carbon in temperate lakes

It has often been hypothesized that the dissolved organic carbon (DOC) pool of algal origin in lakes is more bioavailable than its terrestrial counterpart, but this hypothesis has seldom been directly tested. Here we test this hypothesis by tracking the production and isotopic signature of bacterial respiratory CO2 in 2 week lake water incubations and use the resulting data to reconstruct and model the bacterial consumption dynamics of algal and terrestrial DOC. The proportion of algal DOC respired decreased systematically over time in all experiments, suggesting a rapid consumption and depletion of this substrate. Our results further show that the algal DOC pool was used in proportions and at rates twice and 10 times as high as the terrestrial DOC pool, respectively. On the other hand, the absolute amount of labile terrestrial DOC was on average four times higher than labile algal DOC, accounting for almost the entire long‐term residual C metabolism, but also contributing to short‐term bacterial C consumption. The absolute amount of labile algal DOC increased with chlorophyll a concentrations, whereas total phosphorus appeared to enhance the amount of terrestrial DOC that bacteria could consume, suggesting that the degradation of these pools is not solely governed by their respective chemical properties, but also by interactions with nutrients. Our study shows that there is a highly reactive pool of terrestrial DOC that is processed in parallel to algal DOC, and because of interactions with nutrients, terrestrial DOC likely supports high levels of bacterial metabolism and CO2 production even in more productive lakes.

A global ocean biogeochemistry general circulation model and its simulations

An ocean biogeochemistry model was developed and incorporated into a global ocean general circulation model (LICOM) to form an ocean biogeochemistry general circulation model (OBGCM). The model was used to study the natural carbon cycle and the uptake and storage of anthropogenic CO2 in the ocean. A global export production of 12.5 Pg C yr−1 was obtained. The model estimated that in the pre-industrial era the global equatorial region within ±15° of the equator released 0.97 Pg C yr−1 to the atmosphere, which was balanced by the gain of CO2 in other regions. The post-industrial air-sea CO2 flux indicated the oceanic uptake of CO2 emitted by human activities. An increase of 20–50 μmol kg−1 for surface dissolved inorganic carbon (DIC) concentrations in the 1990s relative to pre-industrial times was obtained in the simulation, which was consistent with data-based estimates. The model generated a total anthropogenic carbon inventory of 105 Pg C as of 1994, which was within the range of estimates by other researchers. Various transports of both natural and anthropogenic DIC as well as labile dissolved organic carbon (LDOC) were estimated from the simulation. It was realized that the Southern Ocean and the high-latitude region of the North Pacific are important export regions where accumulative air-sea CO2 fluxes are larger than the DIC inventory, whereas the subtropical regions are acceptance regions. The interhemispheric transport of total natural carbon (DIC+LDOC) was found to be northward (0.11 Pg C yr−1), which was just balanced by the gain of carbon from the atmosphere in the Southern Hemisphere.

CO&amp;lt;sub&amp;gt;2&amp;lt;/sub&amp;gt; increases &amp;lt;sup&amp;gt;14&amp;lt;/sup&amp;gt;C primary production in an Arctic plankton community

