Suspended particulate organic matter (POM) plays a central role in carbon cycling and trophic transfer in Antarctic waters, yet its sources, composition, and degree of transformation remain poorly constrained in nearshore Antarctic Peninsula. Here, we characterize the elemental (C and N), isotopic (δ13C and δ15N), and lipid biomarker (alcohols, sterols, and fatty acids) composition of suspended POM collected in Admiralty Bay and the Bransfield Strait, south of King George Island, during late austral spring 2019. Surface samples were collected at 12 stations, whereas DCM samples were collected at 6 of these stations. Suspended particulate matter concentrations ranged from 5.4 to 21.9 mg L−1 and showed marked variability between the two sampling campaigns. Molar C/N ratios and lipid biomarker distributions indicate a predominance of marine-derived, phytoplankton-associated OM, largely linked to diatom-rich assemblages with minor contributions from other marine sources. Diagnostic lipid biomarker ratios further suggest that suspended POM was relatively fresh and had undergone limited early diagenetic alteration. Paired comparisons between surface and DCM samples revealed no statistically significant depth-related differences, indicating limited vertical variability in OM composition within the upper water column. The results show that physical forcing primarily regulates particle loads, whereas biological production exerts stronger control on OM composition and quality, highlighting the sensitivity of Antarctic coastal carbon cycling to ongoing environmental changes along the Antarctic Peninsula.
The Tatar Trough lies off the eastern coast of Far Eastern Russia, extending into the northern part of the East Sea (also known as the Sea of Japan). This region is characterized by active tectonics and widespread gas seepage, making it an ideal natural laboratory for investigating the biogeochemical dynamics of gas-rich sediments. In this study, we analyzed two sediment cores, LV67-07HC and LV67-19HC, which were recovered from an active fault zone on the eastern slope during the 2014 SSGH expedition. Using a combination of lipid and nucleic acid analyses along with complementary measurements such as gas and porewater composition, we assessed the ecological roles of the archaeal communities inhabiting these seep environments. Both cores exhibited elevated concentrations of light hydrocarbon gases (primarily methane, CH4) and carbon dioxide, along with a distinct sulfate-methane transition zone (SMTZ). However, notable differences in porewater geochemistry, lipid biomarkers, and microbial community composition were observed between the two cores, reflecting variations in CH4 flux and subsurface fluid migration pathways within the underlying coal-bearing strata. In core LV67-07HC (91–185 cm depth), we observed notably high concentrations of 13C-depleted archaeal lipids in sediments surrounding carbonate accretions. These features suggest enhanced anaerobic methanotrophic activity under relatively higher CH4 flux conditions. Archaeal 16S rRNA gene sequences were dominated by members of the ANME-1 clade, consistent with previous observations in CH4-rich seep environments, and further constrained here within a site-specific, multi-proxy framework. To the best of our knowledge, this study provides new insights into how spatial differences in CH4 sources and subsurface fluid-migration pathways influence carbon cycling and archaeal community structure in CH₄-rich continental margin sediments.
Excessive terrestrial phosphorus (P) loading drives coastal eutrophication, yet estuarine tidal flats serve as critical biogeochemical reactors that intercept these fluxes prior to ocean discharge. However, the diagenetic pathways governing permanent P burial under varying hydrodynamic and biotic conditions remain poorly constrained. Here, we combine geochemical profiles, sequential P extraction (SEDEX), and reactive transport modeling (RTM) to examine P transformation processes along a tidal gradient, comparing unvegetated mudflats with vegetated salt marsh in the Yangtze Estuary. Our results reveal that P burial pathways are regulated by the interplay between local sedimentation rates and vegetation cover. In bare mudflats, rapid deposition transports reactive iron (oxyhydr)oxides (FeOx) below the sulfate-methane transition (SMT), fueling iron-mediated anaerobic oxidation of methane (Fe-AOM). Porewater profiles exhibit coincident peaks of Fe2+ and PO43− in the methanic zone, and RTM confirms that Fe-AOM promotes extensive precipitation of authigenic ferrous phosphate (Fe(II)-P, e.g., vivianite). In contrast, within the vegetated salt marsh, dense roots trap reactive substances, and the rhizosphere iron barrier immobilizes phosphate, severely restricting downward reactive P fluxes and suppressing authigenic Fe(II)-P formation. Although rapid oxidation during sample processing often leads to misclassification of this deep Fe(II)-P as temporary Fe(III)-P in conventional SEDEX protocols, our combined lines of evidence confirm that it functions as a cryptic yet permanent P sink. Given that this authigenic Fe(II)-P is thermodynamically more stable than traditional iron-bound P (Fe-P), the capacity of tidal flats for permanent P sequestration has been chronically underestimated. These findings highlight the need to incorporate spatial heterogeneity and cryptic deep diagenetic processes into global marine P budgets to accurately evaluate the buffering capacity of estuarine wetlands against coastal eutrophication.
