Glacial meltwater determines the balance between autotrophic and heterotrophic processes in a Greenland fjord.

Global warming accelerates melting of glaciers and increases the supply of meltwater and associated inorganic particles, nutrients, and organic matter to adjacent coastal seas, but the ecosystem impact is poorly resolved and quantified. When meltwater is delivered by glacial rivers, the potential impact could be a reduction in light and nutrient availability for primary producers while supplying allochthonous carbon for heterotrophic processes, thereby tipping the net community metabolism toward heterotrophy. To test this hypothesis, we determined physical and biogeochemical parameters along a 110-km fjord transect in NE Greenland fjord, impacted by glacial meltwater from the Greenland Ice Sheet. The meltwater is delivered from glacier-fed river outlets in the inner parts of the fjord, creating a gradient in salinity and turbidity. The planktonic primary production was low, 20-45 mg C m-2 d-1, in the more turbid inner half of the fjord, increasing 10-fold to around 350 mg C m-2 d-1 in the shelf waters outside the fjord. Plankton community metabolism was measured at three stations, which displayed a transition from net heterotrophy in the inner fjord to net autotrophy in the coastal shelf waters. Respiration was significantly correlated to turbidity, with a 10-fold increase in the inner turbid part of the fjord. We estimated the changes in meltwater input and sea ice coverage in the area for the last 60 y. The long-term trend and the observed effects demonstrated the importance of freshwater runoff as a key driver of coastal ecosystem change in the Arctic with potential negative consequences for coastal productivity.

Coupling pathway prediction and fluorescence spectroscopy to assess the impact of auxiliary substrates on micropollutant biodegradation.

Some bacteria can degrade organic micropollutants (OMPs) as primary carbon sources. Due to typically low OMP concentrations, these bacteria may benefit from supplemental assimilation of natural substrates present in the pool of dissolved organic matter (DOM). The biodegradability of such auxiliary substrates and the impacts on OMP removal are tightly linked to biotransformation pathways. Here, we aimed to elucidate the biodegradability and effect of different DOM constituents for the carbofuran degrader Novosphingobium sp. KN65.2, using a novel approach that combines pathway prediction, laboratory experiments, and fluorescence spectroscopy. Pathway prediction suggested that ring hydroxylation reactions catalysed by Rieske-type dioxygenases and flavin-dependent monooxygenases determine the transformability of the 11 aromatic compounds used as model DOM constituents. Our approach further identified two groups with distinct transformation mechanisms amongst the four growth-supporting compounds selected for mixed substrate biodegradation experiments with the pesticide carbofuran (Group 1: 4-hydroxybenzoic acid, 4-hydroxybenzaldehyde; Group 2: p-coumaric acid, ferulic acid). Carbofuran biodegradation kinetics were stable in the presence of both Group 1 and Group 2 auxiliary substrates. However, Group 2 substrates would be preferable for bioremediation processes, as they showed constant biodegradation kinetics under different experimental conditions (pre-growing KN65.2 on carbofuran vs. DOM constituent). Furthermore, Group 2 substrates were utilisable by KN65.2 in the presence of a competitor (Pseudomonas fluorescens sp. P17). Our study thus presents a simple and cost-efficient approach that reveals mechanistic insights into OMP-DOM biodegradation.

Removal of dissolved organic matter from the woodchip bioreactor start-up by foam fractionation.

Denitrifying woodchip bioreactors are passive, low-tech systems primarily designed to remove nitrate from shallow ground waters as well as point source discharges. Despite their capacity to achieve constant nitrate removal over several years, natural aquatic environments may be affected by the leaching of dissolved organic matter (DOM) from fresh woodchips during start-up. Simple on-site measures might reduce the woodchip leachate during start-up and thus add to the overall environmental sustainability of woodchip bioreactor installations. The aim of the study was to investigate whether foam fractionators could provide an effective solution. Water was flowed through fresh laboratory-scale woodchip bioreactors and recirculated through foam fractionators for 11 days. The bioreactors removed nitrate but increased phosphate and ammonia, which were not effectively removed via foam fractionation. However, foam fractionation did remove 37.8 ± 4.7% of the dissolved chemical oxygen demand (CODdiss) leached during the first 11 days of operation. Fluorescence spectroscopy revealed that the DOM composition differed between the foam and water, where the foam fraction contained higher amounts of DOM associated with the highest bioavailability and hence the greatest potential environmental impact. Optimised foam fractionators could therefore help to reduce the environmental impact of DOM leachate from woodchip bioreactors during start-up.

