A Source of Terrestrial Organic Carbon to Investigate the Browning of Aquatic Ecosystems

There is growing evidence that terrestrial ecosystems are exporting more dissolved organic carbon (DOC) to aquatic ecosystems than they did just a few decades ago. This “browning” phenomenon will alter the chemistry, physics, and biology of inland water bodies in complex and difficult-to-predict ways. Experiments provide an opportunity to elucidate how browning will affect the stability and functioning of aquatic ecosystems. However, it is challenging to obtain sources of DOC that can be used for manipulations at ecologically relevant scales. In this study, we evaluated a commercially available source of humic substances (“Super Hume”) as an analog for natural sources of terrestrial DOC. Based on chemical characterizations, comparative surveys, and whole-ecosystem manipulations, we found that the physical and chemical properties of Super Hume are similar to those of natural DOC in aquatic and terrestrial ecosystems. For example, Super Hume attenuated solar radiation in ways that will not only influence the physiology of aquatic taxa but also the metabolism of entire ecosystems. Based on its chemical properties (high lignin content, high quinone content, and low C:N and C:P ratios), Super Hume is a fairly recalcitrant, low-quality resource for aquatic consumers. Nevertheless, we demonstrate that Super Hume can subsidize aquatic food webs through 1) the uptake of dissolved organic constituents by microorganisms, and 2) the consumption of particulate fractions by larger organisms (i.e., Daphnia). After discussing some of the caveats of Super Hume, we conclude that commercial sources of humic substances can be used to help address pressing ecological questions concerning the increased export of terrestrial DOC to aquatic ecosystems.     

Tracing sources and pathways of dissolved nitrate in forest and river ecosystems using high-resolution isotopic techniques: a review

Nitrogen inputs into stream and river ecosystems, and the factors influencing those inputs, are important for various ecological and environmental concerns. Reliable information on where and how nitrogen compounds flow into aquatic ecosystems is indispensable to understanding the nutrient status of these ecosystems. Such information should include the biogeochemical mechanisms and hydrological controls of nutrient leaching into rivers from terrestrial systems such as forests, agricultural fields, and urbanized areas. Advancements in stable isotopomer measurements over the past two decades have expanded the variety of target substances and the precision with which they can be investigated. The high-throughput microbial denitrifier method allows for simultaneous measurement of nitrogen and oxygen isotope ratios and can provide high-resolution spatiotemporal information on both nitrate sources and biogeochemical processes. Although advanced techniques of stable isotope analysis have been used extensively to detect sources and estimate the relative contributions of multi-source systems in various rivers, there are still new horizons in investigating nitrogen transformations. For example, stable isotopes of oxygen (¹⁸O and ¹⁷O) occurring in nitrate due to atmospheric deposition can be used as natural tracers for evaluating internal nitrogen cycling; these isotopes are distinct from the oxygen within microbially generated nitrate in soils and water bodies. Another future challenge is improved use of nitrous oxide isotopomers in evaluating the relative contributions of nitrification and denitrification. Such analysis could provide insight into the nitrogen transformation that occurs under redox conditions at the boundary between terrestrial and aquatic systems, where nitrification and denitrification often occur simultaneously in soil and aquatic environments.     

Evidence for structuring of bacterial community composition by organic carbon source in temperate lakes

Water entering lakes from the surrounding watershed often delivers large amounts of terrestrial-derived dissolved organic carbon (DOC) that can contribute to aquatic bacterial production. However, research suggests that phytoplankton-derived DOC is more labile than its terrestrial counterpart, owing to microbial processing of terrestrial-derived DOC along its flow path to surface waters. The ratio of water colour (absorbance at 440 nm) to chlorophyll a has been suggested as a simple measure of the relative contribution of terrestrial and aquatic primary production to aquatic secondary production. To explore the correlation between primary DOC source and the occurrence of bacterial taxonomic groups, we conducted a survey of bacterial 16S rRNA gene composition in 15 lakes positioned along a water colour : chlorophyll a gradient. Our goal was to identify bacterial taxa occurrence patterns along the colour : chlorophyll a gradient that may indicate a competitive advantage for bacterial taxa using terrestrial or aquatic carbon. We observed a large number of bacterial taxa occurrence patterns suggestive of carbon substrate niche partitioning, especially when relatively highly resolved taxonomic groups were considered. Our survey supports the hypothesis that bacterial taxa partition along a carbon substrate source gradient and highlights carbon source–bacterial interactions that should be explored further.     