Marine pollutants are commonly studied in bulk seawater, where their concentrations, toxicity and biodegradation rates are averaged across space. However, microbial transformation of wastes and pollutants rarely occurs in homogeneous environments. Instead, it is concentrated at interfaces, including phycospheres, plastispheres, marine snow and oil droplets, where steep microscale gradients of substrates, oxygen, redox potential and stressors shape microbial behavior. In this Perspective review, we argue that pollutants should not be viewed solely as substrates to be degraded or stressors to be tolerated, but as chemical triggers whose biological meaning is defined by interface context. At these microhabitats, the same pollutant can function as a carbon source, a signal, or a chronic stressor, activating distinct microbial programs such as attachment, biofilm formation, detoxification, storage allocation, community cooperation and secondary metabolite production. We propose that interface-driven trigger responses provide a unifying explanation for the high variability observed in marine biodegradation outcomes, the frequent uncoupling of degradation from growth, and the emergence of interaction-driven consortia. By reframing marine pollutant transformation as a spatially encoded, trigger-mediated process, this Perspective highlights how microbial metabolism, ecological interactions and valorization pathways are jointly controlled at ocean interfaces. Finally, we outline key predictions and experimental priorities needed to integrate microscale interface processes into models of marine pollution fate and carbon cycling.
Buoyancy control in phytoplankton is a fundamental process for oceanic cycling of elements. Previous studies suggested that the ionic composition of cell saps (cytosol plus vacuole) contribute to buoyancy control by plankton cells: replacement of heavy ions in seawater by lighter ions in the cell saps could lead to enough positive buoyancy to keep the cells within the euphotic zone. This hypothesis has stimulated a number of experimental and theoretical investigations on the densities of cell saps. Unfortunately, these studies often neglected the importance of electric charge balance within cells which renders results largely useless for estimating mass densities. In this paper I review previous work with respect to charge balance and correct some of the ensuing errors in a well-cited (> 100 citations) paper in this field. Rather than estimating cell sap densities from ionic compositions I propose an alternate approach based on salt compositions which allows more robust scaling for solutions of high ionic strength. This method will be applied to a data set that needs only slight adjustment (within measurement uncertainty) to obey charge balance.
Subterranean estuaries modify nutrient transport to coastal waters through submarine groundwater discharge (SGD). In marginal seas with restricted circulation, both surface water and groundwater nutrient inputs may trigger and sustain eutrophication. Here, we use 224Ra to quantify total SGD and related nutrient inputs from beaches across 5 Baltic countries. Groundwater sampled from 17 transects was brackish, hypoxic, and nutrient rich relative to river waters. Nutrient and radium concentrations were highest at the shoreline and rapidly decreased with distance offshore, indicating a terrestrial nutrient source at the land-ocean interface. Groundwater DIN and DIP concentrations were on average ∼4 and ∼21 times higher, respectively compared to nearby rivers. Overall, mean nutrient ratios across all river and groundwater samples deviated from the Redfield ratio, with elevated DIN: DIP (>16:1) and DSi:DIP (>15:1) but reduced DIN:DSi (<16:15). An SGD source was not needed to close the 224Ra mass balance in ∼50% of the transects. Total SGD inputs could be estimated using 224Ra mass balances in ∼50% of sampled transects resulting in mean total SGD of 6 ± 10 m3/day per meter of shoreline across 8 beach-transects. Despite large natural variability and uncertainties, extrapolating beach-scale total SGD-derived fluxes to the 8000 km Baltic shoreline led to total SGD nutrient fluxes comparable to river inputs. Total SGD derived from 224Ra budgets, but not fresh SGD derived from hydrological models, seems to be a relevant but still uncertain component of Baltic coastal nutrient budgets.
