Variability Of Dissolved Organic Carbon Research Articles

Effects of bedrock groundwater discharge on spatial variability of dissolved carbon, nitrogen, and phosphorous concentrations in stream water within a forest headwater catchment

AbstractThe quantitative evaluation of the effects of bedrock groundwater discharge on spatial variability of stream dissolved organic carbon (DOC), dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorous (DIP) concentrations has still been insufficient. We examined the relationships between stream DOC, DIN and DIP concentrations and bedrock groundwater contribution to stream water in forest headwater catchments in warm‐humid climate zones. We sampled stream water and bedrock springs at multiple points in September and December 2013 in a 5 km2 forest headwater catchment in Japan and sampled groundwater in soil layer in small hillslopes. We assumed that stream water consisted of four end members, groundwater in soil layer and three types of bedrock groundwater, and calculated the contributions of each end member to stream water from mineral‐derived solute concentrations. DOC, DIN and DIP concentrations in stream water were compared with the calculated bedrock groundwater contribution. The bedrock groundwater contribution had significant negative linear correlation with stream DOC concentration, no significant correlation with stream DIN concentration, and significant positive linear correlation with stream DIP concentration. These results highlighted the importance of bedrock groundwater discharge in establishing stream DOC and DIP concentrations. In addition, stream DOC and DIP concentrations were higher and lower, respectively, than those expected from end member mixing of groundwater in soil layer and bedrock springs. Spatial heterogeneity of DOC and DIP concentrations in groundwater and/or in‐stream DOC production and DIP uptake were the probable reasons for these discrepancies. Our results indicate that the relationships between spatial variability of stream DOC, DIN and DIP concentrations and bedrock groundwater contribution are useful for comparing the processes that affect stream DOC, DIN and DIP concentrations among catchments beyond the spatial heterogeneity of hydrological and biogeochemical processes within a catchment.

Dissolved organic carbon loss from paddy fields by drainage and roles of common ions

Cations and anions exerted important effects on dissolved organic carbon (DOC) release and retention in arable soils. However, knowledge is limited on DOC loss from rice (Oryza sativa L.) paddies by drainage and its association with common ions under a conventional irrigation and drainage regime. This study aimed to investigate DOC dynamics in the soil-overlying water system and the synergistic behaviors of common ions (Na+, K+, Ca2+, Mg2+, Cl−, NO3− and SO42−) contributing to variations in DOC flux in drainage water at the pre-growing, vegetative and reproductive stages of rice cultivation in the Sanjiang Plain of NE China. Samples of overlying water and soil in three layers (0–20, 20–40, and 40–60 cm) were collected before drainage to evaluate variations of DOC and common ions across rice growing stages and to estimate DOC loss by drainage. Sorption experiments were conducted to determine DOC sorption capacities of surface soil (0–5 cm). Compared with the pre-growing stage, DOC concentrations in overlying water and soil significantly increased at the vegetative stage, while significant increases at the reproductive stage were merely in soil. Net movement of DOC was from soil to overlying water at the pre-growing and reproductive stages, and was close to equilibrium at the vegetative stage. Ca2+ and Mg2+ in surface soils (0–20 cm) displayed stronger capacities explaining DOC dynamics in the soil-overlying water system than the other common ions. As Ca2+ and Mg2+ in surface soils (0–20 cm) increased, DOC in the uppermost soil layers (0–20 and 20–40 cm) and overlying water increased, and DOC sorption capacities in surface soil were enhanced. DOC loss from paddy fields by drainage mainly occurred at the vegetative stage, occupying 62–83 % of the total, and much more attention ought to be paid to control drainage DOC loss during this period. In light of important roles of Ca2+ and Mg2+ in DOC dynamics and then DOC loss by drainage, appropriate measures with respect to addition of Ca2+ and Mg2+ could be taken into consideration in rice cultivation to alleviate paddy field DOC export.

