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Aquatic Microbial Ecology

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AME 42:55-62 (2006)  -  doi:10.3354/ame042055

Net growth of the bloom-forming dinoflagellate Heterocapsa triquetra and pH: why turbulence matters

Harry Havskum*, Per J. Hansen

Marine Biological Laboratory, University of Copenhagen, Strandpromenaden 5, 3000 Helsingør, Denmark

ABSTRACT: The effect of turbulence on pH and the proliferation of the phototrophic dinoflagellate Heterocapsa triquetra was monitored for 10 d. Four turbulent energy dissipation rates ranging from 0.0001 to 1 cm2 s–3 were employed. The net growth rate of H. triquetra was on average 0.42 d–1 at all turbulence levels, as long as cell densities were low and pH stayed below 8.9. When cell densities increased and the pH exceeded 9.0, the net growth rate of H. triquetra decreased in all cases. However, at high cell densities, the pH and net growth rate of H. triquetra depended on the turbulence level. At the highest turbulence level, the net growth rate of H. triquetra was higher and the pH lower than at the lowest turbulence level, because turbulence increased the exchange of CO2 between the medium and the atmospheric air. At the second highest turbulence level, the net growth rate of H. triquetra was also higher than the net growth rate at lower turbulence; this was likely to be due to a higher influx of inorganic carbon, even though pH measurements from the middle of the light period did not differ significantly among turbulence levels. However, during the night, pH decreased more at the second highest turbulence level than at the 2 lowest turbulence levels and thereby allowed a higher net growth rate of H. triquetra. Our results suggested that when forming blooms in eutrophic waters under conditions of high pH, H. triquetra will benefit from turbulent energy dissipation rates between 0.05 and 1 cm2 s–3.

KEY WORDS: Turbulence · pH · Growth · Phytoplankton blooms · Dinoflagellates · Heterocapsa triquetra

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