Labile and Recalcitrant Organic Matter Utilization by River Biofilm Under Increasing Water Temperature

Microbial biofilms in rivers contribute to the decomposition of the available organic matter which typically shows changes in composition and bioavailability due to their origin, seasonality, and watershed characteristics. In the context of global warming, enhanced biofilm organic matter decomposition would be expected but this effect could be specific when either a labile or a recalcitrant organic matter source would be available. A laboratory experiment was performed to mimic the effect of the predicted increase in river water temperature (+4?°C above an ambient temperature) on the microbial biofilm under differential organic matter sources. The biofilm microbial community responded to higher water temperature by increasing bacterial cell number, respiratory activity (electron transport system) and microbial extracellular enzymes (extracellular enzyme activity). At higher temperature, the phenol oxidase enzyme explained a large fraction of respiratory activity variation suggesting an enhanced microbial use of degradation products from humic substances. The decomposition of hemicellulose (β-xylosidase activity) seemed to be also favored by warmer conditions. However, at ambient temperature, the enzymes highly responsible for respiration activity variation were β-glucosidase and leu-aminopeptidase, suggesting an enhanced microbial use of polysaccharides and peptides degradation products. The addition of labile dissolved organic carbon (DOC; dipeptide plus cellobiose) caused a further augmentation of heterotrophic biomass and respiratory activity. The changes in the fluorescence index and the ratio Abs(250)/total DOC indicated that higher temperature accelerated the rates of DOC degradation. The experiment showed that the more bioavailable organic matter was rapidly cycled irrespective of higher temperature while degradation of recalcitrant substances was enhanced by warming. Thus, pulses of carbon at higher water temperature might have consequences for DOC processing.     

Bioavailability and composition of dissolved organic carbon and nitrogen in a near coastal catchment of south-western Australia

The bioavailability and composition of dissolved organic carbon (DOC) and nitrogen (DON) were examined in 10 major sub-catchments of the Swan-Canning estuary, which bisects the city of Perth, in south-western Australia. Catchments contain a mix of forest, agriculture, and urban-dominated land-use, with the degree of development increasing near the city center. We incubated water samples from the 10 sub-catchments for 14 days at 25°C, and measured changes in DOC and DON and dissolved inorganic nitrogen (DIN). A greater proportion of DON (4–44%) was decomposed compared to DOC (1–17%). Both agricultural and urban catchments had high proportions of bioavailable DOC and DON, but overall DOC and DON losses were greatest in urban catchments. Using resin isolation techniques, we found that DOC was concentrated in the hydrophobic (humic) fraction, whereas DON had both hydrophobic and hydrophilic (non-humic) fractions. Hydrophobic DOC content was positively related to DOC decomposition. In contrast, DON decomposition was highly correlated with hydrophilic DON content and inversely related to the hydrophilic DOC/DON ratio, indicating a labile fraction of DON from non-humic sources. Taken together, these relationships suggest that bioavailable DOC may be supplied in part from terrestrial plant material, but bioavailable DON is likely to be from highly labile sources, possibly autochthonous or anthropogenic. Overall, labile DON was greater than initial DIN concentration at seven of ten sites and was even dominant in highly developed catchments. This study highlights the importance of organic N in urbanizing coastal catchments that, in addition to DIN, may serve as a readily available source of N for in-stream and estuarine production.     

Reduced breeding success of Pied Flycatchers Ficedula hypoleuca along regulated rivers

Most large rivers in northern Sweden are regulated to produce hydropower, with subsequent effects on flow dynamics and aquatic insect communities. Several studies have shown that aquatic and terrestrial systems are intimately connected via the export of emergent aquatic insects, but few have assessed how human modifications of aquatic habitats may influence this connection. We compared breeding success of the insectivorous Pied Flycatcher Ficedula hypoleuca in near‐riparian upland forests along two regulated and two free‐flowing large rivers in northern Sweden over 3 years. The regulated rivers showed lower aquatic insect export to the surroundings, as a consequence of regulation‐induced loss of suitable aquatic insect habitats. Survival of Pied Flycatcher nestlings was 10–15% greater along the free‐flowing rivers. Females breeding near the free‐flowing rivers also started egg‐laying earlier and with greater synchrony than those at the regulated rivers, and showed a smaller decrease in weight during breeding than did females along the regulated rivers. However, there were no differences in occupation rate, clutch size or number of successfully hatched juveniles between regulated and free‐flowing rivers. As regulated rivers showed lower abundance of flying aquatic insects, which may also reduce the abundance of terrestrial invertebrate prey, regulation‐induced changes in the export of emergent aquatic insects may explain both directly and indirectly the observed reduction in Pied Flycatcher breeding success along regulated rivers. Large‐scale river regulation may therefore impair the breeding success of insectivorous birds through impacts on prey availability.     