Soil dissolved organic carbon in terrestrial ecosystems: Global budget, spatial distribution and controls

AbstractAimsSoil dissolved organic carbon (DOC) is a primary form of labile carbon in terrestrial ecosystems, and therefore plays a vital role in soil carbon cycling. This study aims to quantify the budgets of soil DOC at biome and global levels and to examine the variations in soil DOC and their environmental controls.LocationGlobal.Time period1981–2019.MethodsWe compiled a global dataset and analysed the concentration and distribution of DOC across 10 biomes.ResultsLarge variations in DOC are found among biomes across space and the soil DOC concentration declines exponentially along soil depths. Tundra has the highest soil DOC concentration in 0–30 cm soils [453.75 (95% confidence interval: 324.95–633.5) mg/kg], whereas tropical and temperate forests have relatively lower DOC concentrations, ranging from 30.20 (24.78–36.80) to 54.54 (49.77–59.77) mg/kg. DOC generally accounts for < 1% of total organic carbon in soils, and DOC in 0–30 cm contributes more than half of the total DOC in the 0–100 cm soil profile. Furthermore, variations in DOC are primarily controlled by soil texture, moisture, and total organic carbon.Main conclusionsA global synthesis is combined with an empirical model to extrapolate the DOC concentration along soil profiles across the globe, and global budgets of DOC are estimated as 7.20 Pg C in the top 0–30 cm and 12.97 Pg C in the 0–100 cm soil profile, respectively, with a considerable variation among biomes. The strong soil texture control but weak total organic carbon (TOC) control on DOC variations suggest that the investigation of physical protection of soil organic carbon might need to expand to consider the labile C in soils. The global maps of DOC concentration serve as a benchmark for validating land surface models in estimating carbon storage in soils.

Contrasting dissolved organic carbon concentrations in the Bransfield Strait, Northern Antarctic Peninsula: insights into ENSO and SAM effects

The marine reservoir of dissolved organic carbon (DOC) is equivalent to the atmospheric carbon reservoir and represents the largest reduced carbon stock in the ocean. However, the role of DOC in the global carbon cycle is not fully understood. In this study, we analyzed the distribution of DOC in the water column of the Bransfield Strait, Northern Antarctic Peninsula. Data were collected during two summer cruises of the NAUTILUS project conducted by the Brazilian High Latitude Oceanography Group in February 2015 and 2016. We investigated the possible effects of the El Niño - Southern Oscillation (ENSO) and Southern Annular Mode (SAM) climate modes on DOC variability based on correlations between DOC and temperature, salinity, and dissolved oxygen. Concentrations of DOC ranged from 40.70 to 122.82 μmol kg−1 in 2015 and 33.12 to 112.20 μmol kg−1 in 2016. The analysis shows that the vertical distribution of DOC in the Bransfield Strait may be controlled by a combination of vertical stratification, vertical export of organic matter and biological activity in the upper mixed layer during summertime. The significant interannual differences observed between DOC concentration and most physicochemical parameters may be linked to ENSO and SAM climate modes. The highest DOC concentration was observed in February 2015, associated with a layer of warm surface water, due to the higher stratification of the water column. The ENSO and SAM climate modes were low and positive in that year, favoring intrusions of waters from the Antarctic Circumpolar Current. In contrast, February 2016 was characterized by record positive ENSO and SAM modes. The intensification of the Weddell Gyre induced the advection of cold, more oxygenated and recently formed shelf waters into the Bransfield Strait, which may have contributed to the lower DOC concentrations. This study contributes to a better understanding of the role of DOC in the global carbon cycle in a region of complex ocean circulation and mixing, and sensitive to global climate change.

Updated Multiple Linear Regression Models for Predicting Chronic Aluminum Toxicity to Freshwater Aquatic Organisms and Developing Water Quality Guidelines.

Multiple linear regression (MLR) models for predicting chronic aluminum toxicity to a cladoceran (Ceriodaphnia dubia) and a fish (Pimephales promelas) as a function of 3 toxicity-modifying factors (TMFs)-dissolved organic carbon (DOC), pH, and hardness-have been published previously. However, the range over which data for these TMFs were available was somewhat limited. To address this limitation, additional chronic toxicity tests with these species were subsequently conducted to expand the DOC range up to 12 mg/L, the pH range up to 8.7, and the hardness range up to 428 mg/L. The additional toxicity data were used to update the chronic MLR models. The adjusted R2 for the C. dubia 20% effect concentration (EC20) model increased from 0.71 to 0.92 with the additional toxicity data, and the predicted R2 increased from 0.57 to 0.89. For P. promelas, the adjusted R2 increased from 0.87 to 0.92 and the predicted R2 increased from 0.72 to 0.87. The high predicted R2 relative to the adjusted R2 indicates that the models for both species are not overly parameterized. When data for C. dubia and P. promelas were pooled, the adjusted R2 values were comparable to the species-specific models (0.90 and 0.88 for C. dubia and P. promelas, respectively). This indicates that chronic aluminum EC20s for C. dubia and P. promelas respond similarly to variation in DOC, pH, and hardness. Overall, the pooled model predicted EC20s that were within a factor of 2 of observed in 100% of the C. dubia tests and 94% of the P. promelas tests. Environ Toxicol Chem 2020;39:1724-1736. © 2020 SETAC.

