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Dissolved organic matter (DOM) in groundwater is fundamentally important with respect to biogeochemical reactions, global carbon cycling, heavy metal transport, water treatability and potability. One source of DOM to groundwater is from the transport of organic matter from the vadose zone by rainfall recharge. Changes in precipitation patterns associated with natural climate variability and climate change are expected to alter the load and character of organic matter released from these areas, which ultimately impacts on groundwater quality and DOM treatability. In order to investigate potential changes in groundwater DOM character after rainfall recharge, we sampled shallow groundwater from a coastal peat-rich sand aquifer in New South Wales, Australia, during an extended period of low precipitation (average daily precipitation rate < 1.6 mm day -1 over the 8 months prior to sampling), and after two heavy precipitation events (84 mm day -1 and 98 mm day -1 respectively). We assess changes in DOM composition after correcting for dilution by a novel combination of two advanced analytical techniques: liquid chromatography organic carbon detection (LC-OCD) and negative-ion electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). We also assess changes in water chemistry pre- and post-rainfall. Post-rainfall, we show that the dilution-corrected amount of highly aromatic DOM molecular formulae (i.e. those categorised into the groups polyphenolics and condensed aromatics) were 1.7 and 2.0 times higher respectively than in pre-rainfall samples. We attribute this to the flushing of peat-derived DOM from buried organic material into the groundwater. We also identify that periods of low precipitation can lead to low hydrophilic/HOC ratios in groundwater (median = 4.9, n = 14). Redundancy analysis (RDA) was used to compare the HOC fraction with FT-ICR MS compound groups. We show that HOC has a more aromatic character in pre-rainfall samples, and is less similar to the aromatic groups in post-rainfall samples. This suggests that the decline in water-borne hydrophobics observed post-rainfall could be associated with preferential adsorption of the hydrophobic aromatic DOM, making post-rainfall samples less treatable for potable water supply. Post-rainfall we also observe significant increases in arsenic (leading to concentrations greater than 3 times the World Health Organisation drinking water limit of 10 μg / L). Increases in coastal rainfall due to climate change may therefore alter the composition of groundwater DOM in coastal peatland areas in ways that may impact DOM bioavailability, and increase arsenic concentrations, reducing the ease of water treatment for human consumption. To the best of our knowledge, this is the first study to identify the chemical and molecular changes of shallow groundwater DOM pre-rainfall and post-rainfall in a sedimentary organic carbon rich environment through multiple analytical techniques., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Shallow lakes are hotspots for carbon processing and important natural sources of methane (CH 4 ) emission. Ebullitive CH 4 flux may constitute the overwhelming majority of total CH 4 flux, but the episodic nature of ebullition events makes determining both quantity and the controlling factors challenging. Here we used the world's longest running shallow-lake mesocosm facility, where the experimental treatments are low and high nutrients crossed with three temperatures, to investigate the quantity and drivers of CH 4 ebullition. The mean CH 4 ebullition flux in the high nutrient treatment (41.5 ± 52.3 mg CH 4 -C m -2 d -1 ) mesocosms was significantly larger than in the low nutrient treatment (3.6 ± 5.4 mg CH 4 -C m -2 d -1 ) mesocosms, varying with temperature scenarios. Over eight weeks from June to August covered here warming resulted in a weak, but insignificant enhancement of CH 4 ebullition. We found significant positive relationships between ebullition and chlorophyll-a, dissolved organic carbon (DOC), biodegradable DOC, δ 2 H, δ 18 O and δ 13 C-DOC, autochthonous dissolved organic matter (DOM) fluorescent components, and a fraction of lipids, proteins, and lignins revealed using ultrahigh-resolution mass spectrometry, and a negative relationship between ebullitive CH 4 flux and the percentage volume inhabited of macrophytes. A 24 h laboratory bio-incubation experiment performed at room temperature (20 ± 2 °C) in the dark further revealed a rapid depletion of algal-DOM concurrent with a massive increased CH 4 production, whereas soil-derived DOM had a limited effect on CH 4 production. We conclude that eutrophication likely induced the loss of macrophytes and increase in algal biomass, and the resultant accumulation algal derived bio-labile DOM potentially drives enhanced outgassing of ebullitive CH 4 from the shallow-lake mesocosms., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