Spatial and temporal aquatic–terrestrial transitions in the temporary Albarine River,France: responses of invertebrates to experimental rewetting

1. In temporary rivers, viewed as coupled terrestrial–aquatic ecosystems, spatial and temporal transition zones between aquatic and terrestrial conditions are common and occur simultaneously. 2. The effects of artificial rewetting on terrestrial and aquatic invertebrate assemblages were examined in dry sediments collected from the Albarine River, France. Rewetted sediments had previously been dry for between 0.1 and 142 days. Dry sediments were collected directly from the streambed (DS) and from riparian gravel bars (RGB). 3. We first predicted that invertebrate responses to rewetting would vary with the duration of the preceding dry period. Second, we predicted convergence of the invertebrate assemblages in DS and RGB sediments with increasing duration of the dry period. Third, we predicted that an aquatic ‘invertebrate seedbank’ (aquatic life stages that persist within streambed sediments during dry periods) would contribute substantially to the resilience of benthic assemblages. 4. Results indicated that the duration of the dry period was the primary driver of aquatic and terrestrial responses to artificial rewetting. The density and richness of aquatic taxa decreased with the duration of the dry period in both DS and RGB sediments, whereas the density of terrestrial invertebrates increased in DS sediments. 5. No convergence between DS and RGB assemblage composition was observed with an increasing dry period. Although there were more aquatic organisms in DS sediments than in RGB sediments, there was no difference in taxonomic richness between sediment types. Even after prolonged dry periods (142 days), there was typically a lower density and taxonomic richness of terrestrial invertebrates in DS sediments than in adjacent RGB sediments. 6. The results suggest that the aquatic invertebrate seedbank could contribute substantially to the resilience of benthic assemblages in the Albarine River, in addition to other mechanisms such as drift and oviposition. Of the taxa in the benthos before and after the summer dry period, 65% were also recovered from artificially rewetted DS sediments. The simultaneous presence of temporal and spatial terrestrial–aquatic transition zones in temporary rivers increases successional diversity (i.e. mosaics of dry and saturated streambed patches at various stages of terrestrial and aquatic succession). This contribution to biodiversity emphasises the need to protect dry reaches of temporary rivers.     

How organic carbon derived from multiple sources contributes to carbon sequestration processes in a shallow coastal system?

Carbon captured by marine organisms helps sequester atmospheric CO₂, especially in shallow coastal ecosystems, where rates of primary production and burial of organic carbon (OC) from multiple sources are high. However, linkages between the dynamics of OC derived from multiple sources and carbon sequestration are poorly understood. We investigated the origin (terrestrial, phytobenthos derived, and phytoplankton derived) of particulate OC (POC) and dissolved OC (DOC) in the water column and sedimentary OC using elemental, isotopic, and optical signatures in Furen Lagoon, Japan. Based on these data analysis, we explored how OC from multiple sources contributes to sequestration via storage in sediments, water column sequestration, and air–sea CO₂ exchanges, and analyzed how the contributions vary with salinity in a shallow seagrass meadow as well. The relative contribution of terrestrial POC in the water column decreased with increasing salinity, whereas autochthonous POC increased in the salinity range 10–30. Phytoplankton‐derived POC dominated the water column POC (65–95%) within this salinity range; however, it was minor in the sediments (3–29%). In contrast, terrestrial and phytobenthos‐derived POC were relatively minor contributors in the water column but were major contributors in the sediments (49–78% and 19–36%, respectively), indicating that terrestrial and phytobenthos‐derived POC were selectively stored in the sediments. Autochthonous DOC, part of which can contribute to long‐term carbon sequestration in the water column, accounted for >25% of the total water column DOC pool in the salinity range 15–30. Autochthonous OC production decreased the concentration of dissolved inorganic carbon in the water column and thereby contributed to atmospheric CO₂ uptake, except in the low‐salinity zone. Our results indicate that shallow coastal ecosystems function not only as transition zones between land and ocean but also as carbon sequestration filters. They function at different timescales, depending on the salinity, and OC sources.     