Modelling spatiotemporal patterns of water quality and its impacts on aquatic ecosystem in the cold climate region of Alberta, Canada

Cold climate regions offer various ecosystem services. The water quality parameters such as dissolved oxygen (DO), water temperature (Tw), and dissolved organic carbon (DOC) have considerable impacts on the aquatic ecosystem species. Any impairments in water quality such as elevated water temperature, and low DO concentrations can limit the survival of aquatic ecosystems and its species, such as walleye, northern pike and salmon. Therefore, a good understanding of the aquatic ecosystem of rivers is essential for effective and sustainable river basin and watershed management of fisheries and aquatic resources. The objectives of this study is to improve a watershed scale module of water quality (DO, DOC and Fecal coliforms (FC) in the SWAT model to examine the spatiotemporal patterns and their impacts on aquatic ecosystem and water quality processes in the Athabasca River Basin (ARB), Alberta, Canada. The calibration and validation results of DO, DOC and FC show that the improved Soil and Water Assessment Tool (SWAT) model achieved successfully with a varied range (satisfactory to vey-good) of accuracy at the daily temporal scales. The results showed that concentrations of DO for the selected stations (spring and summer) reduced far below the thresholds for ecosystems survival. In concurrent reduction with DO, the FC concentration considerably varied in the different monitoring stations of ARB. These results highlight that DO, DOC and FC variability in the ARB may drive changes in water quality and ecosystem services that have to be understood on the specific research scale for designing adaptive management scenarios. This study reveals that the new SWAT model can be applied to other similar regions of the worlds.

Are dissolved organic carbon concentrations in riparian groundwater linked to hydrological pathways in the boreal forest?

Abstract. The riparian zone (RZ), or near-stream area, plays a fundamental role in the biogeochemistry of headwaters. Here, wet, carbon-rich soils can change groundwater chemistry before it enters the stream. In the boreal forest, the RZ plays an especially important role in the export of dissolved organic carbon (DOC) to streams. However, the RZ is not uniform, and spatial variability of riparian groundwater hydrology and chemistry can be large. Terrestrial topographic depressions create hydrological pathways towards focal points in the RZ, which we refer to as “discrete riparian inflow points” (DRIPs). Combining the chemical function of the RZ and the convergence of hydrological pathways, we hypothesize that DRIPs play a disproportionally large role in conveying DOC to small streams. Earlier work has demonstrated that runoff from DRIPs can make up the majority of riparian flow contributions to streams, but it is currently unknown how their groundwater chemistry differs from the rest of the RZ. Therefore, we ask the following question: are DOC concentrations in riparian groundwater linked to hydrological pathways in the boreal forest? To answer this question, we sampled riparian groundwater during six campaigns across three boreal headwater streams in Sweden. The groundwater wells were distributed into 10 DRIP and non-DRIP pairs (60 wells), following transects from the upland (20 m lateral distance from the stream bank) to the near-stream area (<5 m lateral distance from the stream bank). The variability in DOC, pH, and electrical conductivity (EC) was analyzed using linear mixed-effects models (LMMs). We explained the variability using three factors: distance from the stream, seasonality, and DRIP/non-DRIP. Our results showed that DRIPs provided DOC-rich water (34 mg L−1) with relatively low EC (36 µS cm−1). The “non-DRIP” riparian water had 40 % lower DOC concentrations (20 mg L−1) and a 45 % higher EC (52 µS cm−1) on average. Moreover, groundwater chemistry from DRIPs was spatially and temporally relatively homogeneous. In contrast, non-DRIP water transformed distinctly in the last 25 m towards the stream, and the chemical variability was also larger between seasons. We concluded that hydrological pathways and spatial variability in riparian groundwater DOC concentrations are linked, and that DRIPs can be seen as important control points in the boreal landscape. Characterizing DRIPs in headwater catchments can be useful for upscaling carbon inputs in boreal stream ecosystems and for delineating hydrologically adapted buffers for forest management practices.

