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Dissolved organic matter in freshwaters: nature, origins and ecological significance

Scientific meeting








Kavli Royal Society Centre, Chicheley Hall, Newport Pagnell, Buckinghamshire, MK16 9JJ


Theo Murphy international scientific meeting organised by Professor Penny Johnes, Professor Davey Jones, Professor Stephen Maberly and Professor Richard Evershed FRS.

DOM in freshwater

Dissolved organic matter (DOM) plays an important role in ecosystem function. Its flux to waters is rising globally, driven by population growth, agricultural intensification, land use and climate change. This meeting brought together a cross-disciplinary community to debate and discuss current understanding of the nature, origins and ecological significance of DOM in freshwaters, and identify exciting opportunities for future research.

Speaker biographies and abstracts are available below. Recorded audio of the presentations is also available below.

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Schedule of talks

20 January


Session 1

5 talks Show detail Hide detail


Professor Penny Johnes, University of Bristol, UK

09:00-09:15 Welcome by the Royal Society and lead organiser

09:15-10:00 Keynote: DOM in the long arc of environmental science: looking back and thinking ahead

Professor William H McDowell, University of New Hampshire, USA


Dissolved organic matter (DOM) is ubiquitous, enigmatic and influences physical, chemical, and biotic processes throughout the Earth system. A unified model for DOC production and transport has emerged over the last 40 years that links terrestrial and aquatic ecosystems. High DOM concentrations in streams and rivers can be found in regions with large net production of terrestrial DOM (roughly the balance between net primary production and organic matter mineralization), and low retention in sesquioxide- or clay-rich mineral soils. Although high concentrations can be found in surface waters of any biome, the overall levels of DOM vary predictably across biomes with soil C and N dynamics. Because of analytical limitations and the importance of DOM in a wide range of processes, interest in DOM over the past 100 years has been driven by a variety of research questions. From the 1920s until the 1970s, research on DOM was focused on the yellow colour “gelbstoff era” and largely on the effects of this colour on primary productivity in “dystrophic” lakes. This era of DOM work ended with a flurry of papers on stream organic matter budgets and experimental studies of organic matter uptake in the 1970s. From the 1970s until the early 2000s, the “elemental era”, new analytical techniques pioneered by Menzel (DOC) and Solorzano (DON, DOP) fostered interest in measuring the individual chemical elements that make up DOM. This era was marked by widespread development of organic carbon budgets for river reaches and whole watersheds. The current era, 2000 – present, can be thought of as the time when a “unified theory of DOM” was developed, which includes topics such as trophic transfers in aquatic foodwebs, production and evasion of greenhouse gases in aquatic systems, photodegradation of DOM, influence of DOC on N dynamics, and the role of DOM in the production of disinfection byproducts and acid-base reactions. From decade to decade, the dominant research questions have varied, but they are unified by a single overarching thread: a focus on understanding the functional consequences of DOM in aquatic ecosystems. Professor McDowell proposes that it is time to develop a new era focusing on understanding the ecological and evolutionary significance of DOM. This era will be spurred by much better understanding of the temporal dynamics of DOM (both long-term trends and sensor-enabled short-term dynamics), multiple analytical approaches to quantifying the organic chemistry of DOM, and a re-assessment of the origins of DOM. Fundamental questions about the origins of DOM abound. In a world that is constrained by energy and nutrient availability, why do some tree species leach more DOC from their leaves or needles than others? Are increases in stream DOC a return to background conditions, or a temporary “flushing” of DOC accumulated during periods of high atmospheric deposition? Why do phytoplankton, Sargassum, and mangroves each “leak” DOC into water? Are there selective pressures that result in these persistent losses of DOC in terrestrial and aquatic ecosystems? Finally, the fundamental conundrum underlying the study of DOC dynamics in fresh waters must be addressed. Our community largely focuses on detailed study of ambient DOC pools, but arguably those are just the leftovers. What was on the original menu?