Dissolved organic matter (DOM) from alpine glaciers is highly biolabile and plays a vital role in the biogeochemical cycle of meltwater-impacted environments. To unravel the composition and interactions of DOM with the bacterial community in glacier and glacier meltwater, we conducted sampling of two different Tibetan Plateau glaciers and carried out laboratory bio-incubation experiments. The field data revealed that four protein-like components accounted for 86.0 ± 11.9% of the total variability of all six fluorescence components, which suggests a predominantly microbial source of glacial chromophoric DOM (CDOM). The ice and meltwater samples displayed major contributions of molecular formulae associated with lipids and proteins (i.e. high H/C) as revealed by ultrahigh resolution mass spectrometry. Multiple linear regression models revealed that the abundant phyla explain 64.2%, 61.3%, and 65.0% of the variability of microbial and terrestrial humic-like, and protein-like components, respectively. Correlation-based network analysis determined the metabolic niches of the bacterial community members associated with different fluorescence types in biogeochemical processes. Furthermore, laboratory DOM bio-incubation experiments confirmed that sub-components of the CDOM pool differentially participate in bacterial metabolism. We therefore conclude that the bacterial community interacted closely with the compositional variability of DOM in the investigated alpine glacial environments by both producing and consuming of DOM., (Copyright © 2019 Elsevier Ltd. All rights reserved.)

In the mostly pristine Congo Basin, agricultural land-use change has intensified in recent years. One potential and understudied consequence of this deforestation and conversion to agriculture is the mobilization and loss of organic matter from soils to rivers as dissolved organic matter. Here, we quantify and characterize dissolved organic matter sampled from 19 catchments of varying deforestation extent near Lake Kivu over a two-week period during the wet season. Dissolved organic carbon from deforested, agriculturally-dominated catchments was older ( 14 C age: ~1.5kyr) and more biolabile than from pristine forest catchments. Ultrahigh-resolution mass spectrometry revealed that this aged organic matter from deforested catchments was energy-rich and chemodiverse, with higher proportions of nitrogen- and sulfur-containing formulae. Given the molecular composition and biolability, we suggest that organic matter from deforested landscapes is preferentially respired upon disturbance, resulting in elevated in-stream concentrations of carbon dioxide. We estimate that while deforestation reduces the overall flux of dissolved organic carbon by ~56%, it does not significantly change the yield of biolabile dissolved organic carbon. Ultimately, the exposure of deeper soil horizons through deforestation and agricultural expansion releases old, previously stable, and biolabile soil organic carbon into the modern carbon cycle via the aquatic pathway.

Estuaries support the livelihood of ~75% of the world's population and maintain high primary production in coastal waters, which are often subjected to strong tides and anthropogenic disturbances. There is a paucity of information on how the optical composition and bioavailability of chromophoric dissolved organic matter (CDOM) are influenced by tidal oscillations in estuaries with highly urbanized surrounding areas. We examined the semi-diurnal Qiantang Bore, one of the Earth's three most predominant tide bores, and found that dissolved organic carbon (DOC), CDOM absorption a(254) and terrestrial humic-like C1, tryptophan-like C2 and C5, fulvic-like C3, and microbial humic-like C4 decreased markedly with increasing salinity. This suggests that physical mixing of riverine freshwater and saltwater can shape the optical composition of CDOM in the estuary. This was supported by the semi-diurnally and hourly observations at Zhijiang (salinity ~0.1‰, upstream of the estuary) that DOC, bioavailable DOC (BDOC), C1-C2, and C4-C5 increased markedly with decreasing tidal level, while DOC and C1-C5 increased notably with increasing salinity. We further found δ 18 O was enriched with increasing tidal level, while tryptophan-like C2 and C5, and fulvic-like C3 decreased significantly with increasing tidal level at Zhapu (salinity ~7‰, downstream of the estuary). Furthermore, DOC, BDOC, C1, and C4 decreased, while δ 18 O and C3 increased markedly with increasing salinity. Further evidences come from the notably lower mean first principal component (PC1) scores at Zhijiang and Zhapu, both positively associated with anthropogenic tryptophan-like inputs, were observed during ebb than during flood tides, and PC1 at Zhijiang increased notably with increasing salinity. We conclude that anthropogenic inputs contributed primarily to the CDOM pool in the estuary and are mediated by the physical mixing of riverine freshwater and seawater, and ebb tides are often associated with enhanced anthropogenic CDOM with relatively high bioavailability., (Copyright © 2019 Elsevier B.V. All rights reserved.)