Abundant carbon substrates drive extremely high sulfate reduction rates and methane fluxes in Prairie Pothole Wetlands

Inland waters are increasingly recognized as critical sites of methane emissions to the atmosphere, but the biogeochemical reactions driving such fluxes are less well understood. The Prairie Pothole Region (PPR) of North America is one of the largest wetland complexes in the world, containing millions of small, shallow wetlands. The sediment pore waters of PPR wetlands contain some of the highest concentrations of dissolved organic carbon (DOC) and sulfur species ever recorded in terrestrial aquatic environments. Using a suite of geochemical and microbiological analyses, we measured the impact of sedimentary carbon and sulfur transformations in these wetlands on methane fluxes to the atmosphere. This research represents the first study of coupled geochemistry and microbiology within the PPR and demonstrates how the conversion of abundant labile DOC pools into methane results in some of the highest fluxes of this greenhouse gas to the atmosphere ever reported. Abundant DOC and sulfate additionally supported some of the highest sulfate reduction rates ever measured in terrestrial aquatic environments, which we infer to account for a large fraction of carbon mineralization in this system. Methane accumulations in zones of active sulfate reduction may be due to either the transport of free methane gas from deeper locations or the co‐occurrence of methanogenesis and sulfate reduction. If both respiratory processes are concurrent, any competitive inhibition of methanogenesis by sulfate‐reducing bacteria may be lessened by the presence of large labile DOC pools that yield noncompetitive substrates such as methanol. Our results reveal some of the underlying mechanisms that make PPR wetlands biogeochemical hotspots, which ultimately leads to their critical, but poorly recognized role in regional greenhouse gas emissions.     

Denitrification in the river network of a mixed land use watershed: unpacking the complexities

River networks have the potential to permanently remove nitrogen through denitrification. Few studies have measured denitrification rates within an entire river network or assessed how land use affect rates at larger spatial scales. We sampled 108 sites throughout the network of the Fox River watershed, Wisconsin, to determine if land use influence sediment denitrification rates, and to identify zones of elevated sediment denitrification rates (hot spots) within the river network. Partial least squares regression models identified variables from four levels of organization (river bed sediment, water column, riparian zone, and watershed) that best predicted denitrification rates throughout the river network. Nitrate availability was the most important predictor of denitrification rates, while land cover was not always a good predictor of local-scale nitrate concentrations. Thus, land cover and denitrification rate were not strongly related across the Fox River watershed. A direct relationship between denitrification rate and watershed land cover occurred only in the Wolf River sub-watershed, the least anthropogenically disturbed of the sub-watersheds. Denitrification hot spots were located throughout the river network, regardless of watershed land use, with hot spot location being determined primarily by nitrate availability. In the Fox River watershed, when nitrate was abundant, river bed sediment character influenced denitrification rate, with higher denitrification rates at sites with fine, organic sediments. These findings suggest that denitrification occurring throughout an entire river network, from headwater streams to larger rivers, can help reduce nitrogen loads to downstream water bodies.

     

Effect of dissolved organic carbon quality on microbial decomposition and nitrification rates in stream sediments

1. Microbial decomposition of dissolved organic carbon (DOC) contributes to overall stream metabolism and can influence many processes in the nitrogen cycle, including nitrification. Little is known, however, about the relative decomposition rates of different DOC sources and their subsequent effect on nitrification. 2. In this study, labile fraction and overall microbial decomposition of DOC were measured for leaf leachates from 18 temperate forest tree species. Between 61 and 82% (mean, 75%) of the DOC was metabolized in 24 days. Significant differences among leachates were found for labile fraction rates (P < 0.0001) but not for overall rates (P=0.088). 3. Nitrification rates in stream sediments were determined after addition of 10 mg C L^–1 of each leachate. Nitrification rates ranged from below detection to 0.49 μg N mL sediment^–1 day^–1 and were significantly correlated with two independent measures of leachate DOC quality, overall microbial decomposition rate (r=–0.594, P=0.0093) and specific ultraviolet absorbance (r=0.469, P=0.0497). Both correlations suggest that nitrification rates were lower in the presence of higher quality carbon. 4. Nitrification rates in sediments also were measured after additions of four leachates and glucose at three carbon concentrations (10, 30, and 50 mg C L^–1). For all carbon sources, nitrification rates decreased as carbon concentration increased. Glucose and white pine leachate most strongly depressed nitrification. Glucose likely increased the metabolism of heterotrophic bacteria, which then out‐competed nitrifying bacteria for NH₄⁺. White pine leachate probably increased heterotrophic metabolism and directly inhibited nitrification by allelopathy.     