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10:00-10:15 Discussion

10:15-10:45 Sources, processing and fate of aquatic carbon from headwaters to the sea

Professor Chris Evans, Centre for Ecology and Hydrology, UK


Dissolved organic matter (DOM) concentrations have been rising in many European and North American headwaters for over 30 years, with implications for aquatic ecosystem function, drinking water supplies and the flux of carbon from land to ocean. This talk will consider the key drivers of these increases, their spatial extent, and their likely future trajectory. It will also consider the fate and impact of terrestrially derived organic carbon fluxes along the aquatic continuum from headwaters to ocean; the major controls on aquatic DOM processing; and the overall significance of land-water carbon fluxes in the global carbon budget.

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10:45-11:00 Discussion

11:00-11:30 Coffee break

11:30-12:00 The “new normal” of catastrophic tropical cyclone flooding in North Carolina (USA) coastal watersheds: Implications for organic matter and nutrient cycling 

Professor Hans Paerl, University of North Carolina at Chapel Hill, Institute of Marine Sciences, USA


Coastal North Carolina has experienced 35 tropical cyclones and 3 record cyclone-driven flood events in the past two decades (Hurricanes Floyd–1999, Matthew–2016 and Florence–2018), causing catastrophic human impacts from flooding and leading to major alterations of water quality, fisheries habitat and overall ecological conditions of the USA’s second largest estuarine complex, the Albemarle-Pamlico Sound. The increased frequency and magnitudes of these events suggests that we have entered a “new normal” in rainfall associated with these storms. Analysis of continuous cyclone-related rainfall records for coastal NC since 1898 reveals a period of unprecedentedly high precipitation since the late 1990’s. Indeed, six out of seven of the “wettest” storm events over this >120 year record have occurred during the past two decades. The team examined freshwater discharge, nutrient (nitrogen and phosphorus) and carbon inputs, hypoxic potentials and phytoplankton community responses to these episodic events and compared them to seasonal and interannual patterns in the Neuse River Estuary, a major estuarine tributary of the Albemarle-Pamlico Sound system. Results indicate that these events lead to large inputs of organic matter and nutrients, which constitute a significant percentage of annual loadings, the biogeochemical impacts of which are discussed. We have entered a new climatic regime characterised by more frequent and extreme precipitation events, with major ramifications for hydrology, carbon and nutrient cycling, water quality and habitat conditions in this and possibly other coastal regions.

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12:00-12:15 Discussion

12:15-12:45 Salt, Acid, Waste: human impacts on organic matter quantity and quality in river networks

Professor Emily Bernhardt, Duke University, USA

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12:45-13:00 Discussion

13:00-14:00 Lunch


Session 2

4 talks Show detail Hide detail


Professor Richard Evershed FRS, University of Bristol, UK

14:00-14:45 Keynote: frontiers and challenges in isotopic characterisation of DOM

Professor Tim Eglinton FRS, ETH Zurich, Switzerland


Dissolved organic matter (DOM) in natural waters is comprised of highly complex mixtures of components that reflect myriad biotic and abiotic production, modification, translocation and decomposition processes. This complexity presents formidable challenges in our ability to characterise DOM, understand its interactions with other carbon pools, and predict its response to natural and anthropogenic forcing. We currently lack a comprehensive understanding of the dynamics and pathways of DOM production, transformation and transport, as well as links between dissolved and particulate carbon phases. Recent analytical and instrumental advances are yielding detailed information on the chemical composition of DOM, however bridging information gleaned at the molecular level with dynamics observed in bulk DOM remains an elusive challenge. In this context, determination of isotopic characteristics of specific DOM constituents holds potential to provide key information on the sources and cycling of DOM. While diverse source-specific “biomarker” compounds are routinely targeted for stable isotopic and radiocarbon characterisation of particulate organic matter (POM), such approaches are much less well developed for DOM. This presentation will provide examples of insights derived from current molecular 14C investigations of POM in aquatic systems and how they may inform DOM isotopic studies. Some glimpses into the isotopic variability within DOM in a range of environments spanning soils to fluvial systems will be provided. Finally, examples that show how isotopic (14C) gradients in space and time may be exploited to derive novel information on DOM sources and cycling will be presented, with the goal of motivating further research in this area.