Abstract. Responses to ocean acidification in plankton communities were studied during a CO2-enrichment experiment in the Arctic Ocean, accomplished from June to July 2010 in Kongsfjorden, Svalbard (78°56′ 2′′ N, 11°53′ 6′′ E). Enclosed in 9 mesocosms (volume: 43.9–47.6 m3), plankton was exposed to CO2 concentrations, ranging from glacial to projected mid-next-century levels. Fertilization with inorganic nutrients at day 13 of the experiment supported the accumulation of phytoplankton biomass, as indicated by two periods of high chl a concentration. This study tested for CO2 sensitivities in primary production (PP) of particulate organic carbon (PPPOC) and of dissolved organic carbon (PPDOC). Therefore, 14C-bottle incubations (24 h) of mesocosm samples were performed at 1 m depth receiving about 60% of incoming radiation. PP for all mesocosms averaged 8.06 ± 3.64 μmol C L−1 d−1 and was slightly higher than in the outside fjord system. Comparison between mesocosms revealed significantly higher PPPOC at elevated compared to low pCO2 after nutrient addition. PPDOC was significantly higher in CO2-enriched mesocosms before as well as after nutrient addition, suggesting that CO2 had a direct influence on DOC production. DOC concentrations inside the mesocosms increased before nutrient addition and more in high CO2 mesocosms. After addition of nutrients, however, further DOC accumulation was negligible and not significantly different between treatments, indicating rapid utilization of freshly produced DOC. Bacterial biomass production (BP) was coupled to PP in all treatments, indicating that 3.5 ± 1.9% of PP or 21.6 ± 12.5% of PPDOC provided on average sufficient carbon for synthesis of bacterial biomass. During the later course of the bloom, the response of 14C-based PP rates to CO2 enrichment differed from net community production (NCP) rates that were also determined during this mesocosm campaign. We conclude that the enhanced release of labile DOC during autotrophic production at high CO2 exceedingly stimulated activities of heterotrophic microorganisms. As a consequence, increased PP induced less NCP, as suggested earlier for carbon-limited microbial systems in the Arctic.

A mechanistic‐based framework to understand how dissolved organic carbon is processed in a large fluvial lake

Lay AbstractDissolved organic carbon (DOC) is a fundamental component of the biogeochemical cycling of nutrients in aquatic ecosystems and is the main carbon source supporting bacterial production. The efficiency at which heterotrophic (nonphotosynthetic) bacteria convert this substrate into biomass depends mainly on the quality of DOC in the water column. DOC is constantly processed through various physical, chemical, and biological mechanisms that operate simultaneously and alter its quality. It is paramount to understand how these different processes interact to drive the fate of DOC in aquatic ecosystems. Based on field data collected in a large fluvial lake, we developed and validated a mechanistic model that provides a framework to understand the relative contribution of the main processes involved in both labile (DOCL) and semilabile (DOCSL) DOC pool kinetics. The model revealed that during the downstream flow, each category of DOC pool was processed differently by bacteria: DOCL was preferentially used for biomass production, whereas DOCSL completed bacterial carbon demand. Our results also suggest that a decrease in DOCL abundance will further determine the intake of DOCSL.

太湖水体溶解性氨基酸的空间分布特征

Amino acids( AAs) are important bio-available components of organic matter( OM) and are the major forms of organic nitrogen( ON) in aquatic ecosystem. They also represent significant fractions of OM in recent coastal marine sediments,water column particles and dissolved organic matters. They are typically labile composed relative to bulk OM and ON,and account for a considerable portion of the particulate OC and ON recycled in both water column and sediments,and therefore are an important nutrient form. Owing to the different AAs monomers have different biogeochemical stability,AAs are often used as an indicator of organic matter sources and as an index of OM degradation potential. Changes in amino acid composition exhibit consistent trends and demonstrate the degradation of cellular organic matter as it sinks through water column and is incorporated into sediments. Due to the labile character of AAs,the percentage of OC and N represented by amino acid( AA-C / OC% and AA-N / ON%) and the percentage of total amino acids have proven to be useful degradation indicators. They are sensitive at different stages of OM alteration and appear uncompromised by source variation. Dissolved amino acids( DAAs) characteristics were investigated in water of Lake Taihu,a large,shallow and eutrophic freshwater lake in China. The northern lake is characterized by phytoplankton dominance,high water turbidity,and severe Microcystis spp. algal blooms. In contrast,eastern lake is characterized by submersed vegetation,clear water,and diverse communities of fishes and invertebrates. The main upstream land use types of south Taihu were classified by the mountain forests and farmland. DAAs concentrations in Taihu ranged from 0. 27 to 3. 95 μmol / L,with a mean value of 1.38 μmol / L. The large regional differences in concentrations and yields of DAAs suggested that DOM produced in situ is an important source of bio-reactive of organic matter in waters of Taihu. The spatial distributions of DAAs concentrations in Taihu were less variable than those previously reported from other aquatic environments. DAAs concentrations of north Taihu were significantly higher than south lake and east lake,with mean values of( 2. 59±0. 71) μmol / L in north Taihu,( 0. 48±0. 14) μmol / L in south Taihu,and( 0. 48 ± 0. 16) μmol / L in east Taihu,respectively( P 0. 001). Based on the relationship between amino acid yields and percent labile DOC,( 17. 65 ±17. 84) % of fresh DOC in north lake is labile,which is higher than in south lake or east lake. The results showed internal sources mainly from the decomposing of phytoplankton should be the main source of DAAs in Taihu,and DAAs was important fraction of DOM. DAAs compositions were dominated by Phenylalanine( PHE) and lysine( LYS) in north lake,while only LYS in south and east lake.Compositions and concentrations of DAAs can be possible used as an indicator,which could provide useful information about the nutritional status and bio-available DOM in Lake Taihu. But because of high spatial heterogeneity in Taihu,we need to establish DAAs-liable DOC linear model for Lake Taihu,by the means of the algal-dominated and macrophytedominated DOM degradation experiments.