Sewage treatment systems are a common feature across the landscape of the United Kingdom, serving an estimated 96% of the population and discharging approximately eleven billion litres of treated wastewater daily. While large treatment facilities are ubiquitous across the landscape, they are not the only method employed in domestic wastewater treatment. This study investigates whether differences in nutrient export (carbon, nitrogen and phosphorus) and organic matter composition (determined by optical indices, SUVA 254 , S 350-400 and E 2 :E 3 ) from treated effluent could be detected between four of the most common facilities employed in the treatment of wastewater across the UK. Set in the context of the River Wylye, a small headwater catchment, treatment facilities studied included; a septic tank system, small packet treatment works, and two large sewage treatment works, one of which employed phosphorus stripping for phosphorus removal. Inorganic N and P concentrations ranged between 7.51 and 42.4 mg N l -1 and 0.22 and 8.9 mg P l -1 respectively, with DOC concentrations ranging between 1.63 and 11.8 mg C l -1 . Optical indices were comparable to those observed in catchments where organic matter is dominated by autochthonous production, suggesting the dominance of low molecular weight material when compared to values observed across temperate aquatic systems. Combining data from both the Environment Agency and Ordinance Survey we estimate that only 15% of domestic properties not connected to mains sewerage in the study catchment have an Environment Agency consent/exemption permit. This calculation suggests that the quantity of small point sources are significantly underestimated, undermining efforts under current legislation to improve stream ecosystem health., (Copyright © 2018 Elsevier Ltd. All rights reserved.)

Atmospheric nitrogen (N) deposition is an important determinant of N availability for natural ecosystems worldwide. Increased anthropogenic N deposition shifts the stoichiometric equilibrium of ecosystems, with direct and indirect impacts on ecosystem functioning and biogeochemical cycles. Current simulation data suggest that remote tropical forests still receive low atmospheric N deposition due to a lack of proximate industry, low rates of fossil fuel combustion, and absence of intensive agriculture. We present field-based N deposition data for forests of the central Congo Basin, and use ultrahigh-resolution mass spectrometry to characterize the organic N fraction. Additionally, we use satellite data and modeling for atmospheric N source apportionment. Our results indicate that these forests receive 18.2 kg N hectare -1 years -1 as wet deposition, with dry deposition via canopy interception adding considerably to this flux. We also show that roughly half of the N deposition is organic, which is often ignored in N deposition measurements and simulations. The source of atmospheric N is predominantly derived from intensive seasonal burning of biomass on the continent. This high N deposition has important implications for the ecology of the Congo Basin and for global biogeochemical cycles more broadly., Competing Interests: The authors declare no conflict of interest.

This study highlights how Chinese economic development detrimentally impacted water quality in recent decades and how this has been improved by enormous investment in environmental remediation funded by the Chinese government. To our knowledge, this study is the first to describe the variability of surface water quality in inland waters in China, the affecting drivers behind the changes, and how the government-financed conservation actions have impacted water quality. Water quality was found to be poorest in the North and the Northeast China Plain where there is greater coverage of developed land (cities + cropland), a higher gross domestic product (GDP), and higher population density. There are significant positive relationships between the concentration of the annual mean chemical oxygen demand (COD) and the percentage of developed land use (cities + cropland), GDP, and population density in the individual watersheds (p < 0.001). During the past decade, following Chinese government-financed investments in environmental restoration and reforestation, the water quality of Chinese inland waters has improved markedly, which is particularly evident from the significant and exponentially decreasing GDP-normalized COD and ammonium (NH 4 + -N) concentrations. It is evident that the increasing GDP in China over the past decade did not occur at the continued expense of its inland water ecosystems. This offers hope for the future, also for other industrializing countries, that with appropriate environmental investments a high GDP can be reached and maintained, while simultaneously preserving inland aquatic ecosystems, particularly through management of sewage discharge., (Copyright © 2017 Elsevier Ltd. All rights reserved.)

Two lowland catchments in the U.K. were sampled throughout 2010-11 to investigate the dominant controls on dissolved organic matter quantity and composition. The catchments had marked differences in terms of nutrient status, land cover and contrasting lithologies resulting in differences in the dominant flow pathways (groundwater vs. surface water dominated). The Upper Wylye is a chalk stream with a baseflow index of 0.98, draining a catchment dominated by intensive agricultural production. Millersford Brook is a lowland peat catchment with a baseflow index of 0.43, draining a semi-natural catchment with heather moorland and coniferous forest. Samples were collected weekly between October 2010 and September 2011 from eleven sampling locations. Samples were analysed to determine dissolved organic carbon, nitrogen and phosphorus fractions with DOM composition evaluated via the DOC:DON ratio, DOC:DOP ratio, specific UV absorption at 254nm, absorbance ratio (a250:a365) and the spectral slope parameter between 350 and 400nm (S350-400). Significant differences were observed in all determinands between the catchments, over time, and spatially along nutrient enrichment and geoclimatic gradients. Seasonal variation in preferential flow pathways mobilising groundwater-derived DOM were identified as likely controls on the delivery of DOM in the permeable chalk dominated catchment. Steeper S350-400 values and elevated a250:a365 ratios in this catchment suggest material of a lower bulk aromatic C content and molecular weight delivered during the winter months when compared to the summer. DOC:DON ratios were markedly lower in the chalk catchment than the peatland catchment, reflecting the paucity of organic matter within the mineral soils of the chalk landscape, and higher fertiliser application rates. This manuscript highlights that DOM composition varies according to catchment landscape character and hydrological function., (Copyright © 2016 The Authors. Published by Elsevier B.V. All rights reserved.)

