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Dissolved organic matter (DOM) plays an important role in sustaining ecosystem services of mangrove forests through well-described biogeochemical and ecological functions. This study was conducted in the Fukido River (Ishigaki Island, Japan) to better understand the seasonal and episodic changes in DOM concentration and composition in a subtropical mangrove system. Water samples were collected seasonally along a headwater–mangrove–sea transect on 10 occasions from September 2014 through June 2016. DOM was fractionated based on hydrophobicity into two fractions (hydrophobic and hydrophilic) and also analyzed by excitation-emission matrix spectroscopy combined with parallel factor analysis (PARAFAC). Although seasonal changes in DOM concentration and composition were not observed, both hydrophobic and hydrophilic DOM concentrations and levels of the identified three PARAFAC components clearly increased during a typhoon event. It is suggested that episodic increases in freshwater input due to a typhoon caused enhanced leaching of DOM from mangrove litter and dissolution of mangrove soil organic matter (SOM), which was otherwise retained in the mangrove soil by salinity-induced aggregation. The aggregation–dissolution properties of SOM are crucial in determining the magnitude of DOM outwelling and possibly SOM accumulation rate by enhancing advective DOM exchanges. Future studies are needed to evaluate the size of the carbon pool and outwelling of DOM after classifying mangrove forests based on the hydrological regime that influences biogeochemical conditions in the forests. Japan, Okinawa, Ishigaki, Fukido. • Hydrology had a great impact on mangrove river DOM concentration and composition. • Episodic increases in freshwater input increased mangrove-derived DOM. • Dissolution of SOM may be important in determining magnitude of DOM outwelling. • Episodic freshwater input is a key for better understanding mangrove elemental cycles. [ABSTRACT FROM AUTHOR]

The organic-Fe association in Scottish freshwater rivers has received little attention compared with in the estuarine mixing zone. We collected 201 water samples from rivers and lakes in Scotland across different sampling years and seasons. Relationships among the hydrophobic (HPO) fraction of dissolved organic matter (DOM), specific UV absorbance (SUVA254), and dissolved metals (Al and Fe) were examined to better understand their co-transportation in Scottish waters. The average DOM, HPO fraction, Fe, and Al concentrations for all the samples co-varied and were lower during winter than during summer. There was a strong positive correlation between DOM and HPO fraction concentrations (R2 = 0.99, p < 0.0001). A significant positive correlation was also found between the HPO fraction and Fe and Al concentrations. The regression slope indicating the overall relationships between the HPO fraction and Fe concentrations differed by as much as 12 times depending on both the sampling period and the river. These slope differences were not significantly determined by the chemical structures of DOM, SUVA254, or Al and Cu concentrations. These results suggest that the Fe transport capacities vary among the Scottish rivers because of other factors such as seasonal effects (temperature and the level of water table) and a suspended solid concentration in the water column. [ABSTRACT FROM AUTHOR]

An anion exchange resin, diethylaminoethyl (DEAE) Sepharose®, was utilized for the isolation of dissolved organic matter (DOM) from fresh waters as an alternative to the discontinued DEAE cellulose. We used the following two chemically distinct model DOM samples to determine the optimized adsorption conditions onto DEAE Sepharose: the International Humic Substances Society's standard samples, Suwannee River Fulvic Acid (FA) and Pony Lake FA. The optimized conditions consisted of the following: a contact time of 1 h (with shaking), a resin dosage of 1 ml mgC−1, and a dissolved organic carbon (DOC) concentration range of 1–100 mgC l−1. In addition, we examined the distribution of the DOM fractions extracted with DEAE Sepharose and DAX-8 from Lake Biwa (Japan) and Scottish river DOM samples. The majority of DOM (70% and 65%) was extracted by both of the resins. As indicated by 1H NMR, the evapo-concentrate (bulk DOM), the DEAE Sepharose fraction and the DAX-8 fraction from the Scottish DOM sample had substantial similarity in their proton distributions, while those of a clear-colored, low DOC sample (Lake Biwa) showed different NMR spectra. These findings highlight a need to pay more attention to the extraction selectivity of resins for experimentally 'challenging' samples. [ABSTRACT FROM AUTHOR]

Although mangrove forests are one of the most well-known soil organic carbon (SOC) sinks, the mechanism underlying SOC accumulation is relatively unknown. High net primary production (NPP) along with the typical bottom-heavy biomass allocation and low soil respiration (SR) have been considered to be responsible for SOC accumulation. However, an emerging paradigm postulates that SR is severely underestimated because of the leakage of dissolved inorganic carbon (DIC) in groundwater. Here we propose a simple yet unique mechanism for SOC accumulation in mangrove soils. We conducted sequential extraction of water extractable organic matter (WEOM) from mangrove soils using ultrapure water and artificial seawater, respectively. A sharp increase in humic substances (HS) concentration was observed only in the case of ultrapure water, along with a decline in salinity. Extracted WEOM was colloidal, and ≤70% of it re-precipitated by the addition of artificial seawater. These results strongly suggest that HS is selectively flocculated and maintained in the mangrove soils because of high salinity. Because sea salts are a characteristic of any mangrove forest, high salinity may be one of mechanisms underlying SOC accumulation in mangrove soils. [ABSTRACT FROM AUTHOR]