The Molecular Fingerprint of Fluorescent Natural Organic Matter Offers Insight into Biogeochemical Sources and Diagenetic State.

Investigating the biogeochemistry of dissolved organic matter (DOM) requires the synthesis of data from several complementary analytical techniques. The traditional approach to data synthesis is to search for correlations between measurements made on the same sample using different instruments. In contrast, data fusion simultaneously decomposes data from multiple instruments into the underlying shared and unshared components. Here, Advanced Coupled Matrix and Tensor Factorization (ACMTF) was used to identify the molecular fingerprint of DOM fluorescence fractions in Arctic fjords. ACMTF explained 99.84% of the variability with six fully shared components. Individual molecular formulas were linked to multiple fluorescence components and vice versa. Molecular fingerprints differed in diversity and oceanographic patterns, suggesting a link to the biogeochemical sources and diagenetic state of DOM. The fingerprints obtained through ACMTF were more specific compared to traditional correlation analysis and yielded greater compositional insight. Multivariate data fusion aligns extremely complex, heterogeneous DOM data sets and thus facilitates a more holistic understanding of DOM biogeochemistry.

Investigating Fluorescent Organic-Matter Composition as a Key Predictor for Arsenic Mobility in Groundwater Aquifers.

Dissolved organic matter (DOM) is linked to the heterogeneous distribution of elevated arsenic (As) in groundwater used for drinking and irrigation purposes, but the relationship between DOM characteristics and arsenic mobility has yet to be fully understood. Here, DOM from groundwater sampled in the Bengal Basin region was characterized using both conventional bulk emission-excitation (EEM) spectroscopy and high-performance size-exclusion chromatography coupled to spectroscopy (HPSEC-EEM). Notably, application of the novel HPSEC-EEM approach permitted the total fluorescence of individual samples to be independently resolved into its underlying components. This allowed the external validation of the bulk-sample fluorescence decomposition and offered insight into the molecular size distribution of fluorescent DOM. Molecular size distributions were similar for the UVA fluorescent (C310 and C340) as well as the three visible fluorescent (C390, C440, and C500) components. There was a greater visible fluorescence in shallow aquifer samples (10-33 m) with high As (SH, up to 418 µg/L) than in samples from the same depth with lower As (up to 40 µg/L). This indicated a link between DOM quality and As mobility within the shallow aquifer. The deep aquifer samples (170-200 m) revealed DOM characteristics similar to SH samples but had low As concentrations (<4 µg/L), signifying that the deep aquifer is potentially vulnerable to As contamination. These findings pave the way for a more comprehensive assessment of the susceptibility of drinking water aquifers, thereby supporting the management of groundwater resources.

The One-Sample PARAFAC Approach Reveals Molecular Size Distributions of Fluorescent Components in Dissolved Organic Matter.

Molecular size plays an important role in dissolved organic matter (DOM) biogeochemistry, but its relationship with the fluorescent fraction of DOM (FDOM) remains poorly resolved. Here high-performance size exclusion chromatography (HPSEC) was coupled to fluorescence emission-excitation (EEM) spectroscopy in full spectral (60 emission and 34 excitation wavelengths) and chromatographic resolution (<1 Hz), to enable the mathematical decomposition of fluorescence on an individual sample basis by parallel factor analysis (PARAFAC). The approach allowed cross-system comparisons of molecular size distributions for individual fluorescence components obtained from independent data sets. Spectra extracted from allochthonous DOM were highly similar. Allochthonous and autochthonous DOM shared some spectra, but included unique components. In agreement with the supramolecular assembly hypothesis, molecular size distributions of the fluorescence fractions were broad and chromatographically unresolved, possibly representing reoccurring fluorophores forming noncovalently bound assemblies of varying molecular size. Samples shared underlying fluorescence components that differed in their size distributions but not their spectral properties. Thus, in contrast to absorption measurements, bulk fluorescence is unlikely to reliably indicate the average molecular size of DOM. The one-sample approach enables robust and independent cross-site comparisons without large-scale sampling efforts and introduces new analytical opportunities for elucidating the origins and biogeochemical properties of FDOM.

A model of extracellular enzymes in free-living microbes: which strategy pays off?

