High abundance of protein-like fluorescence in the Amerasian Basin of Arctic Ocean: Potential implication of a fall phytoplankton bloom

Changes of dissolved organic matter following salinity invasion in different seasons in a nitrogen rich tidal reach

Dissolved organic matter (DOM) is a heterogeneous mixture of dissolved material found ubiquitously in aquatic systems and dissolved organic nitrogen is one of its most important components. We hypothesised nitrogen species and salinity intrusions affect the DOM changes. Here, using the nitrogen rich Minjiang River as an easily accessible natural laboratory 3 field surveys with 9 sampling sites (S1-S9) were conducted in November 2018, April and August 2019. The excitation emission matrices (EEMs) of DOM were explored with parallel factor (PARAFAC) and cosine-histogram similarity analysis. Four indices including fluorescence index (FI), biological index (BIX), humification index (HIX) and the fluorescent DOM (FDOM) were calculated and the impact of physicochemical properties was assessed. The results suggested that the highest salinities of 6.15, 2.98 and 10.10, during each campaign corresponded to DTN concentrations of 119.29-240.71, 149.12-262.42 and 88.27-155.29 μmol·L^-1, respectively. PARAFAC analysis revealed the presence of tyrosine-like proteins (C1), tryptophan-like proteins or a combination of the peak N and tryptophan-like fluorophore (C2) and the humic-like material (C3). The EEMs in the upstream reach (i.e. S1-S3) were complex with larger spectra ranges, higher intensities and similar similarity. Subsequently, the fluorescence intensity of three components decreased significantly with low similarity of EEMs (i.e. S4-S7). At the downstream, the fluorescence levels dispersed significantly and no obvious peaks were seen except in August. In addition, FI and HIX increased, while BIX and FDOM decreased from upstream to downstream. The salinity positively correlated with FI and HIX, and negatively related to BIX and FDOM. Besides, the elevated DTN had a significant effect on the DOM fluorescence indices. Altogether, salinity intrusion and elevated nitrogen are relevant for the distribution of the DOM, which is helpful for the water management tracing the DOM source according to the on-line monitoring of salinity and nitrogen in estuaries.

Response mechanism of sediment organic matter of plateau lakes in cold and arid regions to climate change: a case study of Hulun Lake, China

Lake organic matter is one of the important forms of terrestrial carbon, and its sedimentary evolution is affected by many factors such as climate and sources. However, few studies have been conducted on the feedback mechanism of the sedimentary evolution of organic matter to climate change in cold and arid lakes. Historical variations and compositions of sources of the sediment organic matter (SOM) of Hulun Lake, a typical lake in the cold and arid region of China, were studied by multiple methods. The interactions and fee7dback mechanisms between the sedimentary evolution of SOM and climate change, and compositions of SOM source change, were also discussed. Overall, the characteristic indexes of the SOM, including total organic carbon (TOC), carbon stable isotope (δ¹³C), carbon to nitrogen ratio (C/N), and fluorescence intensity (FI) of the protein-like component in water extractable organic matter (WEOM), showed obvious and uniform characteristics of periodical changes. The indexes were relatively stable before 1920, and fluctuated from 1920 to 1979. Since the 1980s, values of TOC, δ¹³C, and FI of the protein-like component in WEOM has increased, while C/N decreased. The absolute dominant contribution of terrestrial source to the SOM had changed, and the relative average contribution rate of autochthonous source increased from 17.6% before 1920 to 36.9% after 2000. The increase of temperature, strong evaporation concentration effect, and change of compositions of SOM sources are the important driving factors of the sedimentary evolution of organic matter in Hulun Lake.

Relative contributions of different sources to DOM in Erhai Lake as revealed by PLS-PM

Dissolved organic matter (DOM) is a complex mixture of organic molecules that plays an important role in freshwater lake ecosystems. Excitation emission matrix-parallel factor analysis (EEM-PARAFAC) is an important tool for the identification and source analysis of DOM but is still inadequate for studying the differences and quantifying the contributions of different sources. Here, based on the maximum fluorescence intensities (Fmax) of the four fluorescent components (e.g., protein-like component C1, and the humic-like components C2, C3, C4) identified by EEM-PARAFAC, combined with large-scale data obtained from observations at Erhai Lake, we used partial least squares path modeling (PLS-PM) to improve the understanding of the migration and transformation mechanisms of DOM. The results showed that the phytoplankton and trophic state had greater impacts on DOM, while the impacts of sediment and inflow rivers were less significant. The results of the models that used nitrogen nutrients (N) and phosphorus nutrients (P) to separately indicate the trophic state suggested that the driving force of P on DOM was stronger than that of N in Erhai Lake. Among the four fluorescent components, the protein-like component with the largest relative proportion (41.98%) was mainly affected by phytoplankton, which was consistent with the autogenic characteristics obtained through spontaneous source index (BIX). The duality of the humic-like components was consistent with the duality of DOM through fluorescent index (FI). C3 had a higher sensitivity to the trophic state than the other components, and C2 received the greatest positive contribution from the rivers entering the lake. These results provide an improved insight into the influence of different sources on the behavior of DOM and demonstrate the potential of using PLS-PM to study the complex driving mechanism of aquatic biogeochemical parameters.