Estimation of dissolved organic carbon contribution from hillslope soils to a headwater stream

The downward flux of dissolved organic carbon (DOC) in the vadose zone was examined using “artificial rain” experiments on a soil lysimeter at the White Clay Creek watershed in southeastern Pennsylvania. Refractory DOC (RDOC) and labile biodegradable DOC (BDOC) were transported from the organic rich soil layer and eluted from the base of a soil lysimeter during the experimental water applications. A physically based, distributed model that considers water flow and the transport of heat, RDOC and labile BDOC was developed and used to explore the hypothesis that the delivery of DOC to the water table occurs primarily in the riparian zone. Macropores were considered by adopting a dual permeability modeling approach. The calibrated model successfully replicated the temporal dynamics of a bromide tracer, and of RDOC and labile BDOC eluted from the soil lysimeter during the experiments. An estimated annual DOC delivery to groundwater in the watershed was obtained by scaling up the vertical one‐dimensional model to the watershed. The results suggest that the subsurface contribution of DOC accounts for about 72% of the total annual export from the watershed, and the riparian zone contributes more than 90% of total DOC in groundwater.

Effects of N, P, and organic carbon on stream biofilm nutrient limitation and uptake in a semi‐arid watershed

We examined the effect of agricultural land use on whole‐stream nutrient availability, biofilm standing crop, and biofilm nutrient limitation and uptake in 21 stream locations in southeastern Idaho. Higher stream water concentrations of dissolved organic carbon (DOC), but not nitrate (NO3‐N) or phosphate (PO4‐P), were associated with % agriculture in the watershed. Streambed chlorophyll a (Chl a) and ash free dry weight (AF dry wt) also increased with agricultural land use. Nutrient diffusing substrate (NDS) bioassays, which determine biofilm nutrient limitation, showed that Chl a was NO3‐N limited and suppressed by labile DOC, whereas AF dry wt did not respond to either NO3‐N or labile DOC additions. Together, the different responses of Chl a and AF dry wt suggest potential competition between biofilm autotrophs and heterotrophs for nutrients or other resources. Nutrient uptake, determined with short‐term releases of NO3‐N, PO4‐P, and DOC (as glucose) showed that NO3‐N uptake velocity (Vf) increased with agricultural land use. Despite the N‐limited status of biofilms indicated by the NDS results, uptake of NO3‐N could not be consistently detected at all locations. The differences in response of biofilm to organic carbon enrichment suggests a difference in DOC quality, with labile DOC added with NDS compared to perhaps less‐labile DOC found in‐stream. Linking the interactive responses of biofilm communities to altered nutrient availability is an important step toward understanding whole ecosystem responses to land‐use change.

