Biological and photochemical reactivity of dissolved organic matter in a large temperate river

Abstract Large rivers are critical conduits from continents to oceans as they receive, produce and process huge amounts of dissolved organic matter (DOM). Yet, the relative influence of intrinsic DOM properties and extrinsic environmental properties on these processes at the ecosystem‐level is rarely studied. We assessed DOM optical properties as well as bioreactivity and photoreactivity at 40 sites along a >200 km transect of the freshwater portion of the St. Lawrence River through a series of standardized microbial incubations and exposure to simulated sunlight, and then estimated in situ areal rates of processing. We found that biological and photochemical processes preferentially targeted contrasting pools of DOM, but that DOM composition had an undiscernible effect on in situ degradation rates compared to other environmental factors. Total daily processing across the whole water column ranged from 36.7 to 892.1 mg C m −2 . In situ photochemical degradation was largely driven by intrinsic DOM photoreactivity rather than environmental drivers in the water. In contrast, we found a relatively constant baseline pool of biolabile DOM that appeared to be independent from changes in concentration and environmental conditions. In situ DOM processing was mostly driven by biological degradation (on average 95%), and disproportionately high biodegradation rates (2.5–4x the average) were found in a few shallower sites near effluents or islands, potentially driven by local increases in nutrient concentration and in the proportion of protein‐like DOM. These results illustrate how DOM composition and degradability interact with ambient environmental and morphological properties to dictate an ecosystem‐level reactivity of DOM.