An initial modeling approach was applied to analyze how a single, nonmotile, free-living, heterotrophic bacterial cell may optimize the deployment of its extracellular enzymes. Free-living cells live in a dilute and complex substrate field, and to gain enough substrate, their extracellular enzymes must be utilized efficiently. The model revealed that surface-attached and free enzymes generate unique enzyme and substrate fields, and each deployment strategy has distinctive advantages. For a solitary cell, surface-attached enzymes are suggested to be the most cost-efficient strategy. This strategy entails potential substrates being reduced to very low concentrations. Free enzymes, on the other hand, generate a radically different substrate field, which suggests significant benefits for the strategy if free cells engage in social foraging or experience high substrate concentrations. Swimming has a slight positive effect for the attached-enzyme strategy, while the effect is negative for the free-enzyme strategy. The results of this study suggest that specific dissolved organic compounds in the ocean likely persist below a threshold concentration impervious to biological utilization. This could help explain the persistence and apparent refractory state of oceanic dissolved organic matter (DOM). Microbial extracellular enzyme strategies, therefore, have important implications for larger-scale processes, such as shaping the role of DOM in ocean carbon sequestration.

Controls of dissolved organic matter quality: evidence from a large-scale boreal lake survey.

Inland waters transport large amounts of dissolved organic matter (DOM) from terrestrial environments to the oceans, but DOM also reacts en route, with substantial water column losses by mineralization and sedimentation. For DOM transformations along the aquatic continuum, lakes play an important role as they retain waters in the landscape allowing for more time to alter DOM. We know DOM losses are significant at the global scale, yet little is known about how the reactivity of DOM varies across landscapes and climates. DOM reactivity is inherently linked to its chemical composition. We used fluorescence spectroscopy to explore DOM quality from 560 lakes distributed across Sweden and encompassed a wide climatic gradient typical of the boreal ecozone. Six fluorescence components were identified using parallel factor analysis (PARAFAC). The intensity and relative abundance of these components were analyzed in relation to lake chemistry, catchment, and climate characteristics. Land cover, particularly the percentage of water in the catchment, was a primary factor explaining variability in PARAFAC components. Likewise, lake water retention time influenced DOM quality. These results suggest that processes occurring in upstream water bodies, in addition to the lake itself, have a dominant influence on DOM quality. PARAFAC components with longer emission wavelengths, or red-shifted components, were most reactive. In contrast, protein-like components were most persistent within lakes. Generalized characteristics of PARAFAC components based on emission wavelength could ease future interpretation of fluorescence spectra. An important secondary influence on DOM quality was mean annual temperature, which ranged between -6.2 and +7.5 °C. These results suggest that DOM reactivity depends more heavily on the duration of time taken to pass through the landscape, rather than temperature. Projected increases in runoff in the boreal region may force lake DOM toward a higher overall amount and proportion of humic-like substances.

Radiocarbon dating of fluvial organic matter reveals land-use impacts in boreal peatlands.

This study measured the effects of land use on organic matter released to surface waters in a boreal peat catchment using radiocarbon dating of particulate and dissolved organic carbon (POC and DOC), DOC concentration, stable carbon and nitrogen isotope composition, and optical measurements. Undisturbed sites invariably released modern DOC and POC (<20 years old), and seasonal forcing had little impact on the age distribution. Release of pre-1950 carbon was detected at peat extraction, agricultural and drained sites, and was consistently observed at agricultural and peat extraction areas throughout the seasons. Conventional mean DOC ages reached 3,100 (±122) years before collection. On average, DOC concentrations were up to 38% higher at impacted sites compared to natural areas, but there was no significant effect of land use on surface water DOC concentrations. The study indicates that the true extent of land use impacts is not necessarily detectible through changes in DOC concentration alone: Radiocarbon dating was essential to show that leaching of old soil organic matter at modified sites had replaced, rather than supplemented, the modern DOM that is usually released from pristine peatlands. Relationships between the specific fluorescence intensity of DOM and its radiocarbon age were identified, indicating that optical techniques may provide a method for the detection of changes in DOM age.

Coastal freshening drives acidification state in Greenland fjords.

