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JPR Advance Access published online on April 11, 2008
Journal of Plankton Research, 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. Thompson

CSIRO Marine and Atmospheric Research and the Aquafin CRC, GPO Box 1538, Hobart, 7001, Australia

Pru I. Bonham

CSIRO Marine and Atmospheric Research and the Aquafin CRC, GPO Box 1538, Hobart, 7001, Australia

Kerrie M. Swadling

Tasmanian Aquaculture and Fisheries Institute and the Aquafin CRC, University of Tasmania, Private Bag 49, Hobart, 7001, Australia

Corresponding author: Peter A. Thompson Email: peter.a.thompson{at}csiro.au Tel.: 61 3 6232 5298 Fax: 61 3 6232 5000

Received on October 18, 2007; revised on December 4, 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 to 2005) changes in chlorophyll a, the peridinin:chlorophyll 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 chlorophyll 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 d–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 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 microheterotrophs and reduced their grazing upon some species of dinoflagellates.

Key Words: grazing • pigments • phytoplankton • microheterotrophs • mesozooplankton

Communicating Editor: RP Harris


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