The Yellow River, one of the world's largest river systems, has experienced substantial anthropogenic modifications over recent decades, posing growing threats to the health of coastal ecosystems. However, phytoplankton responses to these changes remain poorly understood due to limited mechanistic insights and inconsistencies across previous studies. Here, we conducted salinity-gradient sampling in the estuarine region to examine the influence of Yellow River discharge on phytoplankton communities during dry (April 2023) and wet (August 2022) seasons. By measuring chlorophyll-a, lipid biomarkers (brassicasterol for diatoms, dinosterol for dinoflagellates), and environmental parameters (temperature, nutrients, total suspended matter-TSM) in the surface seawater along salinity gradients, this study revealed that revealed that the August 2022 cruise-which coincided with the wet season-showed 81% higher river discharge, 47% higher total phytoplankton biomass, and 185% higher diatom biomass compared to the April 2023 cruise. These differences are consistent with enhanced riverine influence during the wet season. Along the salinity gradients, total phytoplankton and diatom biomass in low-salinity regions were 114–295% higher than those in high-salinity regions. Diatoms dominated under high-nutrient and high-TSM conditions, whereas dinoflagellates were more abundant in low-nutrient and low-TSM environments. Correlation analyses indicated that riverine inputs played a dominant role in regulating the distributions of diatom and dinoflagellate by modulating nutrient supply and light availability. Our observations are broadly consistent with key aspects of the Margalef r/K succession framework, suggesting its utility as an interpretive framework for understanding diatom-dinoflagellate patterns in this human altered estuary.
The anthropogenic aerosols transport inorganic and organic nutrients and deposit over the surface ocean. The highest levels and rate of increase in anthropogenic aerosols were reported over the Bay of Bengal, compared to the elsewhere in the globe, and it is hypothesised that their deposition may enhance nutrients and surface primary production in the Bay of Bengal. To examine this, aerosol samples were collected at a monthly interval, along with primary production incubation experiments in the coastal Bay of Bengal, off Visakhapatnam. The concentrations of nitrate (0.02–0.35 μmol m−3), ammonium (0.01–1.31 μmol m−3), and dissolved organic nitrogen (DON; 0.02–0.32 μmol m−3) showed significant temporal variability in the aerosols. The atmospheric nitrogen mainly originated from biomass burning, vehicular and industrial emissions, with large seasonality associated with seasonal reversing winds. Dry depositional flux of inorganic nitrogen was higher in the coastal compared to the offshore Bay of Bengal, supporting 14–42% (mean of 30 ± 9%) of primary production in the surface ocean. This contribution is notably higher than global estimates in other marine regions, associated with the highest aerosol optical depth, highlighting the regional importance of atmospheric nutrient inputs. These results provide strong evidence that atmospheric deposition is a significant nutrient source influencing coastal productivity in the central east coast of India with large seasonality driven by different sources. Therefore, it is strongly recommend to incorporate the atmospheric sources of nitrogen in the numerical models for accurate simulation of nitrogen and carbon cycling in the north Indian Ocean.
