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JPR Advance Access originally published online on April 11, 2008
Journal of Plankton Research 2008 30(7):735-753; doi:10.1093/plankt/fbn044
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© The Author 2008. Published by Oxford University Press. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org

Phytoplankton blooms in the Huon Estuary, Tasmania: top-down or bottom-up control?

Peter A. Thompson1,*, Pru I. Bonham1 and Kerrie M. Swadling2

1 CSIRO Marine and Atmospheric Research and the Aquafin CRC, GPO Box 1538, Hobart 7001, Australia 2 Tasmanian Aquaculture and Fisheries Institute and the Aquafin CRC, University of Tasmania, Private Bag 49, Hobart 7001, Australia

* CORRESPONDING AUTHOR: peter.a.thompson{at}csiro.au

Received on October 18, 2007; accepted on January 4, 2008


  Abstract

The roles of "top-down" and "bottom-up" factors were investigated in terms of their influence on the diatom and dinoflagellate abundances in the microtidal, salt wedge Huon Estuary, Tasmania, Australia. Long-term (1996–2005) changes in Chl a, the peridinin:Chl a ratio and the abundance of autotrophic dinoflagellates were observed to coincide with the warming of regional surface waters. There were significant seasonal differences in pigment-specific net growth rates for Chl a, peridinin and fucoxanthin. Diatoms dominated the spring bloom when species such as Skeletonema costatum had the highest net growth rates and fucoxanthin-specific gross growth rates were ~0.9 day–1. During late summer, the peridinin-specific grazing mortality was significantly less than the fucoxanthin-specific grazing mortality and dinoflagellates increased their dominance of the phytoplankton community. This late summer relaxation of grazing pressure on dinoflagellates was associated with a decline in the overall abundance of microheterotroph (MH) grazers and a peak in the abundance, biomass and estimated grazing rates of mesozooplankton. We suggest the composition of the autumn phytoplankton community was dependent upon a trophic cascade where mesozooplankton, such as Noctiluca scintillans, preyed upon MHs and reduced their grazing upon some species of dinoflagellates.

Corresponding editor: Roger Harris


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