Greenland's fjords and coastal waters are highly productive and sustain important fisheries. However, retreating glaciers and increasing meltwater are changing fjord circulation and biogeochemistry, which may threaten future productivity. The freshening of Greenland fjords caused by unprecedented melting of the Greenland Ice Sheet may alter carbonate chemistry in coastal waters, influencing CO2 uptake and causing biological consequences from acidification. However, few studies to date explore the current acidification state in Greenland coastal waters. Here we present the first-ever large-scale measurements of carbonate system parameters in 16 Greenlandic fjords and seek to identify the drivers of acidification state in these freshening ecosystems. Aragonite saturation state (Ω), a proxy for ocean acidification, was calculated from dissolved inorganic carbon (DIC) and total alkalinity from fjords along the east and west coast of Greenland spanning 68-75°N. Aragonite saturation was primarily >1 in the surface mixed layer. However, undersaturated-or corrosive--conditions (Ω < 1) were observed on both coasts (west: Ω = 0.28-3.11, east: Ω = 0.70-3.07), albeit at different depths. West Greenland fjords were largely corrosive at depth while undersaturation in East Greenland fjords was only observed in surface waters. This reflects a difference in the coastal boundary conditions and mechanisms driving acidification state. We suggest that advection of Sub Polar Mode Water and accumulation of DIC from organic matter decomposition drive corrosive conditions in the West, while freshwater alkalinity dilution drives acidification in the East. The presence of marine terminating glaciers also impacted local acidification states by influencing fjord circulation: upwelling driven by subglacial discharge brought corrosive bottom waters to shallower depths. Meanwhile, discharge from land terminating glaciers strengthened stratification and diluted alkalinity. Regardless of the drivers in each system, increasing freshwater discharge will likely lower carbonate saturation states and impact biotic and abiotic carbon uptake in the future.

Remediation of marine dead zones by enhancing microbial sulfide oxidation using electrodes.

Marine dead zones caused by hypoxia have expanded over the last decades and pose a serious threat to coastal marine life. We tested sediment microbial fuel cells (SMFCs) for their potential to reduce the release of sulfide from sediments, in order to potentially protect the marine environment from the formation of such dead zones. Steel electrodes as well as charcoal-amended electrodes and corresponding non-connected controls of a size of together 24 m2 were installed in a marine harbour, and the effects on water quality were monitored for several months. Both pure steel electrodes and charcoal-amended electrodes were able to reduce sulfide concentrations in bottom water (92 % to 98 % reduction, in comparison to disconnected control steel electrodes). Also phosphate concentrations and ammonium were drastically reduced. SMFCs might be used to eliminate hypoxia at sites with high organic matter deposition and should be further investigated for this purpose.

Tracing Atlantic water transit time in the subarctic and Arctic Atlantic using 99Tc-233U-236U.

The pathway and transport time of Atlantic water passing northern Europe can be traced via anthropogenic radioisotopes released from reprocessing of spent nuclear fuels at Sellafield (SF) and La Hague (LH). These reprocessing derived radioisotopes, with extremely low natural background, are source specific and unique fingerprints for Atlantic water. This study explores a new approach using 99Tc-233U-236U tracer to estimate the transit time of Atlantic water in the coast of Greenland. We isolate the reprocessing plants (RP) signal of 236U (236URP) by incorporating 233U measurements and combine this with 99Tc which solely originates from RP, to estimate the transit time of Atlantic water circulating from Sellafield to the coast of Greenland-Iceland-Faroe Islands. Both being conservative radioisotopes, the temporal variation of 99Tc/236URP ratio in Atlantic water is only influenced by their historic discharges from RP, thus 99Tc/236URP can potentially be a robust tracer to track the transport of Atlantic water in the North Atlantic-Arctic region. Based on our observation data of 99Tc-233U-236U in seawater and the proposed 99Tc/236URP tracer approach, Atlantic water transit times were estimated to be 16-22, 25 and 25 years in the coast of Greenland, Iceland and Faroe Island, respectively. Our estimates from northeast Greenland coastal waters agree with earlier results (17-22 years). Therefore, this work provides an independent approach to estimate Atlantic water transit time with which to compare estimates from ocean modelling and other radiotracer approaches.

Vertical redistribution of principle water masses on the Northeast Greenland Shelf.

The Northeast Greenland shelf (NEGS) is a recipient of Polar Water (PW) from the Arctic Ocean, Greenland Ice Sheet melt, and Atlantic Water (AW). Here, we compile hydrographical measurements to quantify long-term changes in fjords and coastal waters. We find a profound change in the vertical distribution of water masses, with AW shoaling >60 m and PW thinning >50 m since early 2000's. The properties of these waters have also changed. AW is now 1 °C warmer and the salinity of surface waters and PW are 1.8 and 0.68 lower, respectively. The AW changes have substantially weakened stratification south of ~74°N, indicating increased accessibility of heat and potentially nutrients associated with AW. The Atlantification earlier reported for the eastern Fram Strait and Barents Sea region has also propagated to the NEGS. The increased presence of AW, is an important driver for regional change leading to a likely shift in ecosystem structure and function.

