Photomineralization of terrigenous dissolved organic matter in Arctic coastal waters from 1979 to 2003: Interannual variability and implications of climate change (original) (raw)
Related papers
Marine Chemistry, 2009
The photoreactivity of chromophoric dissolved organic matter (CDOM) transported to Arctic shelf environments by rivers has only recently been studied and its quantitative role in Arctic shelf biogeochemistry has received little attention. Sunlight exposure experiments were performed on CDOM collected over a three year period (2002 to 2004) from river, estuary, shelf, and gulf regions of the Western Canadian Arctic. Decreases in CDOM absorption, synchronous fluorescence (SF), and dissolved organic carbon (DOC) concentration were followed after 3 days of exposure, and in two experiments, six optical cutoff filters were used to incrementally remove ultraviolet radiation incident on the samples. Apparent quantum yields for CDOM photobleaching (AQY ble ) and for DOC photomineralization (AQY min ) were computed, as were two AQY spectra (ϕ ble and ϕ min ) for the Mackenzie River and a sample from the Mackenzie Shelf. The photoreactivity of Mackenzie River CDOM was highest after break-up and peak discharge and lowest in late summer. The half-lives of CDOM and DOC were estimated at 3.7 days and 4.8 days, respectively, when Mackenzie River water was exposed to full sunlight. Photobleaching of Mackenzie River CDOM fluorescence after most UV-B wavelengths were removed increased the correlation between the river and offshore waters in the Beaufort Sea. When light attenuation from particle-and CDOMrich river water was considered for the Mackenzie Shelf, our photodegradation models estimated around 10% loss of absorption and b 1% DOC loss, suggesting that sunlight exposure does not substantially degrade CDOM on Arctic shelves.
Frontiers in Marine Science
Arctic landscapes are warming and becoming wetter due to changes in precipitation and the timing of snowmelt which consequently alters seasonal runoff and river discharge patterns. These changes in hydrology lead to increased mobilization and transport of terrestrial dissolved organic matter (DOM) to Arctic coastal seas where significant impacts on biogeochemical cycling can occur. Here, we present measurements of dissolved organic carbon (DOC) and chromophoric DOM (CDOM) in the Yukon River-to-Bering Sea system and two river plumes on the Alaska North Slope which flow into the Beaufort Sea. Our sampling characterized optical and biogeochemical properties of DOM during high and low river discharge periods for the Yukon River-Bering Sea system. The average DOC concentration at the multiple Yukon River mouths ranged from a high of 10.36 mg C L-1 during the ascending limb of the 2019 freshet (late May), 6.4 mg C L-1 during the descending limb of the 2019 freshet (late June), and a low o...
DOM degradation by light and microbes along the Yukon River-coastal ocean continuum
Scientific Reports, 2021
The Arctic is experiencing rapid warming, resulting in fundamental shifts in hydrologic connectivity and carbon cycling. Dissolved organic matter (DOM) is a significant component of the Arctic and global carbon cycle, and significant perturbations to DOM cycling are expected with Arctic warming. The impact of photochemical and microbial degradation, and their interactive effects, on DOM composition and remineralization have been documented in Arctic soils and rivers. However, the role of microbes, sunlight and their interactions on Arctic DOM alteration and remineralization in the coastal ocean has not been considered, particularly during the spring freshet when DOM loads are high, photoexposure can be quite limited and residence time within river networks is low. Here, we collected DOM samples along a salinity gradient in the Yukon River delta, plume and coastal ocean during peak river discharge immediately after spring freshet and explored the role of UV exposure, microbial transf...
