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

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?

Professor William H McDowell, University of New Hampshire, USA

Professor William H McDowell, University of New Hampshire, USA

William H McDowell is Professor of Environmental Science in the Department of Natural Resources and the Environment at the University of New Hampshire in Durham, USA. He has a background in ecosystem ecology and has worked on land-water interactions for over 40 years. He published some of the earliest descriptions of the origin and controls on dissolved organic matter in streams and continues to study organic matter dynamics with a wide range of techniques including in situ optical sensors. Dr McDowell is former Chairperson of the Department of Natural Resources, serves as Director of the NH Water Resources Research Center, and currently holds a UNH Presidential Chair. He is co-chair of the US International LTER Committee. McDowell teaches courses on watershed management and ecosystem ecology. He is author of over 250 publications on biogeochemistry and has been cited in peer-reviewed articles over 18,000 times. He conducts long-term research in New Hampshire, Czech, and Puerto Rican watersheds, with a focus on understanding the interactions between nutrients and dissolved organic matter and linking critical zone structure to biotic processes. He has served as PI of the Luquillo Critical Zone Observatory since 2013.  

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.

Professor Chris Evans, Centre for Ecology and Hydrology, UK

Professor Chris Evans, Centre for Ecology and Hydrology, UK

Professor Chris Evans works on carbon cycling in aquatic and terrestrial ecosystems, with a particular focus on peatlands, greenhouse gases and the role of aquatic carbon fluxes in the global carbon cycle. His field-based research extends from the high latitudes to the tropics, and covers a range of temporal and spatial scales, ecosystems, scientific questions and policy issues. He is currently the CEH co-lead for the NERC LOCATE project on land-ocean carbon cycling, which aims to improve understanding of the processing and fate of terrestrially-derived organic matter along the freshwater-marine continuum. Chris was a Lead Author of the Flooded Lands chapter of the 2019 IPCC Agriculture, Forestry and Other Land-Use Guidelines refinement, and the 2013 IPCC Wetland Supplement, for which he helped to develop methods to incorporate CO₂ and CH₄ emissions from ditches, rivers, ponds and reservoirs in national greenhouse gas emissions inventories. He is a visiting professor at the Swedish University of Agricultural Sciences, where he previously held the position of King Carl XVI Gustaf’s 20th Visiting Professor in Environmental Science for his work on aquatic carbon cycling.

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.

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

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

Hans W Paerl is the Kenan Professor of Marine and Environmental Sciences at the University of North Carolina’s Institute of Marine Sciences. His research addresses microbially-mediated nutrient/carbon cycling and primary production dynamics, environmental controls and management of harmful algal blooms, and assessing effects of human and climatic alterations of biogeochemical cycling, water quality and sustainability of inland, estuarine and coastal marine waters. He has published over 300 peer reviewed articles and book chapters on these subjects. He received the 2003 G. Evelyn Hutchinson Award from the Association of the Sciences of Limnology and Oceanography, and the 2011 Odum Award from the Coastal and Estuarine Research Federation for addressing the causes, consequences and controls of eutrophication in aquatic ecosystems. In 2015, he was named a Fellow of the American Geophysical Union.

 

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.

Professor Timothy Eglinton FRS

Professor Timothy Eglinton FRS

Timothy Eglinton FRS is Professor of Biogeoscience within the Department of Earth Sciences at ETH Zürich. Eglinton obtained a BSc in Environmental Science at the Plymouth Polytechnic, and MSc and PhD in Organic Geochemistry at the University of Newcastle-upon-Tyne.  Following postdoctoral research positions at Delft Technical University (Netherlands) and Oslo University (Norway), he joined Woods Hole Oceanographic Institution in 1989. He was a member of the Scientific Staff there until 2010, when he joined ETH. Professor Eglinton’s research program is centered on tracing the origin, cycling and legacy of biospheric carbon, and to understand the role of organic matter production, remineralization, transport, and burial as a component of the global carbon cycle. He focusses on processes on the continents and in the oceans, as well as the transfer of carbon between these systems. He has pioneered radiocarbon measurements at the molecular level as a tool to examine carbon cycle processes.   More information is available at: http://www.biogeoscience.ethz.ch/.

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.

Professor Elizabeth Minor, University of Minnesota Duluth, USA

Professor Elizabeth Minor, University of Minnesota Duluth, USA

Liz Minor is a full professor in the Department of Chemistry and Biochemistry and at the Large Lakes Observatory at the University of Minnesota Duluth, USA. She received her BS in Chemistry from The College of William and Mary in Virginia (Williamsburg, VA, USA) and her PhD in Marine Chemistry and Geochemistry from the MIT/WHOI Joint Program in Oceanography and Ocean Engineering (Woods Hole, MA, USA). She did post-doctoral work in macromolecular mass spectrometry at FOM-AMOLF (Amsterdam, The Netherlands). Her research group studies carbon cycling in lake, river, and ocean water columns across seasonal to decadal scales. As part of this work, they apply spectroscopy and mass spectrometry approaches to characterize the molecular and isotopic composition of aquatic organic matter components. Minor is the author/co-author of 54 papers on carbon cycling in aquatic systems (with 21 of these focused upon studying large lakes systems). 

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.

Dr Charlotte Lloyd, University of Bristol, UK

Dr Charlotte Lloyd, University of Bristol, UK

Charlotte Lloyd obtained her PhD in Geography from the University of Bristol in 2012, researching the transport of livestock-derived organic matter through hillslopes, using a molecular approach. She then completed post-doctoral work at the University of Bristol as part of the Defra funded Hampshire Avon Demonstration Test Catchment project, where she developed numerical analytical methods for interrogating high temporal resolution water quality data. She moved to the School of Chemistry at the University of Bristol in 2014 in order to join the NERC funded DOMAINE programme where she has developed laboratory analytical methods for the molecular characterisation of dissolved organic matter in freshwater systems. Her work combines molecular chemistry with hydrology in order to enhance our understanding of the impact of organic compounds in the environment.

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.

Professor Thorsten Dittmar, University of Oldenburg, Germany

Professor Thorsten Dittmar, University of Oldenburg, Germany

Thorsten Dittmar is professor for Marine Geochemistry and director of the Institute for Chemistry and Biology of the Marine Environment (ICBM) at the University of Oldenburg. He studied geoecology and marine chemistry (Universities of Bayreuth and Bremen, Germany). Before he was appointed professor in Oldenburg, he was a group leader at the Max Planck Institute (MPI) for Marine Microbiology in Bremen and professor for Chemical Oceanography at Florida State University (USA). He also worked several years at universities in Belém (Brazil) and Seattle (USA). Dittmar’s research focus is the organic geochemistry of the oceanic water column. Most of the organic matter in the ocean resides there in dissolved form, and the oceans are among the largest organic carbon pools on Earth’s surface. Dittmar introduced novel ultrahigh-resolution mass spectrometry techniques in the marine sciences to address long-standing open questions in the oceanic carbon cycle.