Measurement of dissolved organic matter fluorescence in aquatic environments: an interlaboratory comparison.

The fluorescent properties of dissolved organic matter (DOM) are often studied in order to infer DOM characteristics in aquatic environments, including source, quantity, composition, and behavior. While a potentially powerful technique, a single widely implemented standard method for correcting and presenting fluorescence measurements is lacking, leading to difficulties when comparing data collected by different research groups. This paper reports on a large-scale interlaboratory comparison in which natural samples and well-characterized fluorophores were analyzed in 20 laboratories in the U.S., Europe, and Australia. Shortcomings were evident in several areas, including data quality-assurance, the accuracy of spectral correction factors used to correct EEMs, and the treatment of optically dense samples. Data corrected by participants according to individual laboratory procedures were more variable than when corrected under a standard protocol. Wavelength dependency in measurement precision and accuracy were observed within and between instruments, even in corrected data. In an effort to reduce future occurrences of similar problems, algorithms for correcting and calibrating EEMs are described in detail, and MATLAB scripts for implementing the study's protocol are provided. Combined with the recent expansion of spectral fluorescence standards, this approach will serve to increase the intercomparability of DOM fluorescence studies.

Sediment alkaline-extracted organic matter (AEOM) fluorescence: An archive of Holocene marine organic matter origins.

Organic matter (OM) is comprised of a complex mixture of substrates, which are difficult to fully characterize. Therefore a range of analytical approaches is applied to provide a better understanding of the dynamics and biogeochemical cycling of aquatic system. One approach is UV-Visible spectroscopy, which includes measurements of spectral absorption and fluorescence of colored and fluorescent fractions of dissolved OM (DOM, CDOM and FDOM). In this study OM fluorescence is characterized by excitation-emission matrix spectroscopy on alkaline extracted DOM from a Baltic Sea sediment core that spanned 8500 years and fluctuating levels of hypoxia. Our results showed that three underlying fluorescence components had strong correlations with carbon, nitrogen content and δ15N. Our results demonstrate that optical properties of extracted OM from sediments reveal information about OM quality and quantity similar to those of biomarkers, which can be a useful additional tool for investigating OM deposition.

Assessment of drinking water quality at the tap using fluorescence spectroscopy.

Treated drinking water may become contaminated while travelling in the distribution system on the way to consumers. Elevated dissolved organic matter (DOM) at the tap relative to the water leaving the treatment plant is a potential indicator of contamination, and can be measured sensitively, inexpensively and potentially on-line via fluorescence and absorbance spectroscopy. Detecting elevated DOM requires potential contamination events to be distinguished from natural fluctuations in the system, but how much natural variation to expect in a stable distribution system is unknown. In this study, relationships between DOM optical properties, microbial indicator organisms and trace elements were investigated for households connected to a biologically-stable drinking water distribution system. Across the network, humic-like fluorescence intensities showed limited variation (RSD = 3.5-4.4%), with half of measured variation explained by interactions with copper. After accounting for quenching by copper, fluorescence provided a very stable background signal (RSD < 2.2%) against which a ∼2% infiltration of soil water would be detectable. Smaller infiltrations would be detectable in the case of contamination by sewage with a strong tryptophan-like fluorescence signal. These findings indicate that DOM fluorescence is a sensitive indicator of water quality changes in drinking water networks, as long as potential interferents are taken into account.

Evidence of local and regional freshening of Northeast Greenland coastal waters.

The supply of freshwater to fjord systems in Greenland is increasing as a result of climate change-induced acceleration in ice sheet melt. However, insight into the marine implications of the melt water is impaired by lack of observations demonstrating the fate of freshwater along the Greenland coast and providing evaluation basis for ocean models. Here we present 13 years of summer measurements along a 120 km transect in Young Sound, Northeast Greenland and show that sub-surface coastal waters are decreasing in salinity with an average rate of 0.12 ± 0.05 per year. This is the first observational evidence of a significant freshening on decadal scale of the waters surrounding the ice sheet and comes from a region where ice sheet melt has been less significant. It implies that ice sheet dynamics in Northeast Greenland could be of key importance as freshwater is retained in southward flowing coastal currents thus reducing density of water masses influencing major deep water formation areas in the Subarctic Atlantic Ocean. Ultimately, the observed freshening could have implications for the Atlantic meridional overturning circulation.