Cycling of two carbon substrates of contrasting lability by heterotrophic biofilms across a nutrient gradient of headwater streams

Anthropogenic impacts can significantly alter stream nutrient and dissolved organic carbon (DOC) delivery and composition. Nutrient and DOC cycling in headwater streams, however, are linked via a variety of complex feedbacks that are, in part, influenced by DOC composition emphasizing the need to investigate coupled nutrient–DOC interactions. This study assessed differential incorporation and mineralization of 13C labeled glucose and vanillin by heterotrophic microbes within epilithic biofilm communities in four temperate headwater streams spanning a 100-fold range in total dissolved nitrogen and soluble reactive phosphorous concentrations. The substrates were traced via 13C analyses of DOC, dissolved inorganic carbon, bulk biofilm, and individual biofilm phospholipid fatty acids (PLFA) to assess total incorporation of the substrates and the distribution of substrate use within the heterotrophic community. Results indicate greater nutrient uptake by high nutrient streams with glucose additions relative to vanillin additions and support the hypothesis that nutrient retention in high nutrient streams is hampered by a lack of labile C sources. Vanillin-derived C uptake was only detectable in PLFA from the highest nutrient stream and was dominated by eukaryotic organisms, likely including fungi. This suggests biofilms in high nutrient streams are better adapted to access relatively slow turnover substrates perhaps due to their composition and overall structure. PLFA-based glucose use efficiencies were greatest in the lowest nutrient stream supporting the hypothesis that labile DOC sources are used more efficiently by heterotrophs in less impacted streams, while biofilms of high nutrient streams are better adapted to utilizing a wider array of DOC sources. This adaption is likely a result of exposure to the lower quality DOC pools in high-nutrient streams resulting from high DOC uptake supported, in part, by fast turnover autochthonous sources of DOC. Nutrient retention in nutrient-rich streams, however, is still likely limited by readily bioavailable DOC leading to lower nutrient retention and downstream nutrient enrichment.

Manipulation of the Dissolved Organic Carbon Pool in an Agricultural Stream: Responses in Microbial Community Structure, Denitrification, and Assimilatory Nitrogen Uptake

Carbon (C) and nitrogen (N) are strongly coupled across ecosystems due to stoichiometrically balanced assimilatory demand as well as dissimilatory processes such as denitrification. Microorganisms mediate these biogeochemical cycles, but how microbial communities respond to environmental changes, such as dissolved organic carbon (DOC) availability, and how those responses impact coupled biogeochemical cycles in streams is not clear. We enriched a stream in central Indiana with labile DOC for 5 days to investigate coupled C and N cycling. Before, and on day 5 of the enrichment, we examined assimilatory uptake and denitrification using whole-stream 15N-nitrate tracer additions and short-term nitrate releases. Concurrently, we measured bacterial and denitrifier abundance and community structure. We predicted N assimilation and denitrification would be stimulated by the addition of labile C and would be mediated by increases in bacterial activity, abundance, and a shift in community structure. In response to the twofold increase in DOC concentrations in the water column, N assimilation increased throughout the enrichment. Community respiration doubled during the enrichment and was associated with a change in bacterial community structure (based on terminal restriction fragment length polymorphisms of the 16S rRNA gene). In contrast, there was little response in denitrification or denitrifier community structure, likely because labile C was assimilated by heterotrophic communities on the stream bed prior to reaching denitrifiers within the sediments. Our results suggest that coupling between C and N in streams involves potentially complex interactions with sediment texture and organic matter, microbial community structure, and possibly indirect biogeochemical pathways.