Mercury (Hg) dynamics in the Arctic is receiving increasing attention, but further understanding is limited by a lack of studies in Russia, which encompasses the majority of the pan-Arctic watershed. This study reports Hg concentrations and trends in burbot (Lota lota) from the Lena and Mezen Rivers in the Russian Arctic, and assesses the extent to which they differ from those found in burbot in arctic rivers elsewhere. Mercury concentrations in burbot in the Lena and Mezen Rivers were found to be generally lower than in 23 other locations, most of which are in the Mackenzie River Basin (Canada). Mercury concentrations in burbot in the Lena and Mezen Rivers also were found to have been declining at an annual rate of 2.3% while they have been increasing in the Mackenzie River Basin at annual rates between 2.2 and 5.1% during roughly the same time period. These contrasting patterns in Hg in burbot across the pan-Arctic may be explained by geographic heterogeneity in controlling processes, including riverine particulate material loads, historically changing atmospheric inputs, postdepositional processes, and climate change impacts.

Organic matter removal by cultured Sesuvium portulacastrum in constructed floating beds was studied during a 20 day greenhouse experiment and an 8 month field campaign in the polluted Yundang Lagoon (southeastern China). Experiments were traced via dissolved organic carbon (DOC) concentration, fluorescence excitation-emission matrix and absorption spectroscopy. Two 'terrestrial' humic-like, one 'marine' humic-like and one protein-like components were identified by parallel factor analysis. The 'terrestrial' humic-like and protein-like components, DOC and absorption coefficient (a280) decreased during the greenhouse experiment. The intensities of four fluorescence components were all reduced during the field experiment. These results demonstrate the clear potential of floating bed phytoremediation techniques for reducing organic pollution degree in brackish environments. The rhizosphere may play an important role during phytoremediation. Our results show that spectrophotometric measurements such as fluorescence provide a useful tool for examining the removal of different organic moieties during various bioremediation processes., (Copyright © 2013 Elsevier Ltd. All rights reserved.)

The UV-visible and fluorescence excitation-emission matrix spectrophotometric properties of dissolved organic matter (DOM) were compared for the effects of both pH and freeze/thaw on a wide range of freshwater DOM samples from the United Kingdom. It was observed that the spectrophotometric properties of our freshwater samples were sensitive to pH and that the recorded change varies with fluorescence and absorbance intensity, DOC concentration and the wavelength observed. Large and variable responses to pH were particularly severe at extremes of pH, but within the natural levels typically observed in freshwaters the response to pH was limited. For the same sample set large and variable responses were observed when subjected to freeze/thaw. From our data, knowledge of the original properties cannot be used to determine the amount of change that will occur with freezing and subsequent thawing. It is therefore recommended that in future research, to maintain the natural signal of the DOM, analysis is conducted at natural pH and without freezing to facilitate ease of comparison between studies. Our results also have implications for studies that utilise spectrophotometric techniques to investigate long-term trends in dissolved organic carbon in rivers. Spectrophotometric parameters from upland derived samples show varied responses of samples to pH and there is clear potential to complicate trends in the interpretation of long-term water colour data if pH is changing over time in a system or if samples are treated with different storage protocols with respect to acidification and freezing.