Understanding how dissolved organic matter (DOM) and particulate organic matter (POM) are coupled across estuarine mixing zones is central to resolving carbon and nutrient processing in mangrove wetlands. Here we tracked DOM and POM composition and their phase exchange along a salinity gradient in Dongzhai Harbor (Hainan, China) using excitation-emission matrix fluorescence spectroscopy with PARAFAC, complemented by fluorescence indices and partial least squares structural equation modeling (PLS-SEM). Fluorescence intensities of both DOM and POM declined with increasing salinity, consistent with freshwater-seawater mixing and dilution. Four PARAFAC components were resolved in each phase, spanning terrestrial/marine humic-like signals. Terrigenous humic components (C1-C3) decreased seaward, whereas the marine/autochthonous component (C4) increased, indicating a source transition across the mixing gradient. Optical metrics further suggested higher photoreactivity and biodegradability for DOM relative to POM, with implications for nutrient regeneration. Importantly, phase exchange was asymmetric, with a stronger apparent POM-to-DOM linkage than the reverse pathway, suggesting that particulate reworking may replenish the DOM pool, whereas DOM-to-POM transfer may temporarily reduce the immediate bioavailability of dissolved fluorescent components. PLS-SEM supported significant hydrochemical controls on both organic matter composition and DOM-POM exchange pathways. Overall, our fluorescence-based tracking highlights estuarine mixing as a coupled physical-biogeochemical filter that regulates the phase distribution and reactivity of organic matter in mangrove-dominated estuaries.
Iron-bound phosphorus (Fe-P) in aquatic sediment could be a major contributor to eutrophication. However, the complex mineralogical forms of Fe-P, such as vivianite, amorphous Fe(III)-phosphate (Am-FePO4), and Fe (III)-(oxyhydr)oxide-bound P, exhibit distinct stabilities and release potentials that are poorly differentiated by existing sequential extraction procedures. This study introduces a highly selective sequential extraction protocol that utilizes 2 g/L 2,2 '-bipyridine +0.1 M KCl (Bipy-KCl), 10 g/L xylenol orange (XO), and 2 g/L 2,2 '-bipyridine +0.22 M bicarbonate-dithionite (Bipy-BD) to quantitatively distinguish these key Fe-P species. The protocol was optimized under controlled conditions (pH, temperature, and shaking frequency), and achieved high recovery rates (e.g., 99.5% for vivianite, 89.5% for Am-FePO4, and 87.7% for Fe(III)-(oxyhydr)oxide-P) with minimal interference from non-target phases (e.g., Al-P and Ca-P). Compared with other extraction protocols, the new protocol can selectively target and quantify vivianite and Am-FePO4. When applied to coastal sediment from the Pearl River Estuary, the new protocol revealed that Fe-P constitutes 47.2-64.3% of total P (TP), of which Am-FePO4 was the primary species (22.0-28.9% of TP) that persisted even in deeper sediment layers, underscoring its crucial role in P dynamics. This method overcomes the limitations of conventional protocols, which fail to resolve the heterogeneity of Fe-P. In addition, it offers essential technical and mechanistic insights into P retention, mobility, and long-term stability in coastal sediments, thereby supporting more effective strategies for managing eutrophication in marine and estuarine environments.
Mercury (Hg) fractional composition at the base of aquatic food webs plays a critical role in controlling its mobility, bioavailability, and trophic transfer in marine ecosystems. This study investigated total mercury (THg) concentrations and fractional Hg composition in major primary producer compartments of Puck Bay (southern Baltic Sea), including phytoplankton, epiphyton, epilithon, macroalgae, and vascular plants. Suspended particulate matter and surface sediments were also analyzed for comparison. THg concentrations differed significantly among compartments, with markedly higher mean levels in microproducers (88.9 ng g-1) than in macro-producers (9.3 ng g-1). The highest concentrations were observed in epiphyton and phytoplankton, indicating that these compartments may represent important interfaces for Hg incorporation into coastal food webs. Thermodesorption analysis revealed a strong predominance of labile Hg forms (HgF1 + HgF2 + HgF4), which accounted for more than 89% of THg in biological matrices. The organic-bound fraction HgF2 dominated the Hg pool, suggesting strong association with recently produced biomass and potentially including methylmercury (MeHg); however, MeHg was not directly quantified in this study. Environmental parameters, particularly temperature and redox potential, were identified as important factors influencing Hg fraction composition and lability. These findings indicate that microproducers may play an important role in controlling Hg partitioning, mobility, and potential trophic transfer in shallow coastal food webs.
