Optical properties of dissolved organic matter along a salinity gradient from a boreal river estuary to open coastal waters

Aleksandr Berezovski; Dag O. Hessen; Hanne Halkjelsvik Børseth; Tom Andersen

doi:10.1071/MF24244

RESEARCH ARTICLE (Open Access)

Next Contents Vol 76(11)

Optical properties of dissolved organic matter along a salinity gradient from a boreal river estuary to open coastal waters

Aleksandr Berezovski

^A ^* , Dag O. Hessen ^A , Hanne Halkjelsvik Børseth ^A and Tom Andersen ^A

+ Author Affiliations

- Author Affiliations

^A Department of Biosciences, Centre for Biogeochemistry in the Anthropocene, University of Oslo, PO Box 1066, Blindern, NO-0316Oslo, Norway. Email: d.o.hessen@mn.uio.no, tom.andersen@ibv.uio.no

^* Correspondence to: aleksandrberezovski@gmail.com

Handling Editor: Simon Mitrovic

Marine and Freshwater Research 76, MF24244 https://doi.org/10.1071/MF24244

Submitted: 1 November 2024 Accepted: 19 June 2025 Published: 11 July 2025

© 2025 The Author(s) (or their employer(s)). Published by CSIRO Publishing. This is an open access article distributed under the Creative Commons Attribution-NonCommercial 4.0 International License (CC BY-NC)

Abstract

Context

Water darkening is an increasingly relevant problem affecting many coastal ecosystems, and it is exacerbated by human activity and climate change.

Aims

To determine the spectral, isotopic, and quantitative changes in coloured dissolved organic matter (CDOM) across a salinity gradient from the Glomma River to the outer Oslofjord.

Methods

CDOM absorbance was measured on a spectrophotometer and organic carbon in the water column was analysed using stable isotopes.

Key results

Spectral slope and CDOM absorption at 443 nm decreases linearly with an increasing salinity. CDOM absorption exhibited significant variations, with low-salinity samples showing higher light absorption per unit of carbon, but little seasonal variations. δ¹³C of TOC analysis showed a strong positive correlation with salinity.

Conclusion

Dissolved organic matter optical characteristics of coastal waters are tightly linked to CDOM. The distance to the river mouth and to the shore are some of the most important factors affecting water clarity. The δ¹³C of TOC analysis indicated a linear transition from terrestrial to marine carbon sources.

Implications

The study has shown how coastal darkening can be an important ecological issue mainly in near-coastal areas and describes valuable diagnostic tools for tracking it.

Keywords: CDOM absorption, coloured dissolved organic matter, δ¹³C of TOC, estuary, light attenuation, salinity gradient, spectral absorbance.

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