Stream Ecosystems in a Changing Environment © 2016 Elsevier Inc

CHAPTER 6 Dissolved Organic Matter in Stream Ecosystems: Forms, Functions, and Fluxes of Watershed Tea L.A. Kaplana, R.M. Coryb aStroud Water Research Center, Avondale, PA, United States bUniversity of Michigan, Ann Arbor, MI, United States Every rill is a channel for the juices of the meadow. Last year's grasses and flower-stalks have been steeped in rain and snow, and now the brook flows with meadow tea—thoroughwort, mint, flagroot, and pennyroyal, all at one draught. Henry David Thoreau, March 8, 1840 Contents Introduction 242 DOM Sources 245 Autochthonous Inputs 245 Allochthonous Inputs 246 Molecular Characterization of DOM 253 Quantitative Geochemistry 253 Optical Methods 255 DOM Composition and Structure 258 DOM Chemogeography and Chemodiversity 260 DOM Transformations and Fates 262 Oxidative Reactivity of DOM 262 Bio-Reactivity of DOM 262 Conceptual Models of DOM Diagenesis and Substances 266 Pathways and Products of Photooxidation 269 Interactions Between Photochemistry and Biological Degradation 272 Rates of Photooxidation in Waters 275 DOM Contributions to Ecosystem Metabolism 280 DOM Uptake 280 Instream Hydrologic Forcing and DOM Export 281 Stream Ecosystems in a Changing Environment © 2016 Elsevier Inc. http://dx.doi.org/10.1016/B978-0-12-405890-3.00006-3 All rights reserved. 241 242 Stream Ecosystems in a Changing Environment DOM in the Anthropocene 281 Altering Ecosystems 282 Urbanization 283 Impacts of a Changing Environment 284 Summary of Impacts of the Anthropocene on DOM Sources and Processing 286 Future Research Challenges 287 Discussion Topics 289 Acknowledgments 289 References 289 INTRODUCTION In Thoreau’s journal, the American author and naturalist provided an early observation on the sources and complexity of what we have come to call dissolved organic matter (DOM). Here we borrow his evocative imagery of DOM as a cold-water extract of terrestrial organic matter, a watershed tea of biomolecules, and use it as the basis for our conceptual model of the watershed processes that define DOM forms and functions and control DOM fluxes in stream ecosystems (Fig. 1). We now know that the relevant watershed processes involving DOM encompass much more than terrestrial plant sources and their leachates. Nevertheless, we note that in connect- ing meadow tea to specific plant leachates that enter the stream, Thoreau Fig. 1 Terrestrial and aquatic controls on DOM reactivity and oxidation. Flow paths from the terrestrial environment to the stream pass through different soil layers that influence DOM sources and their composition. DOM that is labile has shorter spiraling lengths than semi-labile DOM and the spiraling lengths of all DOM increase as streams and river get larger in a downstream direction. CO2 incorporated into plant biomass by photosynthesis is returned to the atmosphere through plant and soil respiration and photodegradation and bacterial respiration in lotic waters. Dissolved Organic Matter in Stream Ecosystems 243 arguably foreshadowed an early focus of stream ecologists on terrestrial plant sources of DOM (Höll, 1955; Cummins et al., 1972; Lock and Hynes, 1976) and the modern concept of watershed specificity in DOM composition (Jaffé et al., 2012). Through the inclusion of a longitudinal perspective depicting how the processing of DOM changes along a river network, our conceptual model also draws upon the theoretical concepts of the river continuum (Vannote et al., 1980), meta-ecosystems (Loreau et al., 2003), and material spiraling (Webster and Patten, 1979; Newbold et al., 1981) in order to place DOM dynamics within stream ecosystems into the broader spatial context of ecohydrology (Janauer, 2000) and ecosystem ecology. DOM constitutes the largest pool of organic matter in aquatic ecosystems (Schlesinger and Melack, 1981; Wetzel, 1992; Mulholland, 2003; Alvarez-Cobelas et al., 2012), and it is one of the most chemically complex mixtures on Earth (Sleighter and Hatcher, 2008). The diverse biogeochem- ical and ecological roles of DOM involve the modulation of processes that influence the fates of many natural and anthropogenic compounds (Prairie, 2008). DOM: controls geochemical reactions (Waples et al., 2005); regu- lates bacterial nutrient uptake and cycling (Meyer et al., 1988; Qualls, 2000; Bernhardt and Likens, 2002); provides C and energy for bacterial hetero- trophs (del Giorgio and Williams, 2005; Wiegner et al., 2005) that shape the composition of microbial communities (Crump et al., 2003; Judd et al., 2006), alter patterns of microbial metabolism (Berggren and del Giorgio, 2015), and influence microbial biogeography (Findlay et al., 2008); alters the bioavailability of organic pollutants (Kukkonen and Oikari, 1991); in- teracts with drinking water disinfectants