Time resolved data unveils the complex DOM dynamics in a Mediterranean river.

In this study, dissolved organic carbon (DOC) data and optical properties (absorbance and fluorescence) of DOM, weekly collected in the Arno River for 2 years, are used to investigate the main processes determining DOM temporal dynamics in a small Mediterranean river, with torrential hydrology and medium-high human impact, and to quantify the contribution of this river to Med Sea carbon budget. A clear seasonal cycle of DOM, with DOC values ranging between 170 and 490 µM, was observed. Optical properties indicates that DOM quality in the river is different depending on the season; terrestrial humic-like substances prevail in winter, when discharge and floods are the main drivers of DOM concentration and quality, whereas autochthonous protein-like substances prevail in spring and summer, when biological processes dominate. Our results provide a robust estimate of the DOC flux to the Med Sea (9.6 · 109 g DOC yr-1) and of its range of variability (12.95 · 109-5.12 · 109 g DOC yr-1). The 80% of this flux was generally delivered during autumn/winter with significant amounts ascribed to single flood events (up to 26% in 2014). This study, by providing a rich dataset on water quantity and quality and by quantifying the importance of the hydrological regime on DOC transport, represents an important step toward a quantitative modeling of the Arno River.

Organic Copper Speciation by Anodic Stripping Voltammetry in Estuarine Waters With High Dissolved Organic Matter.

The determination of copper (Cu) speciation and its bioavailability in natural waters is an important issue due to its specific role as an essential micronutrient but also a toxic element at elevated concentrations. Here, we report an improved anodic stripping voltammetry (ASV) method for organic Cu speciation, intended to eliminate the important problem of surface-active substances (SAS) interference on the voltammetric signal, hindering measurements in samples with high organic matter concentration. The method relies on the addition of nonionic surfactant Triton-X-100 (T-X-100) at a concentration of 1 mg L-1. T-X-100 competitively inhibits the adsorption of SAS on the Hg electrode, consequently 1) diminishing SAS influence during the deposition step and 2) strongly improving the shape of the stripping Cu peak by eliminating the high background current due to the adsorbed SAS, making the extraction of Cu peak intensities much more convenient. Performed tests revealed that the addition of T-X-100, in the concentration used here, does not have any influence on the determination of Cu complexation parameters and thus is considered "interference-free." The method was tested using fulvic acid as a model of natural organic matter and applied for the determination of Cu speciation in samples collected in the Arno River estuary (Italy) (in spring and summer), characterized by a high dissolved organic carbon (DOC) concentration (up to 5.2 mgC L-1) and anthropogenic Cu input during the tourist season (up to 48 nM of total dissolved Cu). In all the samples, two classes of ligands (denoted as L1 and L2) were determined in concentrations ranging from 3.5 ± 2.9 to 63 ± 4 nM eq Cu for L1 and 17 ± 4 to 104 ± 7 nM eq Cu for L2, with stability constants logK Cu,1 = 9.6 ± 0.2-10.8 ± 0.6 and logK Cu,2 = 8.2 ± 0.3-9.0 ± 0.3. Different linear relationships between DOC and total ligand concentrations between the two seasons suggest a higher abundance of organic ligands in the DOM pool in spring, which is linked to a higher input of terrestrial humic substances into the estuary. This implies that terrestrial humic substances represent a significant pool of Cu-binding ligands in the Arno River estuary.

Seasonal differences in dissolved organic matter properties and sources in an Arctic fjord: Implications for future conditions.

The Arctic Ocean is undergoing drastic changes due to the effects of climate change. Arctic fjords are preferred systems to study these changes as they respond quickly to variations in ocean, land and atmosphere conditions. In this study, we investigated for the first time the seasonal variability of dissolved organic matter (DOM) properties and its origin in an Arctic fjord, which allows for an assessment of the future potential effects of climate change in this environment. We conducted an integrated analysis of the concentrations, optical properties (absorption and fluorescence), and molecular size distributions of DOM in two seasons (October 2017 and April 2018) and in eight to ten stations in Kongsfjorden (Svalbard) along with the related environmental parameters such as chlorophyll-a, inorganic nutrients, particulate organic carbon (POC), temperature, and salinity. Our results showed that, in both seasons, the DOM in the fjord was predominately of autochthonous origin with a seasonally variable terrestrial input. The dissolved organic carbon (DOC) concentrations were consistently higher in October than in April at each station. Fluorescence spectroscopy revealed a marked seasonal variability depending on the DOM fluorophore types and size fractions. In October, humic-like and tryptophan-like substances were dominant whereas in April, tyrosine-like compounds represented, on average, 58% of the DOM fluorescence. This study points out the key role of spring sea ice melting in determining the DOM properties of the fjord in spring.

In-Depth Assessment of the Effect of Sodium Azide on the Optical Properties of Dissolved Organic Matter.

Treatment and preservation of samples are critical issues in measuring the optical properties of dissolved organic matter (DOM) due to their high sensitivity to physical and chemical changes upon sample handling. In this study, we rigorously assessed the potential interferences of sodium azide (NaN3) on DOM absorption and fluorescence. A wide range of different samples were poisoned with varying NaN3 concentrations. Several commonly used optical parameters derived from absorbance and fluorescence spectroscopy were compared at different samples and conditions to assess the interfering effect of NaN3. Our results showed that NaN3 altered the original features of absorbance and fluorescence even at the lowest level of the addition. The absorption coefficients of NaN3-treated samples increased up to 2608% at 254 nm and 66% at 280 nm relative to the untreated control. Fluorescence data revealed both a quenching effect and an enhancement in fluorescence. The effect of NaN3 on fluorescence was highly variable and affected by the NaN3 concentrations added, and the sources and the concentrations of DOM samples. None of these factors exhibited a clear linear behavior with NaN3 levels, making it difficult to develop a correction method. It can be recommended from the findings not to use NaN3 in preserving DOM samples for optical measurements.

Optical and molecular characterization of dissolved organic matter (DOM) in the Arctic ice core and the underlying seawater (Cambridge Bay, Canada): Implication for increased autochthonous DOM during ice melting.

Sea ice contains a large amount of dissolved organic matter (DOM), which can be released into the ocean once it melts. In this study, Arctic sea ice DOM was characterized for its optical (fluorescence) properties as well as the molecular sizes and composition via size exclusion chromatography and Fourier transformation ion cyclotron resonance mass spectrometry (FT-ICR MS). Ice cores were collected along with the underlying seawater samples in Cambridge Bay, an Arctic area experiencing seasonal ice formation. The ice core samples revealed a marked enrichment of dissolved organic carbon (DOC) compared to the seawater counterparts (up to 6.2 times greater). The accumulation can be attributed to in situ production by the autotrophic and heterotrophic communities. Fluorescence excitation emission matrices (EEMs) elaborated with parallel factor analysis (PARAFAC) evidenced the prevalence of protein-like substances in the ice cores, which likely results from in situ production followed by accumulation in the ice. Size exclusion chromatography further revealed the in situ production of all DOM size fractions, with the exception of the humic substance fraction. The majority of DOM in both the ice and seawater consists of low molecular weight compounds (<350 Da) probably derived by the microbial degradation/transformation of freshly produced DOM. Molecular characterization also supported the in situ production of DOM and highlighted the marked difference in molecular composition between sea ice and seawater. This study provides new insights into the possible role of sea ice DOM in the Arctic carbon cycle under climate change.