The Effect of Increased Loads of Dissolved Organic Matter on Estuarine Microbial Community Composition and Function.

Increased river loads are projected as one of the major consequences of climate change in the northern hemisphere, leading to elevated inputs of riverine dissolved organic matter (DOM) and inorganic nutrients to coastal ecosystems. The objective of this study was to investigate the effects of elevated DOM on a coastal pelagic food web from the coastal northern Baltic Sea, in a 32-day mesocosm experiment. In particular, the study addresses the response of bacterioplankton to differences in character and composition of supplied DOM. The supplied DOM differed in stoichiometry and quality and had pronounced effects on the recipient bacterioplankton, driving compositional changes in response to DOM type. The shifts in bacterioplankton community composition were especially driven by the proliferation of Bacteroidetes, Gemmatimonadetes, Planctomycetes, and Alpha- and Betaproteobacteria populations. The DOM additions stimulated protease activity and a release of inorganic nutrients, suggesting that DOM was actively processed. However, no difference between DOM types was detected in these functions despite different community compositions. Extensive release of re-mineralized carbon, nitrogen and phosphorus was associated with the bacterial processing, corresponding to 25-85% of the supplied DOM. The DOM additions had a negative effect on phytoplankton with decreased Chl a and biomass, particularly during the first half of the experiment. However, the accumulating nutrients likely stimulated phytoplankton biomass which was observed to increase towards the end of the experiment. This suggests that the nutrient access partially outweighed the negative effect of increased light attenuation by accumulating DOM. Taken together, our experimental data suggest that parts of the future elevated riverine DOM supply to the Baltic Sea will be efficiently mineralized by microbes. This will have consequences for bacterioplankton and phytoplankton community composition and function, and significantly affect nutrient biogeochemistry.

Using fluorescent dissolved organic matter to trace and distinguish the origin of Arctic surface waters.

Climate change affects the Arctic with regards to permafrost thaw, sea-ice melt, alterations to the freshwater budget and increased export of terrestrial material to the Arctic Ocean. The Fram and Davis Straits represent the major gateways connecting the Arctic and Atlantic. Oceanographic surveys were performed in the Fram and Davis Straits, and on the east Greenland Shelf (EGS), in late summer 2012/2013. Meteoric (fmw), sea-ice melt, Atlantic and Pacific water fractions were determined and the fluorescence properties of dissolved organic matter (FDOM) were characterized. In Fram Strait and EGS, a robust correlation between visible wavelength fluorescence and fmw was apparent, suggesting it as a reliable tracer of polar waters. However, a pattern was observed which linked the organic matter characteristics to the origin of polar waters. At depth in Davis Strait, visible wavelength FDOM was correlated to apparent oxygen utilization (AOU) and traced deep-water DOM turnover. In surface waters FDOM characteristics could distinguish between surface waters from eastern (Atlantic + modified polar waters) and western (Canada-basin polar waters) Arctic sectors. The findings highlight the potential of designing in situ multi-channel DOM fluorometers to trace the freshwater origins and decipher water mass mixing dynamics in the region without laborious samples analyses.

Experimental insights into the importance of aquatic bacterial community composition to the degradation of dissolved organic matter.

Bacteria play a central role in the cycling of carbon, yet our understanding of the relationship between the taxonomic composition and the degradation of dissolved organic matter (DOM) is still poor. In this experimental study, we were able to demonstrate a direct link between community composition and ecosystem functioning in that differently structured aquatic bacterial communities differed in their degradation of terrestrially derived DOM. Although the same amount of carbon was processed, both the temporal pattern of degradation and the compounds degraded differed among communities. We, moreover, uncovered that low-molecular-weight carbon was available to all communities for utilisation, whereas the ability to degrade carbon of greater molecular weight was a trait less widely distributed. Finally, whereas the degradation of either low- or high-molecular-weight carbon was not restricted to a single phylogenetic clade, our results illustrate that bacterial taxa of similar phylogenetic classification differed substantially in their association with the degradation of DOM compounds. Applying techniques that capture the diversity and complexity of both bacterial communities and DOM, our study provides new insight into how the structure of bacterial communities may affect processes of biogeochemical significance.