Porewater geochemistry at submarine groundwater discharge (SGD) sites reflects both fluid mixing and diagenetic reactions, though separating these contributions remains challenging. The Lofoten-Vester & aring;len (LV) margin offshore Norway hosts a deep-water SGD system where glacially recharged groundwater discharges through two submarine canyons (north and south) at similar to 800 m depth, providing a unique setting to disentangle these processes. The study applied multivariate analysis to 75 porewater samples characterized by ten geochemical parameters. Multiple Factor Analysis (MFA) identified two statistically separable axes of variability: a primary mixing gradient (47% variance) separating groundwater and seawater end-members, and a secondary diagenetic axis (22% variance) driven by manganese reduction and alkalinity production. The orthogonality of these axes demonstrates that the geochemical signatures of mixing and diagenesis can be statistically separated. The separation persists even though mixing ultimately regulates reactant supply to the reaction zones. Although the two canyons showed no difference in mixing intensity (p = 0.77), they differed significantly in diagenetic activity (p < 0.001), with the south canyon exhibiting enhanced alkalinity production. Fuzzy C-means (FCM) clustering identified three porewater facies-seawater-dominated, active reaction zone, and groundwater-dominated-stratified by depth, with 16% of samples showing transitional characteristics. Site-specific analysis revealed three distinct discharge modes: steady diffusive mixing, focused high-flux conduits, and compressed reaction zones. This multivariate framework separates physical transport from biogeochemical transformation, offering a transferable approach for characterizing complex SGD environments globally.
Coastal ecosystems are biogeochemically complex, shaped by intense organic matter cycling and shifting redox gradients driven by both in situ processes and benthic-pelagic interactions. The influence of these dynamics on the cycling of rare earth elements (REE) remains unclear, which is concerning given the growing input of potentially harmful anthropogenic REE into coastal environments. The main objective of this study was to investigate the biogeochemical cycling of natural and anthropogenic REE in a highly productive coastal system and to identify the key processes controlling their distribution and transformation. Using a controlled mesocosm experiment simulating the intertidal Wadden Sea, we show that REE dynamics are strongly mediated by the interplay of sediment geochemistry, redox gradients, and phytoplankton bloom development. Shifts from suboxic to euxinic conditions enhance REE sequestration through redox-dependent transformations, highlighting the importance of mineral phases and organic matter dynamics in controlling REE behavior. The REE fractionation was shifting significantly with respect to phytoplankton growth and associated dissolved organic matter. Sediments act as major reservoirs for both natural and anthropogenic REE, controlling their retention and mobilization. Natural REE and anthropogenic samarium and lanthanum were effectively transferred to and retained in sediments, with transfer capacities dependent on carrier phase pool properties and retention strongly depending on sediment composition and redox conditions. In contrast, anthropogenic gadolinium behaved mostly conservatively, non-conservative enrichments were only present at times of highest bloom intensities and organic matter content, suggesting that benthic-derived reactive anthropogenic Gd was stabilized by organic carrier phases inhibiting its transfer to the particulate pool. Sediment disturbances simulating storm-induced resuspension resulted in the release of natural and anthropogenic REE into the water column, temporarily altering their availability. Overall, our findings highlight that coastal zones act as dynamic alteration and remediation interfaces for REE, with sediment retention, biotic processes, and episodic remobilization events controlling their export to the open ocean.
