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?

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.

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.

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.

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.

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.

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.