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RESEARCH ARTICLE
Contents Vol 56(7)

Seasonal production regimes off south-western Australia: influence of the Capes and Leeuwin Currents on phytoplankton dynamics

Christine E. Hanson A B C , Charitha B. Pattiaratchi A and Anya M. Waite A
+ Author Affiliations
- Author Affiliations

A Centre for Water Research, University of Western Australia, 35 Stirling Hwy, Crawley, WA 6009, Australia.

B Present address: Centre for Ecosystem Management, School of Natural Sciences, Edith Cowan University, 100 Joondalup Drive, Joondalup, WA 6027, Australia.

C Corresponding author. Email: c.hanson@ecu.edu.au

Marine and Freshwater Research 56(7) 1011-1026 https://doi.org/10.1071/MF04288
Submitted: 15 December 2004  Accepted: 2 June 2005   Published: 3 November 2005

Abstract

Temporal primary production dynamics were investigated off south-western Australia, where the summer upwelling regime of the Capes Current was compared with early winter conditions characterised by strengthened near-shore Leeuwin Current flow. Seasonal upwelling in this region sourced nitrate levels of ≥1 μm from the nutricline at the base of the Leeuwin Current’s mixed layer, with total water column production reaching a maximum of ~950 mg C m−2 day−1 in the Capes Current. Stable isotope signatures of particulate matter indicated that productivity off south-western Australia was heavily reliant on nitrate as a nitrogen source, with mean δ15N ranging from ~4 to 5 ‰ under both upwelling and non-upwelling (winter) conditions. Unexpectedly, significant nutrient enrichment within the Leeuwin Current (up to 3.1 μm nitrate) occurred during winter, likely as a result of the meandering Leeuwin Current flooding the inner shelf north of the study area and entraining relatively high-nutrient shelf waters in its southwards flow. However, early winter production under these nutrient-replete conditions (mean ± s.d. 310 ± 105 mg C m−2 day−1) was significantly lower than in summer (695 ± 140 mg C m−2 day−1) due to light limitation, both as a result of reduced surface irradiance characteristic of the winter months and significantly higher light attenuation within the water column as compared with summer conditions.

Extra keywords: photosynthetic parameters, physical–biological coupling, primary production, Western Australia.


Acknowledgments

We gratefully acknowledge the assistance of Bridget Alexander, Joanne O’Callaghan and Stéphane Pesant in the field and laboratory; Graham Pateman and the crew of the FV Cape Leeuwin for field operations; Roger Head and Bill Foster for assistance with modifications to the F-probe; the Australian Bureau of Meteorology for wind data; Peter Thompson for advice on experimental design; CSIRO Marine Research (Floreat) for use of the photosynthetron; Alan Pearce (WASTAC) and Mike Steber (DOLA) for satellite imagery; and two anonymous reviewers for comments on the manuscript. This work was supported by an Australian Research Council Small Grant (to C.P.), and an International Postgraduate Research Scholarship/University Postgraduate Award (to C.H.). This is Centre for Water Research reference ED 1876.


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