Dissolved organic matter photolysis in Canadian arctic thaw ponds
The abundant thaw lakes and ponds in the circumarctic receive a new pool of organic carbon as permafrost peat soils degrade, which can be exposed to significant irradiance that potentially increases as climate warms and ice cover shortens. Exposure to sunlight is known to accelerate the transformation of dissolved organic matter (DOM) into molecules that can be more readily used by microbes. We sampled the water from two common classes of ponds found in the ice-wedge system of continuous permafrost regions of Canada, polygonal and runnel ponds, and followed the transformation of DOM over 12 days by looking at dissolved organic carbon (DOC) concentration and DOM absorption and fluorescence properties. The results indicate a relatively fast decay of color (3.4 and 1.6% loss d −1 of absorption at 320 nm for the polygonal and runnel pond, respectively) and fluorescence (6.1 and 8.3% loss d −1 of total fluorescent components, respectively) at the pond surface, faster in the case of humic-like components, but insignificant losses of DOC over the observed period. This result indicates that direct DOM mineralization (photochemical production of CO 2 ) is apparently minor in thaw ponds compared to the photochemical transformation of DOM into less chromophoric and likely more labile molecules with a greater potential for microbial mineralization. Therefore, DOM photolysis in arctic thaw ponds can be considered as a catalytic mechanism, accelerating the microbial turnover of mobilized organic matter from thawing permafrost and the production of greenhouse gases, especially in the most shallow ponds. Under a warming climate, this mechanism will intensify as summers lengthen.
Carbon cycle. Sunlight controls water column processing of carbon in arctic fresh waters
Science (New York, N.Y.), 2014
Carbon in thawing permafrost soils may have global impacts on climate change; however, the factors that control its processing and fate are poorly understood. The dominant fate of dissolved organic carbon (DOC) released from soils to inland waters is either complete oxidation to CO2 or partial oxidation and river export to oceans. Although both processes are most often attributed to bacterial respiration, we found that photochemical oxidation exceeds rates of respiration and accounts for 70 to 95% of total DOC processed in the water column of arctic lakes and rivers. At the basin scale, photochemical processing of DOC is about one-third of the total CO2 released from surface waters and is thus an important component of the arctic carbon budget.
Journal of Geophysical Research: Biogeosciences, 2016
Boreal and northern temperate lakes (hereinafter referred to as northern lakes) are sites of intense processing of dissolved organic carbon (DOC), which is reflected in part in the persistent CO 2 supersaturation of their surface waters. These ecosystems are subject to strong seasonal fluctuations in both irradiance and DOC amount and quality, which in turn should result in temporal shifts in the magnitude of DOC photodegradation. Here we explore the temporal patterns in the magnitude of water column DOC photomineralization and its potential contribution to pelagic CO 2 production in three northern lakes of different DOC content. We performed laboratory DOC photodegradation incubations and combined the resulting rates with field measurements and modeling to reconstruct the annual cycle in depth-integrated DOC photomineralization. We found that areal rates of DOC photomineralization were driven by both irradiance and intrinsic DOC photoreactivity, both of which showed seasonality. Over an annual cycle, depth-integrated DOC photomineralization rates were remarkably similar across lakes, averaging 4.4 (SD = 0.7) g C m À2 yr À1 and daily rates followed an apparent seasonal pattern. The contribution of DOC photomineralization to total pelagic CO 2 production (as the sum of respiration and DOC photomineralization) peaked after ice melt (up to 49%), averaging 14% for the entire open water season. Our study identifies potential hot periods of photochemical activity that result from the interplay between DOC properties and environmental conditions, which should be incorporated into models of lake functioning. There has been significant progress over the past decades in our understanding of DOC photoreactivity and photochemical degradation in aquatic systems. DOC can be photodegraded into smaller (low molecular weight) and often biolabile molecules [
Remote Sensing of Environment, 2022
HAL is a multidisciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Estuarine, Coastal and Shelf Science, 2007
The optical characteristics of coloured dissolved organic matter (CDOM) were analyzed in the Great Whale River and adjacent Hudson Bay (55 N, 77 W) in the eastern Canadian Low Arctic, and in the Mackenzie River and adjacent Beaufort Sea in the western Canadian High Arctic (70 N, 133 W). Sampling was during ice-free open water conditions. Both rivers contained high concentrations of dissolved organic carbon (3 and 6 mg DOC l À1 in the Great Whale River and Mackenzie River, respectively) and CDOM (a 320 of 11 and 14 m À1 ), resulting in a substantial load of organic matter to their coastal seas. There were pronounced differences in the CDOM characteristics of the two rivers, notably in their synchronous fluorescence scans (SFS). The latter showed that the Mackenzie River was depleted in humic materials, implying a more mature catchment relative to the younger, more recently glaciated Great Whale River system. SFS spectra had a similar shape across the freshwatere saltwater transition zone of the Great Whale plume, and DOC was linearly related to salinity implying conservative mixing and no loss by flocculation or biological processes across the salt front. In contrast, there were major differences in SFS spectral shape from the Mackenzie River to the freshwater-influenced coastal ocean, with a marked decrease in the relative importance of fulvic and humic acid materials. The SFS spectra for the coastal Beaufort Sea in SeptembereOctober strongly resembled those recorded for the Mackenzie River during the high discharge, CDOM-rich, snowmelt period in June, but with some loss of autochthonous materials. These results are consistent with differences in freshwater residence time between the Mackenzie River and Great Whale River coastal ocean systems. Models of arctic continental shelf responses to present and future climate regimes will need to consider these striking regional differences in the organic matter content, biogeochemistry and optics between waters from different catchments and different inshore hydrodynamic regimes.