The ability to distinguish water inputs from both natural and anthropogenic sources was investigated in the complex environment of an urban estuary (Tyne) and a relatively pristine estuary (Tweed). We used a data set from a total of 11 estuarine transects, comprising measurements of bulk dissolved organic matter (dissolved organic carbon and nitrogen), dissolved nitrogen (total dissolved nitrogen, ammonium, nitrate+nitrite and dissolved organic nitrogen), optical absorbance measurements (a350, S290-350) and fluorescence excitation emission matrix measurements (fluorophores A, H, B and T intensity and A and H emission wavelength maxima). In order to investigate trends within the numerous parameters measured, multivariate statistics were employed. Principal components analyses showed 63.4% of the variability in the total data set can be explained by two sets of components and 74.9% of the variability by the spectrophotometric measurements alone. In both analyses the first component correlated to the mixing of terrestrial and marine waters and the second component was correlated to sources of pollution such as domestic sewage. Within the data set, river flow and terrestrially derived DOM were significantly correlated, and situations with high river input showed an increase in terrestrial signature in the estuary. Discriminant analyses were also carried out and indicated that 59.8% (total data set) and 53.3% (solely spectrophotometric data) of the samples can be correctly classified into their respective groups (water categories) assigned on the basis of salinity and sampling location. Overall the results clearly show the potential of spectrophotometric techniques to discriminate distinct water categories with different DOM characteristics. In particular, measurement of the fluorophore H emission maxima, the spectral slope parameter, S290-350, and fluorophores T and B intensity enabled discrimination of DOM from riverine, estuarine, marine, and sewage affected water categories. The results presented here indicate the ability of spectrophotometric data alone to distinguish between marine, anthropogenic and terrestrial DOM and distinguish terrestrial DOM from different catchments (Tyne vs. Tweed). With current advances in the in-situ deployment of absorbance and fluorescence spectroscopy it is anticipated that multivariate statistics will gain importance as a cost effective, powerful and diagnostic approach to assessing the distributions of water types and their associated DOM characteristics and fluxes at the land-ocean interface.

Nitrogen isotope ratios (delta(15)N) were used to help elucidate the sources and fate of ammonium (NH(4)(+)) and nitrate (NO(3)(-)) in two northeastern English estuaries. The dominant feature of NH(4)(+) in the heavily urbanised Tyne estuary was a plume arising from a single point source; a large sewage works. Although NH(4)(+) concentrations (ranging from 30-150 microM) near the sewage outfall varied considerably between surveys, the sewage-derived delta(15)N-NH(4)(+) signature was remarkably constant (+10.6+/-0.5 per thousand) and could be tracked across the estuary. As indirectly supported by (15)N-depleted delta(15)N-NO(3)(-) values observed close to the mouth of the Tyne, this sewage-derived NH(4)(+) was thought to initiate lower estuarine and coastal zone nitrification. In the more rural Tweed, NH(4)(+) concentrations were low (<7 microM) compared to those in the Tyne and delta(15)N-NH(4)(+) values were consistent with mixing between riverine and marine sources. The dominant form of dissolved inorganic nitrogen (DIN) in the Tweed was agricultural soil-derived NO(3)(-). A decrease in riverine NO(3)(-) flux during the summer coinciding with an increase in delta(15)N-NO(3)(-) values was mainly attributed to enhanced watershed nutrient processing. In the Tyne, where agricultural inputs are less important compared to the Tweed, light delta(15)N-NO(3)(-) (ca. 0 per thousand) detected in the estuary during one winter survey pointed to a larger contribution from precipitation-derived NO(3)(-) during high river discharge. Regardless of the dominant sources, in both estuaries most of the variability in DIN concentrations and delta(15)N values was explained by simple end-member mixing models, implying very little estuarine processing.

Dissolved organic matter fluorescence, absorbance and dissolved organic carbon were measured from source to sea in the River Tyne catchment, of approximately 2935 km(2) and encompassing areas of contrasting land use. The catchment has three major tributaries: the North Tyne which has good water quality, high dissolved organic carbon concentrations and visible water colour from the high proportion of peat in its upper catchment; the South Tyne which has good water quality with typical riverine dissolved organic carbon concentrations and drains from limestone uplands; and the Derwent, a more urbanized catchment which is increasingly impacted by treated sewage effluent discharges towards its mouth. Thirty sample sites, 23 along the three main tributaries and seven within the estuary, were sampled on six occasions over the period 2002-2003. High absorbance at 340 nm and dissolved organic carbon concentration identify N Tyne waters due to the peaty headwaters, but no downstream trends in these parameters are observed in any of the tributaries, in contrast to the estuary where a rapid decrease is observed in both. Fluorescence in contrast demonstrated downstream trends in both intensity and wavelength, especially in the Derwent as it is increasingly impacted by anthropogenic dissolved organic matter. Elevated protein-like fluorescence intensity also fingerprints sewage effluent within the estuary. The absorbance coefficient at 340 nm was found to have the strongest correlation to dissolved organic carbon concentration, greater than all fluorescence intensity parameters measured. However, fluorescence analysis permits the source of the dissolved organic matter to be determined, and therefore has implications for understanding its fate in estuaries and the ocean.