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RESEARCH ARTICLE
Contents Vol 21(4)

A critical assessment of physicochemical indices used to characterise natural dissolved organic carbon (DOC), their inter-relationships, and the effects of pH

Carolyn Morris https://orcid.org/0000-0002-7254-8433 A * , Samantha Zulian B , Colin J. Brauner https://orcid.org/0000-0002-3695-7707 A , Chris M. Wood https://orcid.org/0000-0002-9542-2219 A C and D. Scott Smith https://orcid.org/0000-0002-3075-4087 B
+ Author Affiliations
- Author Affiliations

A Department of Zoology, University of British Columbia, Vancouver, BC, V6T 1Z4, Canada.

B Department of Chemistry and Biochemistry, Wilfrid Laurier University, 75 University Avenue W, Waterloo, ON, N2L 3C5, Canada.

C Department of Biology, McMaster University, 1280 Main Street West, Hamilton, ON, L8S 4K1, Canada.

* Correspondence to: morris@zoology.ubc.ca

Handling Editor: Kevin Wilkinson

Environmental Chemistry 21, EN24021 https://doi.org/10.1071/EN24021
Submitted: 9 April 2024  Accepted: 8 June 2024  Published: 2 July 2024

© 2024 The Author(s) (or their employer(s)). Published by CSIRO Publishing.

Abstract

Environmental context

Dissolved organic carbon (DOC) is ubiquitous in freshwater and concentrations are rising universally while pH is decreasing with climate change. This study demonstrates the interrelationships among DOC characterisation techniques and the pH-sensitive aspects of these techniques that were previously not well understood. As DOC regulates important processes within ecosystems, understanding DOC behaviour at altered pH and identifying techniques to effectively evaluate DOC composition are critical requirements for monitoring aquatic ecosystem health.

Rationale

Dissolved organic carbon (DOC) is both ubiquitous and heterogeneous in freshwater. Freshwaters are browning universally and pH values are decreasing with climate change. DOCs influence water pH, whereas changes in water pH potentially alter the conformation and function of DOCs. The physicochemical properties of DOCs can be characterised by optical and chemical indices, but the inter-relationships among them, and the effects of low pH, are not well understood.

Methodology

We characterised five naturally sourced DOCs, spanning large differences in origin and composition, at pH 7 and 4, using multiple indices: specific absorbance coefficient at 340 nm, molecular weight index, fluorescence index, octanol–water partition coefficient, molecular charge, proton binding index, size-fractionation, and percentage humic-acid-like, percentage fulvic-acid-like and percentage protein-like components by fluorescence-based parallel factor analysis.

Results

Many of the indices changed between pH 7 and 4 as reflected in the corresponding absorbance and fluorescence profiles. Generally, apparent aromaticity, apparent molecular weight and molecular charge all decreased with low pH, while lipophilicity increased. Key positive correlations occurred between aromaticity and apparent molecular weight, chemical reactivity and apparent molecular weight, and aromaticity and chemical reactivity, and a negative correlation between lipophilicity and molecular charge. These relationships were pH dependent.

Discussion

Our results highlight that physicochemical indices used to characterise DOCs from distinct sources should consider pH and be interpreted carefully. The pH-dependent changes in many of the indices are likely alterations in the conformation, reflected in the optical signatures, rather than changes in the composition of DOCs. In contrast, increased lipophilicity and reduced charge at lower pH are due to actual changes in DOC molecules, resulting from proton binding. The ecological functions of DOCs are dependent on source and will likely change with natural acidification events such as increasing atmospheric CO2.

Keywords: absorbance, acid–base titration, dissolved organic carbon, DOC characterisation, fluorescence signatures, lipophilicity, optical properties, pH-dependent properties.

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