Unified understanding of intrinsic and extrinsic controls of dissolved organic carbon reactivity in aquatic ecosystems

Abstract

Despite our growing understanding of the global carbon cycle, scientific consensus on the drivers and mechanisms that control dissolved organic carbon (DOC) turnover in aquatic systems is lacking, hampered by the mismatch between research that approaches DOC reactivity from either intrinsic (inherent chemical properties) or extrinsic (environmental context) perspectives. Here we propose a conceptual view of DOC reactivity in which the combination of intrinsic and extrinsic factors controls turnover rates and determines which reactions will occur. We review three major types of reactions (biological, photochemical, and flocculation) from an intrinsic chemical perspective and further define the environmental features that modulate the expression of chemically inherent reactivity potential. Finally, we propose hypotheses of how extrinsic and intrinsic factors together shape patterns in DOC turnover across the land‐to‐ocean continuum, underscoring that there is no intrinsic DOC reactivity without environmental context. By acknowledging the intrinsic–extrinsic control duality, our framework intends to foster improved modeling of DOC reactivity and its impact on ecosystem services.

A Temporal Graph Model to Predict Chemical Transformations in Complex Dissolved Organic Matter

“…Dissolved organic matter (DOM) in surface waters is a highly dynamic component of the terrestrial carbon cycle and an important nutrient source for aquatic organisms. − Due to its negative impact on color and odor and contribution to disinfection byproduct formation, it needs to be removed during drinking water production. , Photochemical reactions of DOM play an important role in freshwater, as they modulate radiation (i.e., alter the available light), contribute to highly reactive species (reactive oxygen and hydroxyl radicals), , and change the chemical composition of DOM during stratification periods in lakes or along the flow path of a stream/river. , An increase in the number of sunny days, a widespread decline of spruce forests in temperate mountain ranges in middle Europe, as well as increased dissolved organic carbon (DOC) concentrations in many catchments have been reported. , As a consequence, increased solar irradiation to previously shaded small streams and drinking water reservoirs and enhanced phototransformation of DOM are expected with currently unknown consequences for drinking water production from such waters. , …”

High Voltage: The Molecular Properties of Redox-Active Dissolved Organic Matter in Northern High-Latitude Lakes

“…Dissolved organic matter (DOM) is a complex mixture of soluble organic molecules originating from various aquatic and terrestrial sources that represent a cascade of transport and degradation processes. 1,2 DOM is an important component of many aquatic ecosystems, serving as a nutrient source, 3 transporting inorganic ions, 4 and a substrate for respiration, thereby cycling carbon from aquatic systems to the atmosphere as greenhouse gases. 5−7 The reactivity of DOM, in terms of photo-and biolability, often dictates its ecosystem function and is largely determined by the ambient environmental conditions as well as its composition, including functional groups, oxygenation, heteroatom content, and aromaticity.…”

“…Dissolved organic matter (DOM) is a complex mixture of soluble organic molecules originating from various aquatic and terrestrial sources that represent a cascade of transport and degradation processes. , DOM is an important component of many aquatic ecosystems, serving as a nutrient source, transporting inorganic ions, and a substrate for respiration, thereby cycling carbon from aquatic systems to the atmosphere as greenhouse gases. − The reactivity of DOM, in terms of photo- and biolability, often dictates its ecosystem function and is largely determined by the ambient environmental conditions as well as its composition, including functional groups, oxygenation, heteroatom content, and aromaticity. − For instance, in freshwaters, saturated DOM compounds are often preferentially consumed over aromatic functional groups on short time scales, but aromatic DOM is more photochemically labile, producing smaller, more aliphatic compounds and releasing CO 2 upon degradation . Aromatic DOM can also shade surface waters from sunlight due to its absorptivity and alter aquatic community structures through a process known as browning which has become exacerbated due to anthropogenic climate change. , As warming and precipitation changes intensify, much of the northern high-latitudes will experience water insecurity and surface water pollution .…”

Transport inventories and exchanges of organic matter throughout the St. Lawrence Estuary continuum (Canada)

“…Tributaries transport an OM mix formed of old terrestrial carbon from soil weathering, recent vegetation detritus from the watershed and newly produce vegetation and phytoplankton (Meybeck, 1982;Ward et al, 2017). These constituents are transformed by intrinsic processes that are modulated by environmental conditions (Raymond et al, 2016;Berggren et al, 2022). Upwelled waters can also bring important amount of dissolved organic matter to deep estuaries and shelves (Walsh, 1991;Bauer et al, 2013), but generally low particulate organic matter to estuaries and coasts.…”

“…The large amount of organic matter transported by tributaries toward the sea undergoes active exchanges between dissolved and particulate phases and transformation along the increasing salinity gradient. Biodegradation, photodegradation, flocculation, precipitation/ dissolution are key intrinsic processes changing organic matter nature throughout its course from tributaries to the sea (Massicotte et al, 2017;Berggren et al, 2022). As a result, the composition of particulate and dissolved organic carbon (POC and DOC, respectively) and nitrogen (PON and DON, respectively) is modified at different rates resulting in changes in POC/PON and DOC/DON ratios, (i.e., preferential remineralisation of N leading to higher ratios as OM ages; Bianchi and Canuel, 2011).…”