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

• Published in: Journal of plankton research 2008-07, Vol.30 (7), p.735-753
• Main Authors: Thompson, Peter A., Bonham, Pru I., Swadling, Kerrie M.
• Format: Article
• Language: English
• Subjects: Abundance; Animal and plant ecology; Animal, plant and microbial ecology; Bacillariophyceae; Biological and medical sciences; Blooms; Brackish; Brackish water ecosystems; Diatoms; Dinoflagellata; Dinoflagellates; Estuaries; Estuarine dynamics; Fucoxanthin; Fundamental and applied biological sciences. Psychology; General aspects; Grazing; Growth rate; Mortality; Noctiluca scintillans; Phytoplankton; Salt wedges; Skeletonema costatum; Summer; Synecology; Zooplankton
• ISSN: 0142-7873; 1464-3774
• DOI: 10.1093/plankt/fbn044

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.