Effects of land use on the characteristics and composition of fluvial chromophoric dissolved organic matter (CDOM) in the Yiluo River watershed, China

• Effects of land use on CDOM and DOC in the Yiluo River watershed were quantified. • Atypical CDOM absorption curve exhibited a shoulder near 390 nm. • Water samples showed high tryptophan-like and humic-like fluorophores. • Crop-dominated and built-up areas had high CDOM and DOC concentrations. Chromophoric dissolved organic matter (CDOM) is a key indicator of water quality, biogeochemical state and nutrient content in freshwaters. Land use affects the concentration, quality, source, and age of CDOM in rivers. To investigate the characteristics and composition of fluvial CDOM and the effects of land use on carbon components, 37 sites were sampled during May 2017 in the Yiluo River watershed, China. In addition, water parameters, absorption and fluorescence spectral parameters of CDOM were measured in situ and in the lab. Some samples showed atypical CDOM absorption curves with an absorption shoulder at 390 nm. CDOM absorption at 440 nm [a CDOM (440)] was positively correlated with dissolved organic carbon (DOC) and fluorescence intensity (Fn). Fluorescence excitation-emission spectra indicated the presence of strong humic-like and tryptophan-like fluorophores revealing terrestrial, anthropogenic influence. Low recent autochthonous contribution index (BIX) and humification index (HIX) values indicated that autochthonous production was an important contributor to CDOM levels. Cropland-dominated subwatersheds (SWs) exhibited high a CDOM (440), Fn and DOC and forest-dominated SWs showed low CDOM concentration. At the riparian buffer scale, CDOM in the built-up-dominated areas showed the highest CDOM concentration. Moreover, built-up areas greatly influenced the concentrations of DOC and CDOM quantificationally at both the SW and riparian buffer scales; more than 64% of the overall variation in carbon components at the SW scale and 46% of DOC variation at the riparian buffer scale was explained by built-up areas. Overall, the results of our study showed that the properties of fluvial CDOM are dependent on the surrounding land use, further highlighting the importance of land use for CDOM photoreactivity in SWs and riparian buffer zones.

Changes in global groundwater organic carbon driven by climate change and urbanization

Climate change and urbanization can increase pressures on groundwater resources, but little is known about how groundwater quality will change. Here, we use a global synthesis (n = 9,404) to reveal the drivers of dissolved organic carbon (DOC), which is an important component of water chemistry and substrate for microorganisms that control biogeochemical reactions. Dissolved inorganic chemistry, local climate and land use explained ~ 31% of observed variability in groundwater DOC, whilst aquifer age explained an additional 16%. We identify a 19% increase in DOC associated with urban land cover. We predict major groundwater DOC increases following changes in precipitation and temperature in key areas relying on groundwater. Climate change and conversion of natural or agricultural areas to urban areas will decrease groundwater quality and increase water treatment costs, compounding existing constraints on groundwater resources.

Monitoring dissolved organic carbon by combining Landsat-8 and Sentinel-2 satellites: Case study in Saginaw River estuary, Lake Huron

Dissolved organic carbon (DOC) in aquatic environments is an important cycled pool of organic matter on the Earth. Satellite remote sensing provides a useful tool to determine spatiotemporal distribution of water quality parameters. Previous DOC remote sensing studies in inland water suffered from either low spatial resolution or low temporal frequency. In this study, we evaluated the potential of jointly using Landsat-8 and Sentinel-2 with high spatial resolution to estimate DOC concentrations in Saginaw River plume regions of Lake Huron. Firstly, CDOM (colored dissolved organic matter) was estimated from images using the known models and then DOC can be derived in terms of the good correlations between DOC and CDOM. The results show that Landsat-8 and Sentinel-2 had acceptable accuracy and good consistency in DOC estimation so that jointly using them can improve the observation frequency. In different seasons from 2013 to 2018, DOC was typically higher in spring and autumn but lower in summer. Monthly spatiotemporal variations of DOC in 2018 were also observed. The image-derived DOC spatiotemporal variations show that DOC was covaried with Saginaw River discharge (r = 0.82) and also weakly and negatively correlated with water temperature (r = −0.6). This study demonstrated that using Landsat-8 and Sentinel-2 together can offer the potential applications for monitoring DOC and water quality dynamic in complex inland water.