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14:45-15:00 Discussion

15:00-15:30 Exploring DOM in large lakes: Successes and challenges

Professor Elizabeth Minor, University of Minnesota Duluth, USA


The world’s largest five freshwater lakes include both tropical meromictic and temperate holomictic lakes ranging in age from twenty-five million to ~10,000 years old. They contain >50% of earth’s surface liquid freshwater. Three of the lakes support extensive human populations in their watersheds. Despite their demonstrated importance in economic, cultural, and environmental milieus, large lakes are understudied in terms of carbon cycling, including the roles of dissolved organic matter (DOM)--as dissolved organic carbon (DOC); as chromophoric dissolved organic matter (CDOM); and at the compound class or molecular level. Existing data shows that large lakes have lower DOC and CDOM concentrations and clearer water than the global lake median. However, available CDOM/chlorophyll data (for Lake Superior) shows a ratio higher than mean ocean surface water and much higher than would be predicted by a temperate-lake-based relationship between lake area and CDOM/chlorophyll. Qualitative characterisation indicates that DOM in Lakes Baikal, Superior, and Michigan contains a large proportion of terrestrially-derived but reworked organic matter. No such data exists for Lakes Malawi and Tanganyika. Large lakes present unique challenges for DOM studies as they have differing inorganic matrices (in terms of major ions & oxidation levels), have variable levels of non-chromophoric DOM, and require oceanographic-style efforts to sample at sufficient temporal and spatial resolution. As these lakes are critical resources for humans and key environmental systems on a global scale, and as they are subject to climate change, land-use and lake-use pressures, such efforts should be an upcoming focus for aquatic scientists.

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15:30-15:45 Discussion

15:45-16:00 Tea break

16:00-16:30 Novel methods for compound specific determination of DOM composition and their relation to landscape character using examples from the NERC DOMAINE platform

Dr Charlotte Lloyd, University of Bristol, UK


The flux of dissolved organic matter (DOM) into rivers is rising due to a range of factors, including inputs of organic wastes from livestock production, discharge of sewage effluent, as well as the mobilisation of soil organic matter stores. There is a growing body of research showing that many DOM compounds are bioavailable and can be rapidly assimilated by stream biota, which may have important implications for nutrient cycling and riverine health. With this in mind, it is vital to gain a more comprehensive understanding of the composition of riverine DOM at a molecular level. DOM is an extremely complex mixture of individual compounds and therefore poses some analytical challenges. Most often, DOM is quantified as a bulk nutrient fraction then characterised by parameters such as hydrophobicity, molecular weight, aromaticity and functional group. However, recent advancements in analytical approaches can help allowing more detailed characterisation to compound level. Typically, molecular-scale analysis of DOM has been carried out in a targeted way, where compounds of interest are analysed. However, here, we propose a hierarchical approach to DOM characterisation using a suite of cutting-edge analytical techniques in order to obtain a wide analytical window spanning different size fractions and polarities. A number of UK-based case studies are used to illustrate the power of using this combination of analytical techniques to provide a truly untargeted approach to DOM characterisation.

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16:30-16:45 Discussion

16:45-17:15 Molecular diversity of dissolved organic matter in freshwater and marine systems

Professor Thorsten Dittmar, University of Oldenburg, Germany


Organic remnants of aquatic organisms have accumulated over thousands of years in dissolved form to one of the largest organic carbon pools on our planet’s surface. Dissolved organic matter (DOM) contains more carbon than the entire vegetation on Earth combined. The reasons behind its persistence and its potential for carbon storage in the future are unknown. During growth and upon death, cells release a myriad of organic compounds on which microorganisms grow. A minor fraction of this organic matter decomposes so slowly that it has persisted in the global ocean for millennia. The resulting mixture of largely unknown compounds has reached an extraordinarily high level of molecular diversity. The current paradigm is that microbes cannot decompose this mixture because suitable metabolic pathways have not evolved. Here, Professor Dittmar presents an emerging, alternative concept that assumes the existence of enzymatic machineries that continuously transform any form of DOM, but due to extreme dilution encounters of cells and substrate units are rare events in the ocean. According to this concept, marine microbes appear most powerful in decomposing any form of organic matter, but rates slow down as molecular diversity increases.