Spatial Distribution of Colored Dissolved Organic Matter in the Western Arctic Ocean

Optical properties of colored dissolved organic matter (CDOM) were investigated along a latitudinal transect (67°–77° N) in upper water (<80 m depth) of the western Arctic Ocean. The absorption coefficient at 280 nm was 0.48–1.25 m−1, with the average for the oligotrophic basin area (1.04 ± 0.08 m−1) being slightly higher than that of the productive shelf area (0.95 ± 0.16 m−1), implying a decoupling effect between CDOM concentration and biological productivity in the western Arctic Ocean. The spectral slope coefficient S270–350 was negatively correlated with salinity, indicating that DOM molecular weight increases with salinity, and may be affected by melt-water input. Four fluorescent components were identified by excitation emission matrices elaborated by parallel factor analysis, including three humic-like (C1, C3, and C4) components and one protein-like (C2) component. Significant increases in concentrations of terrestrially derived humic-like C3 and C4 components with salinity were observed in the basin, mainly controlled by the physical mixing of surface fresh water and subsurface inflowing Pacific Ocean water. Terrestrial material carried by Pacific inflow is thus an important factor affecting the distribution of CDOM fluorescence components. The C3 and C4 fluorescence components may be useful as tracers of Pacific water in the western Arctic Ocean.

The impact of the abnormal salinity enrichment in pore water on the thermodynamic stability of marine natural gas hydrates in the Arctic region

In this study, the thermodynamic and structural characteristics of natural gas hydrates (NGHs) retrieved from gas hydrate mounds (ARAON Mound 03 (AM03) and ARAON Mound 06 (AM06)) in the Chukchi Sea in the Arctic region were investigated. The gas compositions, crystalline structure, and cage occupancy of the NGHs at AM03 and AM06 were experimentally measured using gas chromatography (GC), ¹³C nuclear magnetic resonance (NMR), Raman spectroscopy, and powder X-ray diffraction (PXRD). In the NGHs from AM03 and AM06, a significantly large fraction of CH₄ (> 99%) and a very small amount of H₂S were enclathrated in small (5¹²) and large (5¹²6²) cages of sI hydrate. The NGHs from AM03 and AM06 were almost identical in composition, guest distributions, and existing environment to each other. The salinity of the residual pore water in the hydrate-bearing sediment (AM06) was measured to be 50.32‰, which was much higher than that of seawater (34.88‰). This abnormal salinity enrichment in the pore water of the low-permeability sediment might induce the dissociation of NGHs at a lower temperature than expected. The saturation changes in the NGHs that corresponded with an increase in the seawater temperature were also predicted on the basis of the salinity changes in the pore water. The experimental and predicted results of this study would be helpful for understanding the thermodynamic stability of NGHs and potential CH₄-releasing phenomena in the Arctic region.

Subsea permafrost as a potential major source of dissolved organic matter to the East Siberian Arctic Shelf