A nonionic macroporous resin, Amberlite ® XAD-8, or its substitute, Supelite™ DAX-8, is used when isolating or quantifying aquatic humic substances (AHS). However, the effect of salinity on the adsorption behavior of AHS onto the resin is yet to be confirmed, rendering the possibility of salinity-induced changes in the values of quantified amounts or characteristics of AHS obtained from a salty system. To verify the results of quantification and isolation of AHS using the resin in different salinity systems, the effect of salinity on such quantitative analyses of AHS has been examined. It has been concluded that the salinity effect is in general trivial and will not hinder comparison of results regardless of sample solution salinity. [ABSTRACT FROM AUTHOR]

Aquatic humic substances (AHSs) are major constituents of dissolved organic matter (DOM) in freshwater bodies. We performed quantitative analyses of AHSs in Japanese lake and wetland waters, focusing mainly on clear waters with low carbon contents, by using a resin isolation technique based on the carbon concentration in the AHSs of each sample. The proportion of AHS to DOM in the clear waters ranged from 38.4 to 64.1 %; these proportions are lower than those widely assumed for freshwater of 20-80 %. Moreover, the proportions of AHSs in DOM were not constant, so regression analysis cannot be used to predict the AHS concentration from the DOM concentration. Thus, AHS and DOM concentrations must be determined separately for each water sample. [ABSTRACT FROM AUTHOR]

Aquatic humic substances (AHS) have been intensively studied because they have a number of significant biogeochemical functions in water chemistry. In order to evaluate such functions, which are regulated by their quantity and quality, a quantitative understanding of AHS is necessary. This study shows the seasonal and vertical changes in the AHS concentration in Lake Biwa, the largest monomictic lake in Japan, over 4 years. The concentration of dissolved organic matter carbon (DOM-C) and AHS carbon (AHS-C) ranged from 0.85–1.6 mg C/l and 0.32–0.71 mg C/l, respectively. The percentages of AHS-C in the DOM-C ranged from 32% to 65%. At all depths, the DOM-C decreased and the AHS-C slightly increased or remained at nearly the same level, suggesting that the quality of the water may have changed mainly due to changes in the composition of the organic matter. Although the AHS-C to DOM-C ratio fluctuated and had no seasonal tendency, the average AHS-C ratio increased during the 4 year monitoring period. Because AHS can influence the entire food web starting with phytoplankton, unraveling the mechanism by which they accumulate remains an important goal. [ABSTRACT FROM AUTHOR]

Around one-third of the world's most carbon-rich ecosystems, mangrove forests, have already been destroyed in Thailand owing to coastal development and aquaculture. Improving these degraded areas through mangrove plantations can restore various coastal ecosystem services, including CO2 absorption and protection against wave action. This study examines the biomass of three coastal mangrove plantations (Avicennia alba) of different ages in Samut Prakarn province, Central Thailand. Our aim was to understand the forest biomass recovery during the early stages of development, particularly fine root biomass expansion. In the chronosequence of the mangrove plantations, woody biomass increased by 40% over four years from 79.7 ± 11.2 Mg C ha-1 to 111.7 ± 12.3 Mg C ha−1. Fine root biomass up to a depth of 100 cm was 4.47 ± 0.33 Mg C ha−1, 4.24 ± 0.63 Mg C ha−1, and 6.92 ± 0.32 Mg C ha−1 at 10, 12, and 14 year-old sites, respectively. Remarkably, the fine root biomass of 14-year-old site was significantly higher than those of the younger sites due to increase of the biomass at 15–30 cm and 30–50 cm depths. Our findings reveal that the biomass recovery in developing mangrove plantations exhibit rapid expansion of fine roots in deeper soil layers. [ABSTRACT FROM AUTHOR]

The molecular composition of soil organic matter (SOM) is important in understanding the elemental cycling of mangrove forests. The aim of the study was to investigate the SOM compositional changes with time in a mangrove mineral soil, which is rarely reported in the literature. A 1-m soil core was collected in the subtropical Fukido River mangrove forest (Okinawa, Japan) and separated into 25-cm sections. Humic acids (HA) and fulvic acids (FA) were extracted from the soil, and their relative abundance and chemical characteristics were analyzed by elemental analysis, stable carbon analysis, spectrophotometric measurement, liquid-state 13C nuclear magnetic resonance spectroscopy, and radiocarbon dating. HA exhibited a clear depth trend in the chemical characteristics, with lower H/C and N/C ratios, higher aromaticity, and higher phenolic 13C NMR peaks in the deeper sections. FA did not show such a clear depth trend, and was enriched in O-containing functional groups. A combination of radiocarbon dating and structural analysis of HA indicated that the rapid structural changes in HA (H/C and N/C) occurred during the first years–decades, followed by a gradual change over a time scale of several hundred (~500) years, and these structural changes were best explained by an increase in the relative phenolic C contents. These results show that mangrove SOM is a complex mixture of organic fractions with different biogeochemical reactivities. Image 1 • We assessed SOM compositional changes with time (~500 yrs) in a mangrove mineral soil. • The compositional changes occurred as first rapid changes followed by later slow ones. • An increase in the relative phenolic C contents best explained these two-step changes. • Mangrove SOM is a complex mixture of organic fractions with different reactivities. [ABSTRACT FROM AUTHOR]