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17:15-17:30 Discussion

17:30-18:30 Poster session

21 January


Session 3

4 talks Show detail Hide detail


Professor Susan Waldron, University of Glasgow, UK

09:00-09:45 Keynote: Low molecular weight DOM: Small concentrations but large fluxes

Professor Davey Jones, Bangor University, UK

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09:45-10:00 Discussion

10:00-10:30 Landscape controls on DOM delivery to rivers of contrasting environmental character

Dr Christopher Yates, University of Bristol, UK


Dissolved organic matter (DOM) plays an important role in freshwater biogeochemistry. The stoichiometry of organic matter has proved a useful tool in assessing compositional changes in the complex and dynamic pool of organic compounds that comprise DOM. What is not clear in the existing literature is whether DOM composition in streams can be reliably estimated from a knowledge of the landscape stores of DOM in soils and biota. To investigate the influence of catchment character on the quality and quantity of DOM in freshwaters, Dr Yates presents data from a national scale monitoring programme, collected as part of the NERC-DOMAINE Large Grant programme (NE/K010689/1). One hundred sites were sampled draining subcatchments of contrasting soil type, hydrology, and land cover between November 2015 and December 2017. The composition of the C, N, and P pool was determined as a function of the inorganic nutrient species (NO3-, NH4+, PO43-) and dissolved organic nutrient fraction (DOC, DON, and DOP) concentrations in each sample. DOM quality was assessed by molar DOC:DON and DOC:DOP ratios, and SUVA254. Catchment soil C:N ratio correlated significantly with DOC:DON, DOC:DOP, and SUVA254 with significant correlations observed between both the DOC:DON and DOC:DOP molar ratios and SUVA254. Dr Yates and his group infer from this that soil character, specifically the C:N ratio of the soil organic matter pool, has a significant influence on the composition of DOM in streams draining through these landscapes.

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10:30-10:45 Discussion

10:45-11:15 Coffee break

11:15-11:45 Environmental controls on DOM composition in lakes: the role of climate and hydrology

Dr Anne Kellerman, Florida State University, USA

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11:45-12:00 Discussion

12:00-12:30 Molecular signatures of the Anthropocene

Dr Robert Spencer, Florida State University, USA


In the Anthropocene, a major uncertainty in our ability to predict how climate change and land cover change will affect the global carbon cycle centres on how dissolved organic matter (DOM) moves across the terrestrial‐aquatic interface and how the quantity and quality of DOM mobilized from soils to aquatic environments will change under these impacts. Climate change induced permafrost thaw in the Arctic is mobilizing ancient dissolved organic carbon (DOC) into headwater streams; however, DOC exported from the mouth of major arctic rivers appears predominantly modern. Here the researchers highlight that ancient (>20,000 years BP) permafrost DOC is rapidly utilised by microbes and the permafrost DOM unique molecular signatures, including high levels of aliphatics were rapidly utilised by microbes. In the tropics agriculturally driven land cover change resulted in aged DOM exported from deforested catchments that was energy rich and enriched in nitrogen- and sulphur-containing formulae. Given the molecular composition and biolability, the researchers suggest that organic matter from deforested landscapes is preferentially respired upon disturbance, resulting in elevated in-stream concentrations of carbon dioxide. Thus, 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. Taken together the team highlights the unique molecular signatures of climate change in the Arctic, and land cover change in the tropics, and draws attention to headwater streams as sentinels of anthropogenic change.