Effects of a native earthworm species (Amynthas morrisi) and Eisenia fetida on metal fractions in a multi-metal polluted soil from South China

The effects of the native species Amynthas morrisi and the composting earthworm Eisenia fetida were evaluated on the forms of Zn, Cd, Pb and Cu in a long-term contaminated Chinese soil, with and without added organic matter (OM), in a 60-day laboratory experiment (25 °C field capacity). Four metal fractions were extracted using a sequential procedure: water-soluble exchangeable and carbonate-bound fraction (WAEX), Fe and Mn oxides fraction (FMOX), organic compounds (ORGA), and residual non-extractable fraction (RESI). The sum of available fractions (AVA = WAEX + FMOX + ORGA) and the total contents of the surface casts and non-ingested soil were calculated. In all treatments, all casts of the two earthworm species had higher pH and DOC (Dissolved organic carbon) contents and lower Eh values than those of their respective controls. Amynthas morrisi casts contained higher concentrations of available forms (AVA) of Cd (+84.1%) and Zn (+39.9%) and lower concentrations of available Cu (−10.1%) than those in the control soil (P < 0.05). Organic matter addition limited the variations of pH, Eh and DOC and had the opposite effect to earthworms on metal speciation. However, in the soils of all experimental units and compared with control soil, earthworms increased organically bound Pb (+16.0%) and Cu (+46.1%, A. morrisi; 64.0%, E. fetida), whereas other extractable forms of elements decreased, depending on the metals and their chemical forms (P < 0.05). Amynthas morrisi had similar effects on metal dynamics to the exotic E. fetida and reproduced when organic matter was added; both are therefore good candidates for soil phytoremediation technologies based on increased earthworm activities.

Quantifying DOC and Its Controlling Factors in Major Arctic Rivers during Ice-Free Conditions using Sentinel-2 Data

The six largest Arctic rivers (Yenisey, Lena, Ob’, Kolyma, Yukon, and Mackenzie) drain the organic-rich Arctic watersheds and serve as important pools in the global carbon cycle. Satellite remote sensing data are considered to be a necessary supplement to the ground-based monitoring of riverine organic matter circulation, especially for the ice-free periods in high-latitudes. In this study, we propose a remote sensing retrieval algorithm to obtain the chromophoric dissolved organic matter (CDOM) levels of the six largest Arctic rivers using Sentinel-2 images from 2016 to 2018. These CDOM results are converted to dissolved organic carbon (DOC) concentrations using the strong relationship (R2 = 0.89) between the field measurements of these two water constituents. The temporal-spatial distributions of the DOC in the six largest Arctic rivers during ice-free conditions are depicted. The performance of the retrieval algorithm verifies the capacity of using Sentinel-2 data to monitor riverine DOC variations due to its improved spatial resolution, better band placement, and increased observation frequency. River discharge, watershed slopes, human activities, and land use/land cover change drove much of the variation in the satellite-derived DOC. The seasonality, geography, and scale would affect the correlation between DOC concentration and these influence factors. Our results could improve the ability to monitor DOC fluxes in Arctic rivers and advance our understanding of the Earth’s carbon cycle.

Impact factors of dissolved organic carbon and the transport in a river-lake continuum in the Tibet Plateau of China

As a significant fraction of carbon in inland waters, dissolved organic carbon (DOC) plays a crucial role in carbon cycling at a global scale. Understanding the linkages between variations in DOC and its dominant factors in lakes is critical for estimating its concentration on a larger scale. This study characterized the concentrations of DOC in the lakes of Tibet Plateau and explored the major underlying influencing factors. The overall mean concentration of DOC in these closed lakes had a mean value of 21.80 ± 30.82 mg/L (mean ± S.D.) and was higher than open lakes (F = 174.1, p < 0.05). The potential drivers for the concentration of DOC in the lakes of Tibet Plateau were conducted by the correlation analysis of DOC with the observed landscape, water quality variables, and climatic factors. The multiple regression model showed that 78% of DOC concentrations across the lakes could be explained by Shannon evenness index (SHEI) of landscape, slope, and landscape dominance index (LDI), of which SHEI explained the most substantial variations for the concentration of DOC. The changes in the concentrations of DOC in river-lake interaction were analyzed in Qinghai Lake watershed. The concentration of DOC in the Qinghai Lake water was higher than the surrounding rivers. Slope and the density of soil organic matter (SOM) in the Qinghai Lake watershed were responsible for the higher DOC in lake than rivers. The landscape had a significant influence on DOC in the lakes, and the dominant factor was SHEI. The results observed from this investigation further supplement the information of DOC in the inland waters with plateau lakes. Further, this study is expected to improve the understanding of the transport of DOC in the river-lake ecosystem.