River-dominated ocean margins are carbon processing hot spots, but the spatiotemporal dynamics of the benthic biogeochemical cycling of Mn, Fe, and S under different discharge regimes and the impact of these variations on carbon cycling are not well characterized. Here, a comparative analysis of pore water profiles of redox-sensitive chemical species reveals the biogeochemical restructuring that occurred over a summer-spring-summer sequence on the Louisiana continental shelf in the northern Gulf of Mexico. Increases in depth-averaged pore water inventories, including exceptionally high Mn(II) concentrations, were associated with high river discharge and sediment inputs, suggesting that Mn reduction plays an important role in Louisiana continental shelf sediments, except during below-average discharge periods in summer when iron and sulfate reduction are promoted. The river effect was greatest and most consistent proximal to the Mississippi River and extended at least similar to 60 km westward when high winds and discharge enhanced plume dispersal, whereas the area where seasonal bottom water hypoxia often develops remained largely isolated from any riverine influence. A river imprint on biogeochemical patterns was also observed south of Atchafalaya Bay, suggesting the area periodically receives inputs from the bay throughout the year. Results demonstrate that Mn(II) often traces these hydrodynamic processes and that river loads of Mn (oxyhydr)oxides buffer the redox state of shelf sediments and moderate the biogeochemical response to low discharge and hypoxia. Stores of river-derived Mn may therefore function as an environmental redox capacitor. Based on the patterns observed between the two summers, a period of Mn reduction may persist for similar to 20 days after Mississippi River discharge subsides to below the threshold for net mobilization of sediment to the shelf.
Multi-parameter analysis of chemical conditions in sediment microenvironments is of great importance to highlight key biogeochemical processes and conditions at interfaces. Yet, many imaging approaches only allow for determining one analyte at a time. Here we present a new planar optical sensor combination to investigate simultaneously the two-dimensional distribution and dynamics of sediment pore-water chemical conditions at a millimeter resolution using planar optodes (PO) of pH and O2 in combination with Diffusive Equilibrium in Thin film gels (DET) for mapping dissolved iron (Fe2+/Fe3+) or nitrite-nitrate. We used the combined sensor imaging approach to simultaneously map pH/dissolved iron and O2/nitrite-nitrate distributions in the rhizosphere of the seagrass Zostera marina. This combination enabled the imaging of seagrass rhizosphere acidification via H+ ions leakage or, indirectly, via enhancement of microbial processes upon O2 leakage, and the remobilization of iron and nitrogen.
Copper (Cu) speciation in seawater is frequently measured using competitive ligand exchange cathodic stripping voltammetry (CSV) or anodic stripping voltammetry (ASV), but no comprehensive comparison of the results obtained by both techniques have been done to date. In this study, Cu complexation by the model organic matter Suwannee River fulvic acid (SRFA) was studied using these two techniques. The complexation capacity was similar for both techniques, but the conditional stability constants differed markedly, with log K'Cuvalues of 7.65 +/- 0.17 for ASV, and up to 11.0 +/- 0.2 for CSV. As a result, Cu speciation at 10 nM Cu and 5 mg L-1 SRFA, is predicted as 90.00% or 99.99% organically complexed Cu, respectively, for ASV and CSV, resulting in 100 times different inorganic Cu concentrations. The complexation of Cu by SRFA was also studied using partial ultrafiltration, and the observed degree of complexation was closer to that predicted by ASV. A thorough analysis of possible analytical bias that could have influenced the results was done, as well as a literature review of studies that have compared ASV or CSV with another speciation technique. On this basis, it was inferred that equilibrium methods based on ligand competition (such as CSV and competitive equilibration with MnO2) predict much higher Cu complexation by organic ligands than any other methods, which may be explained by kinetic limitations of these techniques. Other methods, such as ASV or cation-exchange resins, may underestimate complexation but, in turn, provide biologically meaningful results according to the literature.