to produce disinfectant byproducts (Marhaba and Van, 2000; Jung et al., 2014; Li et al., 2014); chelates metals (McKnight and Bencala, 1990; Kuhn et al., 2015); influences both light penetration (Fee et al., 1996) and the quality of light within a water column (Kirk, 2011); and provides behavioral cues for planktonic aquatic larvae (Harder and Qian, 2000) and anadromous fishes (Hasler and Wisby, 1951; Scholz et al., 1976). The study of DOM has expanded enormously, achieving new insights regarding sources (Tipping et al., 2010; Inamdar et al., 2011; Lambert et al., 2011), composition (Kim et al., 2006), structure (Hedges et al., 2000), oxidative reactivity (Volk et al., 1997; Bertilsson and Tranvik, 1998; Cory et al., 2013, 2014; Sleighter et al., 2014), patterns within river networks (Mosher et al., 2015; Creed et al., 2015), global patterns of distribution (Jaffé et al., 2012), and its potential as a metric for both restoration (Stanley et al., 2012) and ecosystem function (Parr et al., 2016). Mechanistic understandings are emerg- ing concerning source variability (Inamdar et al., 2011), the influences of 244 Stream Ecosystems in a Changing Environment hydrology (Mei et al., 2012, 2014; Koch et al., 2013; McLaughlin and Kaplan, 2013; Pereira et al., 2014; Sawyer et al., 2014), geomorphology (Yamashita et al., 2010a), landscape-level phenomena (Petrone, 2010; Williams et al., 2010; Lu et al., 2013), and seasonal (Ågren et al., 2007; Miller and McKnight, 2010) and interannual variability (Holmes et al., 2012). This rapidly expanding understanding of DOM has benefited from the development and application of new tools that provide measures of the natural abundances of stable (Schiff et al., 1990; St-Jean, 2003) and radioactive (Bauer et al., 1992; Raymond and Bauer, 2001; Butman et al., 2015) isotopes, detailed molecular-level composition (Hertkorn et al., 2007; Hockaday et al., 2009), optical properties (Cory and McKnight, 2005), and the extensive and intensive temporal dynamics of solutes (Spencer et al., 2007; Downing et al., 2009; Jollymore et al., 2012; Wilson et al., 2013). New data processing and statistical methods have been developed for handling the massive data sets generated by some of these methods, including visualization graphics for complicated mass spectra (Kim et al., 2003), an automated compound identification algorithm (Kujawinski and Behn, 2006; Koch et al., 2007), parallel factor analysis (Stedmon and Bro, 2008), and two-dimensional correlation analysis (Abdulla et al., 2013). As more is learned about the connections between organic matter cycling in inland waters and the global carbon cycle (Cole et al., 2007; Battin et al., 2009; Aufdenkampe et al., 2011; Lauerwald et al., 2012), including the evasion of CO2 into the atmosphere (Richey et al., 2002; Butman and Raymond, 2011; Raymond et al., 2013; Kokic et al., 2014; Borges et al., 2015) and its burial, primarily in lakes (Tranvik et al., 2009), but also within large rivers and upon their flood plains (Aufdenkampe et al., 2011), research into the role of DOM as an energy resource in stream ecosystems has taken on a new urgency. It has become an intensive area of focus in aquatic sci- ences and has been recognized appropriately as a component of climate change (IPCC, 2014). This chapter presents our current understanding of DOM in lotic ecosystems with a particular emphasis on the reactivity of DOM constituents to microbial and photochemical oxidation. We begin with a review of sources and the hydrologic influences on terrestrial source loadings, quality, and fluxes. We explore the molecular complexity of DOM, present a conceptual model of DOM diagenesis and structure, and discuss how structure and composition influence biologically and photochemi- cally driven oxidative processes. These topics are addressed from a broad ecosystem-level and watershed perspective, relating the cycling of C and N, and the flow of energy to stream ecosystem metabolism. This approach provides a context for discussing the roles of catchment morphology and how hydrology influences the spatial scales of transformation and export within Dissolved Organic Matter in Stream Ecosystems 245 stream and river networks. With this background as a foundation, we discuss the impact of the Anthropocene on DOM sources and dynamics, including changes in land cover, land use, and global climate. Lastly, we identify what we consider to be future challenges for extending knowledge of DOM in streams and rivers, and we provide a list of possible discussion topics. DOM SOURCES Autochthonous Inputs There are streams and rivers in which aquatic primary productivity rep- resents a major energy source, and presumably this translates into in-stream sources of DOM. Such ecosystems include

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