Controls on dissolved organic matter (DOM) degradation in a headwater stream: the influence of photochemical and hydrological conditions in determining light-limitation or substrate-limitation of photo-degradation

Abstract

Abstract. We investigated how absorption of sunlight by chromophoric dissolved organic matter (CDOM) controls the degradation and export of DOM from Imnavait Creek, a beaded stream in the Alaskan Arctic. We measured concentrations of dissolved organic carbon (DOC), as well as concentrations and characteristics of CDOM and fluorescent dissolved organic matter (FDOM), during ice-free periods of 2011-2012 in the pools of Imnavait Creek and in soil waters draining to the creek. Spatial and temporal patterns in CDOM and FDOM in Imnavait Creek were analyzed in conjunction with measures of DOM degradation by sunlight and bacteria and assessments of hydrologic residence times and in situ UV exposure. CDOM was the dominant light attenuating constituent in the UV and visible portion of the solar spectrum, with high attenuation coefficients ranging from 86 ± 12 m −1 at 305 nm to 3 ± 1 m −1 in the photosynthetically active region (PAR). High rates of light absorption and thus light attenuation by CDOM contributed to thermal stratification in the majority of pools in Imnavait Creek under low-flow conditions. In turn, thermal stratification increased the residence time of water and DOM, and resulted in a separation of water masses distinguished by contrasting UV exposure (i.e., UV attenuation by CDOM with depth resulted in bottom waters receiving less UV than surface waters). When the pools in Imnavait Creek were stratified, DOM in the pool bottom water closely resembled soil water DOM in character, while the concentration and character of DOM in surface water was reproduced by experimental photo-degradation of bottom water. These results, in combination with water column rates of DOM degradation by sunlight and bacteria, suggest that photo-degradation is the dominant process controlling DOM fate and export in Imnavait Creek. A conceptual model is presented showing how CDOM amount and lability interact with incident UV light and water residence time to determine whether photo-degradation is "light-limited" or "substrate-limited". We suggest that degradation of DOM in CDOM-rich streams or ponds similar to Imnavait is typically light-limited under most flow conditions. Thus, export of DOM from this stream will be less under conditions that increase the light available for DOM photo-degradation (i.e., low flows, sunny days).

Photochemically Induced Changes of Dissolved Organic Matter in a Humic-Rich and Forested Stream

“…The upscaled measured value of the cumulated global radiation was comparable to the monthly sum for August 2018 of the DWD (Section 3.1) [75]. Similar values of global radiation was measured by [55] for arctic regions in the years 2011 and 2012. The bacteria were potentially damaged under these conditions showing negligible carbon processing activity.…”

“…As yet, global radiation has been combined and presented only with parameters different from DOM quality. Published in 2015, differences between summer 2011 and 2012 and between streams or ponds (light-limited/flow conditions) were related to global radiation using meteorological data [55]. Here, we correlated DOM quality parameters to cumulated solar radiation (see methods Section 2.8.4).…”

Land‐use controls on carbon biogeochemistry in lowland streams of the Congo Basin

“…We suggest this is unlikely and that photodegradation is a minor fate of deforested DOM in these streams for two reasons. First, the study streams were small and low-order, which means low water residence times and thus limited exposure to solar radiation (Cory, Harrold, Neilson, & Kling, 2015). Second, increased erosion in deforested sites results in high turbidity, which would physically limit photochemistry in these sites.…”

Groundwater Flow and Exchange Across the Land Surface Explain Carbon Export Patterns in Continuous Permafrost Watersheds

“…During low discharge, when flow is dominated by groundwater input, some stream concentrations approached and dropped below minimum observed groundwater concentrations. This can be explained in part by longer water residence times within the stream that would result in greater DOC processing and removal, for example, by bacterial respiration or photomineralization of DOC in surface water (Cory et al, 2014(Cory et al, , 2015. In contrast, during precipitation events or snowmelt the thin TLA can quickly saturate and result in overland flow.…”