The East China Sea (ECS) is one of the world's largest marginal seas and has a complex water mass structure. However, quantitative knowledge of its water mass structure in autumn remains limited. In this study, dissolved rare earth elements (REEs) from the GEOTRACES GP06-CN cruise (October 2015) were used as tracers to resolve water masses. REEs showed strong correlations with salinity (P < 0.0001) at salinities >25, indicating semi-conservative behavior. A multi-endmember mixing model incorporating potential temperature, salinity, and heavy REEs revealed that the Taiwan Warm Current (TWC) and Kuroshio Surface Water dominated in autumn, contributing 50 +/- 30% and 20 +/- 23%, respectively. Kuroshio Subsurface Water was identified, characterized by upwelling at the shelf edge (similar to 28.7 degrees N) and weak northwestward intrusion onto the shelf. Above the euphotic layer, Changjiang Diluted Water (CDW) and TWC were the major nutrient sources, with CDW supplying 24 +/- 33% of dissolved inorganic nitrogen (DIN) and TWC supplying 45 +/- 27% of dissolved inorganic phosphorus (DIP). Based on Delta DIP (the difference between observed values and predicted from water mass contributions) and the Redfield C/P ratio, autumn net community production was estimated at 0.18 +/- 0.13 g C/m(2)/day (range: 0.02-0.43 g C/m(2)/day). A sensitivity analysis suggests that TWC intrusion reduced phosphorus limitation by 47 +/- 24% (range: 9-61%). Low-oxygen waters were observed in bottom waters on the inner shelf (CDW: 39 +/- 5%) and middle shelf (TWC: 45 +/- 1%), with oxygen consumption along the TWC pathway estimated at 2.39 +/- 2.89 mu mol/kg/day. This study highlights the structure and mixing of autumn water masses in the ECS, demonstrating the key roles of TWC and CDW in nutrient transport and the development of low-oxygen conditions.
Dissolved organic nitrogen (DON) is likely comprised of multiple compound classes with varying reactivities and turnover times, resulting in numerous roles in ocean biogeochemistry. Here, we present measurements of total DON and solid-phase extracted DON (SPE-DON) concentrations and delta 15N values from sampling sites across the global ocean. An optimized SPE protocol was developed to maximize total DON recovery, with a recovery of 41.0 +/- 9.4% for surface DON across the global ocean and 56.6 +/- 11.2% for deep water DON from the Sargasso Sea. SPE-DON concentrations were 1.8-1.9 mu M across most sampling sites, in contrast to greater variation in total DON concentration (4.0-5.9 mu M). However, in the equatorial upwelling zones, SPE-DON concentrations were slightly (0.3-0.4 mu M) higher than in other regions. The delta 15N of total DON in surface waters correlated well with the delta 15N of nitrate supplied to the euphotic zone from the subsurface. SPE-DON delta 15N was also correlated with nitrate delta 15N, but SPE-DON delta 15N values were confined to a narrower range compared to those of total DON. The combined concentration and delta 15N data indicate that while SPE-DON is biased toward long-lived DON, it still retains some reactive components introduced through regional inputs in the upper ocean, and even some of its longer-lived components may be labile on the timescales of deep ocean circulation. The longer average turnover time of SPE-DON suggests that greater molecular polarity and/or charge directly increase or are otherwise correlated with the biogeochemical lability of different DON pools.
Global coral coverage has declined significantly, and environmental pollution is a factor that cannot be ignored. This study presents a comprehensive investigation into the bioaccumulation of rare earth elements (REEs) and other trace elements in corals from two distinct coral reef areas in the South China Sea (SCS). Results revealed that the concentrations of trace elements in coastal corals (tissues + skeleton) from Sanya were generally higher than those in offshore Xisha corals, with median REEs values of 1.35 mu g g(-1) dw and 0.22 mu g g(-1) dw, respectively. The predominant trace elements in corals from both regions were B, Ni, Zn, and Cu, while the dominant REEs plus yttrium (REY) were Ce, La, Y, and Nd. Trace metal (including REY) concentrations were significantly higher in massive, densely structured Porites than in branching Pocillopora and Acropora. The standardized coral REY patterns showed the depletion of heavy REEs and enrichment of light REEs bioaccumulation, as well as positive anomalies of La and Eu, weak negative anomaly of Ce and high Y/Ho ratio (>50). The results demonstrated that REY accumulated more easily from seawater than from sediment. This study indicates that the geochemical characteristics of REY in corals can serve as a reliable alternative proxy to assess the environmental quality of local seawater and sediment, and may provide new insights into the coral bioaccumulation patterns of trace metals, including REEs.