Characterization of chromophoric dissolved organic matter in lakes across the Tibet-Qinghai Plateau using spectroscopic analysis

Fluorescent dissolved organic matter (FDOM) has a significant influence on global inland water carbon cycling and biogeochemical processes. The alpine lakes on Tibet-Qinghai Plateau (TQP) are exposed to high-intensity ultraviolet radiation, long retention time with less anthropogenic influences. Yet, limited studies were conducted on FDOM of lakes on the highest plateau in the world with unique aquatic environments. Spatiotemporal variations of FDOM components from 63 lakes (32 fresh lakes, N = 135; 31 brackish lakes, N = 109) were grouped according to salinity across the TQP collected during 2014–2017. FDOM were examined using excitation-emission matrix spectra (EEM) and fluorescence regional integration (FRI). CDOM absorption and FDOM fluorescence indices, average fluorescence intensities of the five fluorescent components (φi,i = I, II, III, IV, V) and total fluorescence intensities (φT) are significantly different between brackish and freshwater lakes (p < 0.05). High correlations (−0.51** < r < −0.86**, p < 0.01) were revealed between FDOM and dissolved organic carbon (DOC) concentrations with salinity > 10‰. A comparatively prolonged retention time and terrestrial allochthonous inputs caused higher DOC accumulation in brackish water. Strong UV radiation resulted in an important effect on CDOM photo-absorption characteristics, and that contributed to DOC variability and fate. Likewise, average CDOM spectroscopic indices in each basin with different land covers showed a moderate correlation between DOC and normalized humic-like φV for 20 basins (r = −0.46**, p < 0.01). The results suggest that studies on the highest alpine lakes across the TQP should focus on the impact of the evapo-concentration, ultraviolet irradiance and landscape features within different basins on FDOM characteristics. It demonstrated that the FDOM was dominated by the allochthonous sources in brackish lakes from the TQP and had a similarity to those observed in marine environments.

Dissolved organic carbon dynamics in the East China Sea and the northwest Pacific Ocean

Abstract. Oceanic dissolved organic carbon (DOC) represents one of the largest carbon reservoirs on Earth, and its distribution and biogeochemical cycles play important roles in carbon cycling and other biogeochemical processes in the ocean. We report the distribution and concentrations of DOC for water samples collected from the shelf-edge and slope regions in the East China Sea (ECS) and the Kuroshio Extension (KE) in the northwestern North Pacific during two cruises in 2014–2015. The DOC concentrations were 45–88 µM in the ECS and 35–65 µM in the KE. In addition to biological processes that are estimated to account for 7 % and 8 %–20 % in shaping the DOC distribution in the ECS and KE regions, respectively, the DOC distribution is largely controlled by hydrodynamic mixing of different water masses. By comparing the DOC results with dissolved inorganic carbon (DIC) and dissolved inorganic radiocarbon (Δ14C-DIC) measured from the same water samples, we further demonstrate that the intrusion of the Kuroshio Current could dilute the DOC concentrations at stations in the outer shelf and slope regions of the ECS. The concentrations of DOC in the KE were significantly lower in surface waters than in the ECS, and a relatively low and stable DOC level (∼40 µM) was found in deep water (below 1500 m) at all stations. Based on the previously reported DIC and Δ14C-DIC values for the stations, the observed spatial variations of DOC in the upper 700 m among the stations in the KE were mainly influenced by mixing of the two water masses carried by the Kuroshio and Oyashio, the two dominant western boundary currents in the region. The hydrodynamic processes are thus important factors in the distribution and dynamics of DOC in the KE region.

Summer and winter coccolithophore blooms in the Black Sea and their impact on production of dissolved organic matter from Bio-Argo data