人工遮光和营养添加对河流反硝化活性和反硝化细菌群落结构的影响

河流生态系统受到人类活动例如河岸带森林植被毁损和农业活动施肥等的干扰日益加剧,而这些活动使河流接收的光照增多、河流的氮磷营养盐浓度增加。微生物的反硝化作用是河流去除氮的有效途径。在汉江的一级支流金水河上游核心保护区内选取6条溪流开展野外控制实验,利用营养添加模拟河流中营养的增加,遮盖河面来模拟源头溪流的隐蔽状态,来研究河流沉积物中微生物的反硝化作用对光照和营养改变的响应,并利用高通量测序(Mi Seq)技术研究在两种处理下河流沉积物中nir S型反硝化细菌的群落结构变化。结果显示:营养元素添加促进了沉积物的反硝化活性,河面遮盖抑制了沉积物的反硝化活性。营养添加和遮盖两种处理均降低了控制实验区域内脱氯单胞菌属(优势菌属)的相对丰度,同时也降低了该区域nir S型反硝化菌群落的Chao多样性。本研究初步证实了光照增加和河流的营养增加提高了河流沉积物反硝化活性,并为提高河流的脱氮能力提供科学依据。     

Denitrification in coastal ecosystems: methods, environmental controls, and ecosystem level controls, a review

In this review of sediment denitrification in estuaries and coastal ecosystems, we examine current denitrification measurement methodologies and the dominant biogeochemical controls on denitrification rates in coastal sediments. Integrated estimates of denitrification in coastal ecosystems are confounded by methodological difficulties, a lack of systematic understanding of the effects of changing environmental conditions, and inadequate attention to spatial and temporal variability to provide both seasonal and annual rates. Recent improvements in measurement techniques involving ¹⁵ N techniques and direct N₂ concentration changes appear to provide realistic rates of sediment denitrification. Controlling factors in coastal systems include concentrations of water column NO ₃ ^– , overall rates of sediment carbon metabolism, overlying water oxygen concentrations, the depth of oxygen penetration, and the presence/absence of aquatic vegetation and macrofauna. In systems experiencing environmental change, either degradation or improvement, the importance of denitrification can change. With the eutrophication of the Chesapeake Bay, the overall rates of denitrification relative to N loading terms have decreased, with factors such as loss of benthic habitat via anoxia and loss of submerged aquatic vegetation driving such effects.     

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.     

Low river flow alters the biomass and population structure of a riparian predatory invertebrate

1. Low flows in rivers are predicted to increase in extent and severity in many areas in the future, yet the consequent impacts of river drying on terrestrial communities via (i) changes to riparian microclimatic conditions and (ii) the identity and abundance of emerging aquatic insects available to riparian predators have not been quantified. 2. We investigated the influence of low river flow on a riparian fishing spider, Dolomedes aquaticus, in five New Zealand rivers containing permanently flowing and drying reaches and, in one river, along a longitudinal drying gradient. 3. The biomass of aquatic insects, potential prey for D. aquaticus, declined with low river flows while the abundance of potential terrestrial prey remained similar at all sites. In the replicate rivers, and along the longitudinal drying gradient, spider biomass was lower, and size classes were skewed towards more small and fewer large spiders, in drying sites. A desiccation experiment in the laboratory indicated high sensitivity of the spiders, with prey presence increasing spider survival. 4. Differences in the spatial distribution, biomass and population size structure of spiders were observed along the longitudinal drying gradient and disappeared within 16 days of the water returning to all sites. 5. In total, low river flow affected the biomass of D. aquaticus, as well as their size class structure and spatial distribution. This indicates that low river flows have the potential to affect adjacent terrestrial ecosystems.     

