Dissolved organic matter dynamics in the pristine Krka River estuary (Croatia)

Abstract

In many estuarine systems, riverine discharge is the main source of dissolved organic matter (DOM). In contrast, karstic Krka River, with self-purifying ability, is characterized by very low DOM concentrations (~30 µM) and a diluting effect on DOM in the estuary. Interestingly, within the estuary a marked in-situ production is observed, particularly in summer. Pristine nature of the estuary makes it a natural laboratory to study the DOM dynamics, originated from 3 sources: the river the seawater and in-situ production. With this work we aim at reporting the first information on DOM dynamics in the Krka estuary in two contrasting seasons, winter and summer 2019. The quality of DOM was assessed using absorbance and fluorescence measurements coupled with PARAFAC analysis. During winter, the estuary is characterized by extremely low DOC concentrations, for which optical properties indicate that is mainly of terrestrial origin. In summer, due to the low riverine discharge and high temperature, DOC concentrations are higher than in winter and a marked increase in absorption and protein-like fluorescence is observed in the estuary. The permanent stratification determines DOC accumulation in the surface layer with values up to 147 µM, suggesting a decoupling between production and removal processes. Our work opens intriguing questions about the main processes responsible for DOC accumulation in this system and highlights the need of new studies combing chemical and biological information.

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

“…However, a similar increase is observed for all other components, showing a positive correlation with ΣL. Even though in situ production can also cause the increase of humic-like components ( Romera-Castillo et al, 2010 ; Marcinek et al, 2020 ), without additional biological information we cannot conclude that the correlation between ΣL and the DCu in late summer is related to any extent to a biological response to Cu-stress.…”

“…Absorbance at 254 nm ( A 254 ) was expressed as the absorption coefficient ( a 254 ) in m −1 ( Stedmon and Nelson, 2015 ). The specific UV absorbance at 254 nm (SUVA 254 ) was calculated by dividing the absorption coefficient at 254 nm by the DOC concentration (m 2 g −1 C) and used as an indicator of the percentage of CDOM in the total DOM pool ( Marcinek et al, 2020 ).…”

“…Knowledge about sources and chemical identity of detected ligands is scarce ( Vraspir and Butler, 2009 ) and its acquisition is hindered by very complex chemical composition of natural DOM ( Repeta, 2015 ). Useful qualitative information about the DOM pool in the aquatic environment (e.g., average aromaticity degree, molecular weight, and occurrence of humic-like, fulvic-like, and protein-like substances) can be gained indirectly via UV/Vis and fluorescence spectroscopic studies of its optically active fractions (colored and fluorescent dissolved organic matter, CDOM and FDOM) ( Yamashita et al, 2011 ; Osburn et al, 2012 ; Lee et al, 2018 ; Galletti et al, 2019 ; Marcinek et al, 2020 ). Including this information in speciation studies could improve our understanding of the sources and composition of metal-binding organic ligands ( Sanchez-Marin et al, 2010 ; Slagter et al, 2017 ; Watanabe et al, 2018 ; Wong et al, 2019 ; Whitby et al, 2020 ).…”

Redox Speciation of Vanadium in Estuarine Waters Using Improved Methodology Based on Anion Exchange Chromatography Coupled to HR ICP-MS System

“…Halocline is usually positioned between 1.5 and 3 m, with the thickness of the halocline layer varying between a few cm up to 1 m [ 12 , 54 ]. Special characteristics make this sampling location ideal for evaluating the mobility and fate of trace metals and natural organic matter [ 12 , 53 , 55 , 56 , 57 ]. Trace metals have been previously studied, although mainly in reference to their total concentrations [ 12 , 51 ].…”

“…In addition, some of the recent studies on sediments of the Krka River estuary showed detectable anthropogenic input (due to the nautical tourism) in local restricted areas, although the estuary generally is not considered polluted [51,52,58]. The sampling sites were selected to represent typical estuarine zones of varying anthropogenic input, physico-chemical gradients and biological activity [57,59]. Sampling was conducted on November 2019 and January of 2020 at three different locations along the Krka River estuary (Figure 7): Skradin bridge (denoted as SB), Martinska (denoted as MA) and Vrnaža port (denoted as VP), as well as two end-member locations: the Krka River water above the Skradinski Buk waterfalls (E1) and coastal seawater (E2).…”