Does Photomineralization of Dissolved Organics Matter in Temperate Rivers?

Abstract

Dissolved organic matter (DOM) is one of the largest fluxes of carbon in inland water ecosystems and a central component of riverine metabolism. DOM, which is approximately 50% dissolved organic carbon (DOC) by mass (Dittmar & Stubbins, 2014;Krogh, 1934), is a nutrient and energy source, capable of transporting metals and pollutants, and a driver of light availability in water bodies (Kaplan & Cory, 2016;Schlesinger & Melack, 1981). Large amounts of riverine DOC enter the ocean annually, where it is an important source of reduced carbon (Raymond & Spencer, 2015). Although the amount of DOC entering the ocean is relatively well known, its representative chemical composition, reactivity, and the seasonality of its input are not as

Global Controls on DOC Reaction Versus Export in Watersheds: A Damköhler Number Analysis

“…DOC $${t}_{s}$$ measured from 20 combined photooxidation/bioassays were comparatively much lower (median: 314 hr) (Figure 8), indicating quicker mineralization when considering in situ photo‐ and biotic oxidations together. Particularly, photodegradation is a widely recognized process that promotes microbial breakdown (Allesson et al., 2016; Lu et al., 2013) though either photo‐ or microbial oxidation alone is insignificant for DOC removal in river networks (del Giorgio & Pace, 2008; Maavara et al., 2021). DOC $${v}_{f}$$ chosen for Da scaling in this analysis (0.038 m d −1 , corresponding of a $${t}_{s}$$ of 720 hr or WRT of a 6–7 order river network) (Wollheim et al., 2015) falls between DOC uptake determined from the solute addition and dark bioassay methods (Figure 8).…”

“…DOC cycling in river networks involves multiple processes that contribute simultaneously to its gains/losses within rivers (Battin et al., 2008; Bernhardt et al., 2018; Kaplan & Cory, 2016). In situ photooxidation (Maavara et al., 2021) and biotic mineralization (del Giorgio & Pace, 2008; Raymond & Bauer, 2000) are the two main processes contributing to permanent DOC removal and CO 2 emission from river systems. The magnitude of DOC removal rate in river networks is however difficult to quantify due to the complex interplays among multiple biotic/abiotic processes that contribute to DOC concentration variations in spatially distributed river networks (Lv et al., 2019; Wollheim et al., 2015).…”

Watershed DOC uptake occurs mostly in lakes in the summer and in rivers in the winter

“…Despite having shown that photomineralization can be important in river networks relative to biomineralization, its importance is close to negligible relative to the magnitude of DOC fluxes in both lakes and rivers. Maavara et al (2021) considered the importance of photomineralization in the Connecticut River watershed's river reaches in terms of its ability to eliminate DOC from the water column and found that per‐reach removal ( R i , Eq. 13) was 0.026–0.18% of the DOC flux at median flows.…”

“…Complete photooxidation of DOC to dissolved inorganic carbon (DIC) (i.e., photomineralization, ψ), in each river reach and lake segment is based on the formulation (Koehler et al 2012;Cory and Kling 2018;Maavara et al 2021):…”

“…We develop a spatially explicit mass balance model of DOC sources, sinks, and transport through the Connecticut River watershed in the northeastern United States and southern Canada. An existing body of literature dedicated to quantifying Connecticut River watershed metabolism, carbon dynamics, and hydrology make the Connecticut an ideal watershed for such an exercise, as ample information is available to parameterize a model (Hosen et al 2019; Brinkerhoff et al 2021; Hosen et al 2021; Maavara et al 2021; Yoon et al 2021; Aho et al 2021 a , b ). We track terrestrial DOC loading, production via GPP, consumption by microbial uptake (biomineralization) and photomineralization and the cascading downstream transport through 98,254 stream reaches and lake segments at an average length resolution of 511 m, from first order through 8 th ‐order streams, rivers, ponds, lakes, and reservoirs.…”