CSIRO PUBLISHING Volume 49, 1998 © CSIRO Australia 1998 A journal for the publication of original contributions in physical oceanography, marine chemistry, marine and estuarine biology and limnology www.publish.csiro.au/journals/mfr All enquiries and manuscripts should be directed to Marine and Freshwater Research CSIRO PUBLISHING PO Box 1139 (150 Oxford St) Collingwood Telephone: 61 3 9662 7618 Vic. 3066 Facsimile: 61 3 9662 7611 Australia Email: ann.grant@publish.csiro.au Published by CSIRO PUBLISHING for CSIRO Australia and the Australian Academy of Science & Marine Freshwater Research