Bio-Argo measurements of the backscattering coefficient ( bbp ) were used to investigate the time-depth evolution of coccolithophore blooms in the Black Sea. Five years of Bio-Argo data obtained in 2014–2018 revealed two distinct peaks of bbp corresponding to the winter and early summer coccolithophore blooms. The latter started in the upper mixed layer (UML) in April–May and was characterized by the highest coccolithophore concentrations. During the most extensive summer bloom in 2017, its estimates reach 10 × 10 6 cells l −1 . The summer blooms occupied the UML (0–10 m) and a seasonal thermocline (10–30 m). The lower boundary of the bloom was related to the position of isopycnal 1014 kg m −3 , which deepens in May–July due to summer heating. Consequently, the coccolithophore bloom deepened to 20–30 m and terminated rapidly in July. Bloom termination was accompanied by a significant rise in light attenuation ( kd ) in the sea basin. This peak was attributed to the release of dissolved organic carbon (DOC), which was possibly related to viral lysis and the exudation of lipids from coccolithophore cells. Data on the kd was used to estimate the seasonal variability in DOC in the Black Sea. Maximal estimated values of DOC were observed at 15–35 m depth in June-August and coincided with the early summer coccolithophore bloom termination. The winter coccolithophore bloom started in October–November in the UML. The maximum bbp was observed in January. High values of bbp were observed down to a depth of 60 m during the maximal deepening of the mixed layer. The winter blooms were distinctly observed in MODIS satellite images, where they were characterized by high reflectance and relatively low chlorophyll concentrations. The estimated coccolithophore concentration in winter was lower than that in summer, but column-averaged bbp values were comparable. The winter coccolithophore bloom reached a peak within one month after the autumn peak of chlorophyll A, indicating the possible importance of the nutrients recycled after the diatom autumn bloom. In contrast to summer, the maximum DOC observed at the surface preceded the winter coccolithophore bloom, and the mass DOC production was probably attributed to the excretion and lysis of the non-calcified phytoplankton cells. • 5 year Bio-Argo data showed regular winter and summer coccolithophore blooms. • Summer bloom starts in May in the upper 20 m and deepen in June–July to 20–30 m. • Winter bloom starts in October–November and peaks in January, occupying 0-60 m layer. • Estimated column-integrated cell concentrations in winter and summer are comparable. • Peak of DOC is observed in August at 15–35 m depth during termination of summer bloom.

Multi-decadal trends and influences on dissolved organic carbon distribution in the Barataria Basin, Louisiana from in-situ and Landsat/MODIS observations

Barataria Basin, a deltaic marsh-estuary system in the northern Gulf of Mexico (nGOM), is an important source of dissolved organic matter (DOM) to the coastal ecosystem. To assess DOM dynamics, monthly measurements of dissolved organic carbon (DOC) concentrations and colored DOM (CDOM; an optical proxy for DOC) absorption were obtained over a three-year period (2008–2011) along a transect encompassing the upper, middle and lower regions of the Barataria Basin estuary. While CDOM absorption coefficient at 355 nm (ag355) generally decreased and spectral slope at 275–295 nm (S275–295) increased along the transect from the upper to lower basin, deviations were observed associated with hydrologic and meteorological influences. An application of wavelet analysis to the time-series of ag355 and various freshwater sources revealed a spatiotemporal resonance of ag355 with freshwater input from the Mississippi River (MR), the controlled freshwater discharge from the Davis Pond freshwater diversion, and rainfall in the lower, middle and upper basins, respectively. Field measurements and Landsat/MODIS data spanning across different seasons between 2008 and 2011 were used to develop optimized ag355 empirical algorithms for the high river flow, cold front, and normal conditions. Power law models relating a band ratio (green/red) of a satellite sensor to ag355 showed good performance for the MODIS-Aqua (R2~0.81–0.83, N = 581) and Landsat-5 TM (R2~0.72–0.79, N = 304) imagery. ag355-DOC relationships (R2~0.77–0.84, N = 306) applied to historical Landsat-5 TM (1985–2011) and MODIS-Aqua imagery (2002−2012) showed strong environmental influences on multi-decadal CDOM and DOC spatiotemporal trends. Satellite-derived DOC agreed reasonably well with in-situ measured DOC (averaged R2 = 0.71; RMSE = 2.13). While the time-series DOC showed seasonality and decreasing gradients from the upper to lower basin, an overall increase in DOC was observed in the upper and middle basins from 1985 to 2012. However, in the lower basin, DOC increased from 1985 to 2006 but decreased from 2007 to 2012, likely due to the impact of two major hurricanes in 2005. Furthermore, relationships between satellite-derived DOC and land cover variations (1985–2011) along the wetland-estuary interfaces derived from Landsat-5 TM supervised classification showed an increase in DOC with corresponding increase in developed area in the upper basin, while in the lower basin, DOC increased by 41% (R2 = 0.54) between 1985 and 2006 corresponding to a 17% decrease in salt marsh area, suggesting strong land use/land loss impact on the long-term DOC trends in Barataria Basin.