Sources of organic carbon supporting the food web of an arid zone floodplain river

SUMMARY 1. Many Australian inland rivers are characterised by vast floodplains with a network of anastomosing channels that interconnect only during unpredictable flooding. For much of the time, however, rivers are reduced to a string of disconnected and highly turbid waterholes. Given these features, we predicted that aquatic primary production would be light-limited and the riverine food web would be dependent on terrestrial carbon from floodplain exchanges and direct riparian inputs.
2. To test these predictions, we measured rates of benthic primary production and respiration and sampled primary sources of organic carbon and consumers for stable isotope analysis in several river waterholes at four locations in the Cooper Creek system in central Australia.
3. A conspicuous band of filamentous algae was observed along the shallow littoral zone of the larger waterholes. Despite the high turbidity, benthic gross primary production in this narrow zone was very high (1.7–3.6 g C m⁻² day⁻¹); about two orders of magnitude greater than that measured in the main channel.
4. Stable carbon isotope analysis confirmed that the band of algae was the major source of energy for aquatic consumers, ultimately supporting large populations of crustaceans and fish. Variation in the stable carbon and nitrogen isotope signatures of consumers suggested that zooplankton was the other likely major source.
5. Existing ecosystem models of large rivers often emphasise the importance of longitudinal or lateral inputs of terrestrial organic matter as a source of organic carbon for aquatic consumers. Our data suggest that, despite the presence of large amounts of terrestrial carbon, there was no evidence of it being a significant contributor to the aquatic food web in this floodplain river system.     

Impacts of Labile Organic Carbon Concentration on Organic and Inorganic Nitrogen Utilization by a Stream Biofilm Bacterial Community

In aquatic ecosystems, carbon (C) availability strongly influences nitrogen (N) dynamics. One manifestation of this linkage is the importance in the dissolved organic matter (DOM) pool of dissolved organic nitrogen (DON), which can serve as both a C and an N source, yet our knowledge of how specific properties of DOM influence N dynamics are limited. To empirically examine the impact of labile DOM on the responses of bacteria to DON and dissolved inorganic nitrogen (DIN), bacterial abundance and community composition were examined in controlled laboratory microcosms subjected to various combinations of dissolved organic carbon (DOC), DON, and DIN treatments. Bacterial communities that had colonized glass beads incubated in a stream were treated with various glucose concentrations and combinations of inorganic and organic N (derived from algal exudate, bacterial protein, and humic matter). The results revealed a strong influence of C availability on bacterial utilization of DON and DIN, with preferential uptake of DON under low C concentrations. Bacterial DON uptake was affected by the concentration and by its chemical nature (labile versus recalcitrant). Labile organic N sources (algal exudate and bacterial protein) were utilized equally well as DIN as an N source, but this was not the case for the recalcitrant humic matter DON treatment. Clear differences in bacterial community composition among treatments were observed based on terminal restriction fragment length polymorphisms (T-RFLP) of 16S rRNA genes. C, DIN, and DON treatments likely drove changes in bacterial community composition that in turn affected the rates of DON and DIN utilization under various C concentrations.     

Sources of dissolved organic matter (DOM) in a Rocky Mountain stream using chemical fractionation and stable isotopes

Dissolved organic matter (DOM) is an important vehicle for the movement of nutrients from terrestrial to aquatic systems. To investigate how the source and composition of aquatic DOM change in both space and time, we used chemical, spectroscopic, and isotopic analyses to characterize DOM in a headwater catchment in the Colorado Front Range. Streamwater samples for DOM analyses were collected from 2 sites, a lightly vegetated alpine site and a forested, subalpine site, in the North Boulder Creek catchment during the snowmelt runoff season (May–September). Concentrations of dissolved organic carbon (DOC) peaked on the ascending limb of the snowmelt hydrograph at both the alpine (2.6 mg C l⁻¹) and the subalpine sites (7.0 mg C l⁻¹) and decreased sharply on the descending limb of the hydrograph. Fractionation of DOM into operationally defined humic and non-humic components showed that the fulvic acid content of DOC decreased through the season at both sites and that spectroscopic (fluorescence and ultraviolet) properties of the humic DOM fraction shifted in a manner consistent with an increase in the proportion of humic DOM derived from instream sources as compared to terrestrial catchment sources. Humic and non-humic fractions of DOM isolated near peak flow in June and during low flows in September showed a seasonal enrichment in ¹⁵N and ¹³C as well as a seasonal decrease in the ratio of aromatic to aliphatic carbon, both of which were correlated with a decrease in the C:N ratio of the DOM fractions. These results suggest that seasonal shifts in the isotopic and chemical characteristics of DOM are a result of changes in catchment sources of DOM. In particular, it appears that DOM production in alpine lakes is an important contributor to the streamwater DOM load during late season low flows, especially in the alpine reach of the catchment. Our results further suggest that stable isotopes of C and N are useful tools, particularly when combined with ancillary data such as elemental analyses and catchment discharge, for evaluating sources and transformations of DOM at the catchment scale.     