This study characterized dissolved organic matter (DOM) obtained from 47 lakes and 2 streams on ice-free areas at Lützow-Holm Bay and Amundsen Bay in East Antarctica (n = 74), where few biogeochemical studies have been historically conducted. Samples were analyzed for basic water chemistry and by resin fractionation, UV–vis spectroscopy, and excitation emission matrix spectroscopy combined with parallel factor analysis (EEM-PARAFAC). Salinity of the samples ranged very broadly from fresh to hypersaline as a result of evaporative concentration. There was a clear positive correlation between log-salinity and the spectral slopes of DOM (S 275–295), an indicator of photodegradation. Thus, we interpreted the correlation as a progression of photodegradation by prolonged water retention time. Of the identified seven PARAFAC components, three ubiquitous humic-like components decreased as photodegradation progressed, while a photorefractory UVC humic-like component increased its relative abundance. A non-humic component, traditionally defined as Peak N, did not show a trend depending on photodegradation, and its level was high in nutrient-rich lakes, presumably due to high in-situ production. We found robust correlations between the relative abundance of the ubiquitous humic-like components and that of the Peak N component in the bulk DOM irrespective of water types or ice-free areas. We proposed there were common processes that generated the ubiquitous humic-like components from the Peak N component in the Lützow-Holm Bay and Amundsen Bay lakes and streams, such as bacterial processing of primary production-derived DOM and photochemical transformation of microbial DOM. • Water chemistry and DOM composition of 47 Antarctic lakes and 2 streams were studied. • Photodegradation substantially influenced the distribution of fluorescent DOM. • Among seven PARAFAC components, four were humic-like components. • Photochemical generation of ubiquitous humic-like fluorophores was proposed. [ABSTRACT FROM AUTHOR]

Mangrove‐fringed estuaries are intertidal ecosystems discharging significant amounts of dissolved organic matter (DOM) into coastal oceans. DOM in these ecosystems is derived from autochthonous production, fluvial input, and mangrove porewater outwelling; however, differentiating between these sources remains challenging. Our incomplete understanding of the biogeochemical factors controlling DOM dynamics and its relationship with nutrient and trace metal cycling still hinders the formulation of elemental budgets in coastal environments. Here, we relate the DOM composition in a mangrove‐fringed estuary in North Brazil (Amazonia) to the redox conditions at the formation sites. We combined molecular DOM analyses via ultrahigh‐resolution mass spectrometry (FT‐ICR‐MS) with parallel factor analysis of excitation–emission fluorescence matrices (EEM‐PARAFAC), nutrient and redox‐sensitive trace metal analyses. During low tide, the influx of oxygen‐depleted porewater carried terrigenous DOM, inorganic nutrients, and trace metals into the mangrove‐fringed creeks. Precipitation of metal(hydr)oxides and microbial turnover controlled nutrient and trace metal dynamics in the estuary. The highest inorganic nitrogen concentrations within the upper mangrove‐fringed estuary indicated outwelling from mangrove sediments as an essential source. Phosphate concentrations were highest within the lower mangrove‐fringed estuary, where available phosphate likely exceeded precipitation with iron(hydr)oxides. We tracked the DOM transport to the coastal ocean using a novel molecular index derived from sulfidic porewater (ISuP). Outwelling of recalcitrant DOM from mangrove habitats is relevant in the context of blue carbon storage. Therefore, applying our molecular proxy (ISuP), together with trace‐metal and optical DOM analyses, is a powerful approach for differentiating contributions of diverse DOM sources in highly complex coastal ecosystems. [ABSTRACT FROM AUTHOR]

Humic substances (HS) are the primary constituents of dissolved organic matter (DOM) and play pivotal roles in aquatic systems. Optical indices of DOM, such as specific UV absorbance (SUVA254), the fluorescence index (FI) and biological index (BIX), have gained wide interest because of their ease of use. In this study, we explored the relationship between HS and the indices in the Trat River Basin (eastern Thailand) from headwaters to the river mouth through the distinct dry and rainy seasons to examine whether changes in index values reflect variability in the relative contribution of HS to DOM, or %HS. The results show that %HS and the indices did not exhibit significant linear relationships (FI and BIX, P > 0.05), or the relationships changed seasonally (SUVA254). However, analyzing the indices versus %HS did show clear DOM composition changes by season with more humic-like or terrestrial material in the rainy season. Relationships between DOM and dissolved iron (dFe) concentrations were also explored. Separating the relationships of DOM versus dFe into HS versus dFe and non-HS versus dFe provides us the opportunity to better understand which fraction contributes more to dFe mobilization. The results indicate stronger positive linear relationships between HS and dFe concentrations independent of river tributary. Overall, this study highlights the importance of quantifying HS for the study of DOM dynamics or compositional changes along a river transect as well as for DOM-induced iron mobilization. [ABSTRACT FROM AUTHOR]