Production of fluorescent dissolved organic matter in Arctic Ocean sediments
Little is known about the production of fluorescent dissolved organic matter (FDOM) in the anoxic oceanic sediments. In this study, sediment pore waters were sampled from four different sites in the Chukchi-East Siberian Seas area to examine the bulk dissolved organic carbon (DOC) and their optical properties. The production of FDOM, coupled with the increase of nutrients, was observed above the sulfate-methane-transition-zone (SMTZ). The presence of FDOM was concurrent with sulfate reduction and increased alkalinity (R 2 > 0.96, p < 0.0001), suggesting a link to organic matter degradation. This inference was supported by the positive correlation (R 2 > 0.95, p < 0.0001) between the net production of FDOM and the modeled degradation rates of particulate organic carbon sulfate reduction. The production of FDOM was more pronounced in a shallow shelf site S1 with a total net production ranging from 17.9 to 62.3 RU for different FDOM components above the SMTZ depth of ca. 4.1 mbsf, which presumably underwent more accumulation of particulate organic matter than the other three deeper sites. The sediments were generally found to be the sources of CDOM and FDOM to the overlying water column, unearthing a channel of generally bio-refractory and pre-aged DOM to the oceans. The Arctic Ocean is intimately associated with global carbon and hydrological cycling as well as climate change. A large stock of organic carbon (1100–1500 Pg C), including permafrost, is contained in its drainage basin area, which is sensitive to climate change 1. The Arctic Ocean also influences formation of North Atlantic Deep Water which drives Atlantic thermohaline circulation. The fluvial discharge into the Arctic Ocean is reported to be 25–36 Tg C yr −1 for dissolved organic carbon (DOC) and around 12 Tg C yr −1 for particulate organic carbon (POC) 1–3. It was estimated that roughly over 80–90% of the organic carbon, carried by fluvial runoff, is buried in shallow coastal sediments 4. Despite the large fraction of the organic carbon contained in the global oceanic sediments (ca. 700 Pg C) 5 , much is unknown about the dynamics of sediment pore water dissolved organic matter (PW-DOM). It was previously reported that sediment oxygen consumption rates were much faster in coastal sediments (> 200 mM O 2 m −2 d −1) than in deep sea sediments (ca. 0.02 mM O 2 m −2 d −1) 6 , implying a higher biological respiration occurring in coastal sediments. The huge difference of more than four orders of magnitude of oxygen consumption rates may reflect the huge differences of organic matter supply. With the prediction of ice-free summer in the Arctic Ocean over the next few decades 7 , it is urgent to understand the dynamics of sediment DOM in this area as increasing organic carbon loading via fluvial runoff can be expected with global warming underway. Whether the organic carbon burial rates will go up or down, however, is unknown at present. Although the origins of sediment organic matter are diverse including riverine runoff, coastal erosion, sea ice, aeolian input, and autochthonous primary productivity in the Arctic 8 , a substantial fraction is known to be of terrigenous origin 9–12. While solar irradiation is usually the major sink for terrigenous chromophoric DOM (CDOM), autochthonous organic matter produced by surface primary productivity is generally considered susceptible to microbial utilization 13–15. Meanwhile, the majority (ca. 80%) of the organic matter sinking to seafloor is channeled into benthic respiration 16. Once settling into sediments, the remainder organic matter undergoes a