Phosphorus and DOC availability influence the partitioning between bacterioplankton production and respiration in tidal marsh ecosystems

The organic carbon consumed by aquatic bacteria (BCC) is partitioned between bacterial production (BP) and respiration (BR), but the factors that determine BCC and its partition into BP and BR are not well understood. We explored the coupling between BR, BR and BCC, and their links to dissolved organic carbon (DOC) and nutrient availability in natural and restored tidal marshes and in the adjoining waters of Delaware Bay estuary. Labile DOC (LDOC) ranged from 3% to 22% of the DOC pool, and explained more of the variance in both BR and BCC than did bulk DOC. Bacterial growth efficiency (BGE) was highly variable (0.09-0.58), and natural Spartina alterniflora marshes had consistently higher BGE than both restoration marshes and tidal floodwaters. BGE was negatively related to the ratio of LDOC to total dissolved phosphorous, which was highest in natural marshes. The enhancement of BP observed in the marshes relative to the estuarine floodwaters had different origins: In natural marshes it was mostly due to increases in BGE, whereas in restored marshes it followed increased BCC. These results highlight the importance of P in regulating microbial metabolism in coastal areas, and the need to understand the pathways that lead to BP in these systems.

Reactivity continuum of dissolved organic carbon decomposition in lake water

We determined microbial decomposition of dissolved organic carbon (DOC) over 3.7 year long dark bioassays of six Swedish lake waters. The overall lost DOC fraction was similar in clearwater lakes (34.8 ± 2.4%) and in brownwater lakes (37.8 ± 1.9%). Reactivity continuum modeling revealed that the most labile DOC fraction, degrading at rates &gt;0.01 d−1, was larger in the clearwater lakes (11.1 ± 1.2%) than in the brownwater lakes (0.8 ± 0.1%). The initial apparent first‐order decay coefficientk was fivefold larger in the clearwater lakes (0.0043 ± 0.0012 d−1) than in the brownwater lakes (0.0009 ± 0.0003 d−1). Over time, k decreased more steeply in the clearwater lakes than in the brownwater lakes, reaching the k of the brownwater lakes within 5 months. Finally, k averaged 0.0001 d−1 in both lake categories. In the brownwater lakes, colored dissolved organic matter (CDOM) absorption decayed with an initial k twice as large (0.0018 ± 0.0008 d−1) as that of DOC. The initial kwas inversely correlated with initial specific UV absorption and CDOM absorption and positively correlated with initial tryptophan‐like fluorescence as proxy for autochthonous DOC. Exposure to simulated sunlight at the end of the incubations caused loss of color in the clearwater lakes and loss of DOC in the brownwater lakes, where subsequent mineralization was also stimulated. The DOC lost in the absence of photochemical processes fell below previously reported watershed‐scale losses in Sweden by 25% at most. This suggests that a major part of the in situ DOC loss could potentially be attributed to dark reactions alone.

Chemical and biological characterization of dissolved organic matter derived from Norway spruce litter divided into fractions according to molecular size

Dissolved organic matter (DOM) derived from the litter layer under Norway spruce (Picea abies (L.) Karst) was separated into different molecular size fractions (<1, 1–10, 10–100, and >100 kDa) with tangential ultrafiltration technique. The chemical characteristics of the molecular size fractions were studied. The degradability of size fractions was assessed as the degradability of DOC and availability of a fraction to soil bacteria and fungi. The molecular size fractions appeared to differ chemically and microbiologically. The smallest molecular size fraction had the highest pH and lowest DOC/DON ratio. The two smallest molecular size fractions contained more carbohydrates and phenolic compounds than the two largest fractions. In all fractions DOC was degraded. The smallest fraction was the most degradable. It had the highest evolution of CO2–C and loss of DOC during the incubation and the highest estimated proportion of labile DOC. The NPK addition increased the evolution of CO2 in the total solution and in all fractions indicating nutrient shortage in all fractions. The smallest molecular size fraction was the most available to soil bacteria and the least available to soil fungi. Due to observed differences between the molecular size fractions tangential ultrafiltration may be useful in characterizing DOM.