Seasonality of Dissolved Organic Carbon in the Upper Northeast Pacific Ocean

AbstractFrom seasonal cruises in the NE Pacific Ocean during 2017, we (1) determined dissolved organic carbon concentrations; (2) calculated net community production (NCP) from nitrate drawdown; and (3) established relationships between NCP and seasonal dissolved organic carbon (DOC) accumulation in the upper 75 m. The fraction of NCP that accumulated as DOC, hereafter referred to as the net dissolved production ratio, was calculated for several stations during spring and summer. The net dissolved production ratio was about 0.26 at the oceanic station Ocean Station Papa during different seasons and years. Using nitrate concentration profiles obtained from Bio‐Argo floats during 2009–2018 operating near Ocean Station Papa, we calculated NCP at high temporal resolution and then applied the 0.26 constant in order to (4) estimate DOC variability for the 9‐year period. We found strong seasonality near Ocean Station Papa, with NCP maxima during summers ranging from 0.3 to 2.9 mol C/m2 and surface DOC concentrations estimated from 56 μmol/kg in winters to 73 μmol/kg in summers. There was a 10‐fold interannual variability in the seasonally accumulated inventory of DOC, ranging from 0.078 to 0.75 mol C/m2. This study reinforces the value of deploying floats equipped with chemical sensors in order to better understand marine biogeochemical cycles, especially when high resolution data cannot be obtained otherwise. Given that ~26% of NCP accumulates as DOC in the central Gulf of Alaska, the remaining balance of ~74% is available for export as sinking biogenic particles.

Biodegradability of riverine dissolved organic carbon in a Dry-hot Valley Region: Initial trophic controls and variations in chemical composition

Aquatic carbon (C) loss is mostly driven by in-situ mineralization of dissolved organic carbon (DOC) and leads to autonomous carbon dioxide (CO2) emissions to the atmosphere. However, limited studies have examined the biodegradability of riverine DOC, especially for the initial trophic controls of DOC biodegradation (%BDOC) and variations in dissolved organic matter (DOM) chemical composition. In this study, we employed a laboratory incubation experiment to determine the riverine %BDOC, temperature sensitivity (Q10 value), nutrients and DOM optical properties in a subtropical river in the Dry-hot Valley Region. We observed the %BDOC at 30 °C (52 ± 19.8%) was significantly higher than that at 20 °C (34 ± 19.9%) with an average Q10 value of 2 ± 1.7. Decreases in a254, a350 values indicated that the aromatic and lignin fractions were decomposed, while increases in E2:E3 and decreases in E4:46 values suggested that large molecular DOMs were incompletely degraded and accumulated after 56-day incubations. The %BDOC was negatively correlated with the DOC:TDP ratio, which suggested that P availability was a dominant control in aquatic microbial respiration. Our results conclude that temperature and trophic status regulate biodegradability of DOC, and also stimulate the incomplete degradation of recalcitrant fraction in DOMs.

Drivers of interannual and intra‐annual variability of dissolved organic carbon concentration in the River Thames between 1884 and 2013

AbstractThe world's longest record of river water quality (River Thames—130 years) provides a unique opportunity to understand fluvial dissolved organic carbon (DOC) concentrations dynamics. Understanding riverine DOC variability through long‐term studies is crucial to capture patterns and drivers influencing sources of DOC at scales relevant for decision making. The Thames basin (United Kingdom) has undergone massive land‐use change, as well as increased urbanisation and population during the period considered. We aimed to investigate the drivers of intra‐annual to interannual DOC variability, assess the variability due to natural and anthropogenic factors, and understand the causes for the increased DOC variability over the period. Two approaches were used to achieve these aims. The first method was singular spectrum analysis, which was used to reconstruct the major oscillatory modes of DOC, hydroclimatic variables, and atmospheric circulation patterns and to visualise the interaction between these variables. The second approach used was generalised additive modelling, which was used to investigate other non‐natural drivers of DOC variability. Our study shows that DOC variability increased by 80% over the data period, with the greatest increase occurring from the beginning of World War II onwards. The primary driver of the increase in DOC variability was the increase in the average value of fluvial DOC over the period of record, which was itself linked to the increase in basin population and diffuse DOC sources to the river due to land‐use and land‐management changes. Seasonal DOC variability was linked to streamflow and temperature. Our study allows to identify drivers of fluvial intra‐annual and interannual DOC variability and therefore empowers actions to reduce high DOC concentrations.