Seasonal changes in the dissolved free amino acid and DOC concentrations in a hypertrophic African reservoir and its inflowing rivers

Dissolved free amino acid (DFAA) concentration and composition and dissolved organic carbon (DOC) concentration were measured over 16 months at three depths in hypertrophic Hartbeespoort Dam, South Africa and in its two perenially inflowing rivers. The range of DFAA concentrations in the reservoir and both rivers were similar with dominant DFAA consisting of serine, glycine, alanine and ornithine in all three systems. The range of DOC concentrations in the rivers was 1.5–11.1 mg l^–1, the major river (Crocodile) having about twice the DOC concentration of the Magalies River. The DFAA/DOC ratios ranged between 0.02–1.1% in the Crocodile River and 0.13–3.7% in the Magalies River. DFAA and DOC concentrations were positively correlated to the Magalies River flow, but for the Crocodile River, which received domestic and industrial effluents, DOC was inversely correlated to flow. The source of DFAA in both rivers was mainly terrestrial, in contrast to the main DOC source in the Crocodile River which was the effluents. The DFAA load of the Crocodile River ranged between 0.22 and 208 kg C d^–1.DOC (5.0–24.8mg l^–1) in Hartbeespoort Dam generally decreased with depth but DFAA (15–4800 nmol l^–1) concentration showed no clear trend. The DFAA/DOC ratios varied between 0.02 and 2.9%. DFAA concentrations were correlated (r = 0.3, n = 30, p = 0.04) with bacterial numbers at 0 and 10 m only while no significant correlations were found with bacterial production, chlorophyll a concentration and phytoplankton primary and EDOC (extracellular DOC) production at any depth. The rate of bacterial utilization of DFAA was low compared with data from other lakes. Diurnal phytoplankton production of DFAA in the euphotic zone of the whole lake was calculated to vary between 268 and 30 780 t C d^–1 indicating autochthonous DFAA sources were dominant to allochthonous DFAA sources. The autochthonous production of DFAA was > 2 × gross bacterial production of the euphotic zone indicating that although DFAA concentrations were frequently < 10 g C l^–1, the rate of DFAA production exceeded bacterial requirements.     

Dissolved Organic Carbon in Alaskan Boreal Forest: Sources, Chemical Characteristics, and Biodegradability

The fate of terrestrially-derived dissolved organic carbon (DOC) is important to carbon (C) cycling in both terrestrial and aquatic environments, and recent evidence suggests that climate warming is influencing DOC dynamics in northern ecosystems. To understand what determines the fate of terrestrial DOC, it is essential to quantify the chemical nature and potential biodegradability of this DOC. We examined DOC chemical characteristics and biodegradability collected from soil pore waters and dominant vegetation species in four boreal black spruce forest sites in Alaska spanning a range of hydrologic regimes and permafrost extents (Well Drained, Moderately Well Drained, Poorly Drained, and Thermokarst Wetlands). DOC chemistry was characterized using fractionation, UV–Vis absorbance, and fluorescence measurements. Potential biodegradability was assessed by incubating the samples and measuring CO₂ production over 1 month. Soil pore water DOC from all sites was dominated by hydrophobic acids and was highly aromatic, whereas the chemical composition of vegetation leachate DOC varied significantly with species. There was no seasonal variability in soil pore water DOC chemical characteristics or biodegradability; however, DOC collected from the Poorly Drained site was significantly less biodegradable than DOC from the other three sites (6% loss vs. 13–15% loss). The biodegradability of vegetation-derived DOC ranged from 10 to 90% loss, and was strongly correlated with hydrophilic DOC content. Vegetation such as Sphagnum moss and feathermosses yielded DOC that was quickly metabolized and respired. In contrast, the DOC leached from vegetation such as black spruce was moderately recalcitrant. Changes in DOC chemical characteristics that occurred during microbial metabolism of DOC were quantified using fractionation and fluorescence. The chemical characteristics and biodegradability of DOC in soil pore waters were most similar to the moderately recalcitrant vegetation leachates, and to the microbially altered DOC from all vegetation leachates.