Arctic subsea permafrost contains more organic carbon than the terrestrial counterpart (~1400 Pg C vs. ~1000 Pg C) and is undergoing fast degradation (at rates of ~10 to 30 cm yr^-1 over the past 3 decades) in response to climate warming. Yet the flux of organic carbon sequestered in the sediments of subsea permafrost to overlying water column, which can trigger enormous positive carbon-climate feedbacks, remain unclear. In this study, we examined the dissolved organic matter (DOM) diffusion to bottom seawaters from East Siberian Sea (ESS) sediments, which was estimated at about 943-2240 g C m^-2 yr^-1 and 10-55 g C m^-2 yr^-1 at the continuous-discontinuous transition zone of subsea permafrost and the remainder shelf and slope sites, respectively. The released DOM is characterized by prevailing dominance (≥ 98%) of low molecular weight (M_n < 350 Da) fractions. A red-shifted (emission wavelength >500 nm) fluorescence fingerprint, a typical feature of sediment/soil DOM, accounts for 4-6% and 7-8% in the fluorescence distributions of seawaters and pore waters, respectively, on ESS shelf. Statistical analysis revealed that seawaters and pore waters possessed similar DOM composition. The estimated total benthic efflux of dissolved organic carbon (DOC) was ~0.7-1.0 Pg C yr^-1 when the estimate was scaled up to the entire Arctic shelf underlain with subsea permafrost assuming the width of continuous-discontinuous transition zone is 1 to 10 m. This estimation is consistent with the established ~10-30 cm yr^-1 degradation rates of subsea permafrost by estimating its thaw-out time. Compiled observation data suggested that subsea permafrost might be a major DOM source to the Arctic Ocean, which could release tremendous carbon upon remineralization via its degradation to CO₂ and CH₄ in the water column.

Tracing riverine dissolved organic carbon and its transport to the halocline layer in the Chukchi Sea (western Arctic Ocean) using humic-like fluorescence fingerprinting

Dissolved organic carbon (DOC) and the fluorescence properties of dissolved organic matter (FDOM) were investigated using parallel factor analysis (PARAFAC) for seawater samples collected in the Chukchi Sea (65°N-78°N, 170°E-160°W) during summer 2017. River water (f_river) and sea-ice meltwater (f_{sea ice melt}) fractions were also derived using oxygen isotopes ratios (δ¹⁸O) to examine the influence of sea ice on riverine DOM. The spatial distributions of f_river, riverine DOC, and the humic-like fluorescent component (C1) showed an overall south-north gradient, with higher values in the northern Chukchi Sea in summer. Pronounced accumulation of river water and riverine DOM was also observed in the anticyclonic Beaufort Gyre at the eastern stations of the northern Chukchi Sea in association with a long water residence time. Estimated riverine DOC in the surface layer accounted for 27 ± 9% (range: 17-47%) of the total DOC in the southern Chukchi Sea, and 39 ± 6% (range: 32-49%) and 31 ± 4% (range: 25-37%) for the eastern and western stations of the northern Chukchi Sea, respectively. Humic-like C1 showed negative and positive relationships with sea-ice meltwater-corrected salinity (S_{sim_corrected}) and f_river, respectively. However, Arctic river waters with distinct humic-like C1 characteristics were likely mixed in the northern Chukchi Sea. The vertical distributions of riverine DOC, humic-like C1 fluorescence, and f_river generally decreased with water depth, reflecting the strong influence of riverine DOM in the surface layer. Although riverine DOM and f_river were dominant in the upper 50 m of the water column, they were also pronounced in the upper halocline (50-200 m), in which f_{sea ice melt} dropped below zero. Our results indicated the existence of brine rejected from growing sea ice, and that sea-ice formation was a key factor for the transport of riverine DOM to the upper halocline layer in the northern Chukchi Sea.

Spectral Characterization of Dissolved Organic Matter in Seawater and Sediment Pore Water from the Arctic Fjords (West Svalbard) in Summer

Fjords in the high Arctic, as aquatic critical zones at the interface of land-ocean continuum, are undergoing rapid changes due to glacier retreat and climate warming. Yet, little is known about the biogeochemical processes in the Arctic fjords. We measured the nutrients and the optical properties of dissolved organic matter (DOM) in both seawater and sediment pore water, along with the remote sensing data of the ocean surface, from three West Svalbard fjords. A cross-fjord comparison of fluorescence fingerprints together with downcore trends of salinity, Cl−, and PO43− revealed higher impact of terrestrial inputs (fluorescence index: ~1.2–1.5 in seawaters) and glaciofluvial runoffs (salinity: ~31.4 ± 2.4 psu in pore waters) to the southern fjord of Hornsund as compared to the northern fjords of Isfjorden and Van Mijenfjorden, tallying with heavier annual runoff to the southern fjord of Hornsund. Extremely high levels of protein-like fluorescence (up to ~4.5 RU) were observed at the partially sea ice-covered fjords in summer, in line with near-ubiquity ice-edge blooms observed in the Arctic. The results reflect an ongoing or post-phytoplankton bloom, which is also supported by the higher levels of chlorophyll a fluorescence at the ocean surface, the very high apparent oxygen utilization through the water column, and the nutrient drawdown at the ocean surface. Meanwhile, a characteristic elongated fluorescence fingerprint was observed in the fjords, presumably produced by ice-edge blooms in the Arctic ecosystems. Furthermore, alkalinity and the humic-like peaks showed a general downcore accumulation trend, which implies the production of humic-like DOM via a biological pathway also in the glaciomarine sediments from the Arctic fjords.