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12:30-12:45 Discussion

12:45-13:45 Lunch


Session 4

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Professor Stephen Maberly, Centre for Ecology and Hydrology, UK

13:45-14:30 Keynote: the fate and role of terrestrial DOM in inland waters, from molecular to global scale

Professor Lars Tranvik, Uppsala University, Sweden


Dissolved organic matter (DOM) in inland waters plays a substantial role in the global carbon cycle, and thus potentially affects climate as well. This presentation is an overview of the dynamics and fluxes of carbon involving DOM, from micro-scale to global scale. DOM is a heterogeneous mixture of decomposition products, and the reactivity and controls of the molecular composition are a central topic in aquatic biogeochemistry. Furthermore, DOM contributes substantially to evasion of carbon dioxide and methane to the atmosphere, but is also an important precursor of carbon that is buried in sediments. The loss of DOM from the water column is mediated via microbial and photochemical mineralization, as well as sedimentation upon formation of particles by flocculation or by sorption to minerals. The factors that constrain and promote loss of DOM from the water column will be discussed, and compared across different habitats, from soil to sea.

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14:30-14:45 Discussion

14:45-14:55 Discussion

14:45-15:15 Can we predict dissolved organic nitrogen and phosphorus use by river phytoplankton?

Dr Eleanor Mackay, UK Centre for Ecology & Hydrology, UK


Dissolved organic matter loading to freshwaters is increasing. Consequently, increasing dissolved organic nutrient concentrations may be contributing to the enrichment of rivers and lakes leading to changes in algal productivity and community composition. However, while we have evidence to suggest that many different compounds are bioavailable to algae, we know much less about how this bioavailability varies with environmental conditions, such as the ambient nutrient concentration of a site or acclimation of the algal community. This study presents results from two different spatial scales of the DOMAINE project, assessing bioavailability of different dissolved organic nitrogen and phosphorus compounds to river phytoplankton. Firstly, a statistical model was developed to predict dissolved organic nutrient use based on nutrient concentration data from six sites along a nutrient gradient in two contrasting rivers. The relative bioavailability of different organic nitrogen and phosphorus compounds and the ambient river nutrient concentrations were determined over the course of one year at these sites. This model was then applied to a one off survey of 25 sites from differing geologies and land uses across Great Britain to assess whether our understanding of dissolved organic nutrient use can be readily up-scaled using a simple model. Results indicate that nutrient limitation, dissolved organic compound bioavailability and algal productivity differ among sites, but consistent patterns of bioavailability are closely linked to the level of nutrient enrichment.

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15:15-15:30 Discussion

15:30-15:45 Tea break

15:45-16:15 The challenging task of getting the carbon cycling and microbial ecology in high-alpine streams on the global map of carbon cycling and microbial ecology

Professor Tom J Battin, École Polytechnique Fédérale de Lausanne, Switzerland


Mountains cover one quarter of the land surface and contribute more than one third to the global runoff. Despite this, the biogeochemistry and microbial ecology of the streams that drain the roof of our planet are relatively poorly understood at present. A general lack of interest in these ecosystems combined with logistical difficulties linked to their study figure among the prime causes for them still being blank spots on the map of aquatic biogeochemistry. In this presentation, Professor Battin will unveil high-mountain streams as relevant for global carbon dioxide fluxes and propose scenarios for how export fluxes of dissolved organic matter (DOM) may change as mountain glaciers shrink. He will also show how the chemical composition of DOM may change across an alpine stream network, particularly as it crosses the treeline. In this context, he shall discuss the relevance of hydrological parameters for DOM biogeochemistry, an angle that appears particularly relevant given that hydrological regimes are prone to alterations because climate change. Finally, he will shed new light on the potential role of the microbial life for biogeochemical fluxes in glacier-fed streams. The aim of this presentation is thus to highlight the relevance of mountain streams for the large-scale biogeochemistry. This appears important to us because ecosystems interfacing with the cryosphere are predicted to be particularly vulnerable to global warming.

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16:15-16:30 Discussion

16:30-17:15 Panel discussion/Overview (future directions)

Dissolved organic matter in freshwaters: nature, origins and ecological significance

Theo Murphy international scientific meeting organised by Professor Penny Johnes, Professor Davey Jones, Professor Stephen Maberly and Professor Richard Evershed FRS.

Kavli Royal Society Centre, Chicheley Hall Newport Pagnell Buckinghamshire MK16 9JJ
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