Photodegradation, interaction with iron oxides and bioavailability of dissolved organic matter from forested floodplain sources
Julia A. Howitt A B C D , Darren S. Baldwin A , Gavin N. Rees A and Barry T. Hart BA Murray-Darling Freshwater Research Centre, PO Box 991, Wodonga, Victoria 3689, Australia.
B Water Studies Centre, Monash University Clayton, Victoria 3800, Australia.
C School of Life and Environmental Science, Deakin University, PO Box 423, Warrnambool, Victoria 3280, Australia.
D Corresponding author. Email: jhowitt@csu.edu.au
Marine and Freshwater Research 59(9) 780-791 https://doi.org/10.1071/MF07225
Submitted: 28 November 2007 Accepted: 29 June 2008 Published: 7 October 2008
Abstract
Photochemical degradation of dissolved organic matter (DOM) can influence food webs by altering the availability of carbon to microbial communities, and may be particularly important following periods of high DOM input (e.g. flooding of forested floodplains). Iron oxides can facilitate these reactions, but their influence on subsequent organic products is poorly understood. Degradation experiments with billabong (= oxbow lake) water and river red gum (Eucalyptus camaldulensis) leaf leachate were conducted to assess the importance of these reactions in floodplain systems. Photochemical degradation of DOM in sunlight-irradiated quartz tubes (with and without amorphous iron oxide) was studied using gas chromatography and UV-visible spectroscopy. Photochemical reactions generated gaseous products and small organic acids. Bioavailability of billabong DOM increased following irradiation, whereas that of leaf leachate was not significantly altered. Fluorescence excitation-emission spectra suggested that the humic component of billabong organic matter was particularly susceptible to degradation, and the source of DOM influenced the changes observed. The addition of amorphous iron oxide increased rates of photochemical degradation of leachate and billabong DOM. The importance of photochemical reactions to aquatic systems will depend on the source of the DOM and its starting bioavailability, whereas inputs of freshly formed iron oxides will accelerate the processes.
Additional keywords: aquatic carbon cycle, excitation-emission matrix (EEM), humic substances, Murray River.
Acknowledgements
We thank Dr Diane McKnight and Dr J. Robin Fulton from the University of Colorado, Boulder for advice on fluorescence corrections and Zygmunt Lorenz of the Murray-Darling Freshwater Research Centre (MDFRC) for assistance with the correction macro. We also thank Dr Patricia Bowen (MDFRC) for providing dried leaf material for use in the leachate experiments and Professor Gerry Quinn (Deakin University) for assistance with statistical analysis, and several anonymous reviewers for their suggestions on how the manuscript could be improved. This work was undertaken with the financial support of an Australian Postgraduate Award from Monash University and a top-up scholarship from the Cooperative Research Centre for Freshwater Ecology.
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