Mass Budget of Methylmercury in the East Siberian Sea: The Importance of Sediment Sources

Biological concentrations of methylmercury (MeHg) are elevated throughout the Arctic Ocean; however, to date, the major sources and the spatial variability of MeHg are not well quantified. To identify the major inputs and outputs of MeHg to the Arctic shelf water column, we measured MeHg concentrations in the seawater and sediment samples from the East Siberian Sea collected from August to September 2018. We found that the MeHg concentrations in seawater and pore water were higher on the slope than on the shelf, while the MeHg concentrations in the sediment were higher on the shelf than on the slope. We created a mass budget for MeHg and found that the benthic diffusion and resuspension largely exceed other sources, such as atmospheric deposition and river water input. The major sinks of MeHg in the water column were dark demethylation and evasion. When we extrapolated our findings on benthic diffusion to the entire Arctic shelf system, the annual MeHg diffusion from the shelf sediments was estimated to be 23,065 ± 939 mol yr^-1, about 2 times higher than previously proposed river discharges. Our study suggests that the MeHg input from shelf sediments in the Arctic Ocean is significant and has been previously underestimated.

Depth-dependent variations of dissolved organic matter composition and humification in a plateau lake using fluorescence spectroscopy

In aquatic systems, dissolved organic matter (DOM) plays an important role in regulating the reactivity and transport of environmental pollutants. Fluorescence excitation-emission matrix (EEM) analysis combined with fluorescence regional integration (FRI) and parallel factorial analysis (PARAFAC) were applied to investigate the composition, humification degree and depth-dependent variation of DOM in Lake Baihua, China. The results showed that humic-like materials with more than 60% of percentage fluorescence response (P_i,n) were dominant in DOM. The molecular complexity and structural condensation of PARAFAC components showed the order of C1 (humic-like components) > C3 (protein-like components) > C4 (fulvic-like components) > C2 (fulvic-like components). The lower maximum fluorescence intensities (F_max) of components in surface layers were attributed to photo-oxidation of DOM by radiation. With depths less than 16 m, the F_max increases and decreases were attributed to accumulation of recalcitrant humic substances and microbial/abiotic degradation of particulate DOM, respectively. A combination of biological and humification indices could be used as indicators for distinguishing different degrees of humification and sources of DOM. DOM from Lake Baihua mainly originated from an aquatic bacterial source that consisted of an important/intermediate autochthonous component with a weak humic character. The fluorescence indices of P_I,n/P_II,n, P_I+II+IV,n/P_III+V,n, C2, C1/C2 and C2/(C3+C4) were more suitable to determine the humification degree. Stronger humic characters and higher humification degree for DOM were present at depths of 10-13 m. The fluorescence spectroscopy combined with fluorescence indices is convenient to investigate depth-dependent DOM characteristics and to assess water quality or pollution risk in lake systems.

Surface accumulation of low molecular weight dissolved organic matter in surface waters and horizontal off-shelf spreading of nutrients and humic-like fluorescence in the Chukchi Sea of the Arctic Ocean

Polar regions play unique roles in global overturning circulation, carbon cycling, and climate change. In this study, seawater dissolved organic matter (DOM) was characterized for the Chukchi Sea in the Arctic Ocean in the summer season. The seawater generally contains high concentrations of dissolved organic carbon (DOC, up to 92 μM C) and tyrosine-like fluorescence (up to 0.21 RU), and it was enriched with heteroatomic molecular formula with nitrogen-containing and sulfur-containing formulas counting 2246 (~41% of total identified molecular formula) and 1838 (~34%), respectively. Significant correlations were observed between salinity and the absorption coefficient at 254 nm, between chlorophyll-a and DOC as well as the tyrosine-like component, C_270/302 (C_ex/em maxima), and between biological index and two protein-like components, C_275/338 and C_305/344. A comparison between surface waters and close-to-seafloor deep waters suggested a trend of the accumulation of low molecular weight (LMW) fraction (~54-74%, nominal average molecular weight M_n < ~350 Da) in the surface waters. Another interesting finding from spatial data was an obvious horizontal off-shelf spreading of nutrients and humic-like fluorescence. This study sheds novel insights of DOM characteristics and dynamics in the highly productive polar sea.