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RESEARCH ARTICLE
Contents Vol 56(8)

Diel changes of inorganic chemistry in a macrophyte-dominated, softwater stream

Robert J. Wilcock A C and Steven C. Chapra B
+ Author Affiliations
- Author Affiliations

A National Institute of Water and Atmospheric Research, PO Box 11-115, Hamilton, New Zealand.

B Civil and Environmental Engineering Department, Tufts University, Medford, MA 02155, USA.

C Corresponding author. Email: r.wilcock@niwa.co.nz

Marine and Freshwater Research 56(8) 1165-1174 https://doi.org/10.1071/MF05049
Submitted: 18 March 2005  Accepted: 6 October 2005   Published: 22 November 2005

Abstract

Diel fluctuations of conductivity and alkalinity were measured in a macrophyte-dominated stretch of the Piako River, a rural, lowland, softwater stream in New Zealand. Both quantities exhibited elevated levels at dawn and depressed levels in the early evening suggesting that the variations might be connected with the diel cycles of macrophyte photosynthesis and respiration. A chemical analysis was used to determine which ions induced the diel variations. For low-flow periods with minimal allochthonous inputs, the changes in conductivity were correlated with calcium, magnesium and the ionised components of total inorganic carbon (bicarbonate, HCO3–, and carbonate, CO32–). The changes in alkalinity were correlated with fluctuations of calcium and magnesium. The latter result was not anticipated based on solubility product calculations. Diel cycles of groundwater inputs explained 60% of average conductivity variations and 30% of average alkalinity variations between dusk and dawn. Other mechanisms also contribute to the observed changes and we speculate that localised calcite production and dissolution may be occurring.

Extra keywords: conductivity, diurnal, ion balance, pH, photosynthesis, Piako River.


Acknowledgments

We are grateful for helpful input and suggestions made by Burns Macaskill regarding water chemistry observations. We thank Kaj Sand-Jensen, University of Copenhagen, and Colin Neal, Centre for Ecology and Hydrology, Wallingford, for helpful suggestions that have improved the manuscript, and the anonymous reviewer who drew our attention to groundwater as a possible explanation for our observations. Our thanks to John Nagels for providing field data. Part of the work was funded by NIWA Visiting Scientist awards (for SCC) and by the Foundation for Research Science and Technology contract No. C01X0305.


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