Grazing impact of microzooplankton on a diatom bloom in a mesocosm as estimated by pigment-specific dilution technique |
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Authors: | Koji Suzuki Atsushi TsudaHiroshi Kiyosawa Shigenobu TakedaJun Nishioka Toshiro SainoMasayuki Takahashi C.S. Wong |
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Affiliation: | a Hydrospheric Atmospheric Research Center, Nagoya University, Chikusa-ku, Nagoya 464-8601, Japan b Hokkaido National Fisheries Research Institute, Kushiro 085-0802, Japan c Marine Biological Research Institute, Shinagawa-ku, Tokyo 142-0042, Japan d Graduate School of Agricultural and Life Science, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo 113-8657, Japan e Biology Department, Central Research Institute of Electric Power Industry, Abiko 270-1166, Japan f Department of Biology, University of Tokyo, Meguro-ku, Tokyo 153-0041, Japan g Institute of Ocean Sciences, Sidney, British Columbia, Canada V8L 4B2 |
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Abstract: | To investigate the impact of microzooplankton grazing on phytoplankton bloom in coastal waters, an enclosure experiment was conducted in Saanich Inlet, Canada during the summer of 1996. Daily changes in the microzooplankton grazing rate on each phytoplankton group were investigated with the growth rates of each phytoplankton group from the beginning toward the end of bloom using the dilution technique with high-performance liquid chromatography (HPLC). On Day 1 when nitrate and iron were artificially added, chlorophyll a concentration was relatively low (4.3 μg l−1) and 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes were predominant in the chlorophyll biomass. However, both the synthetic rates and concentrations of 19′-hexanoyloxyfucoxanthin declined before bloom, suggesting that 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes weakened. Chlorophyll a concentration peaked at 23 μg l−1 on Day 4 and the bloom consisted of the small chain-forming diatoms Chaetoceros spp. (4 μm in cell diameter). Diatoms were secondary constituents in the chlorophyll biomass at the beginning of the experiment, and the growth rates of diatoms (fucoxanthin) were consistently high (>0.5 d−1) until Day 3. Microzooplankton grazing rates on each phytoplankton group remarkably increased except on alloxanthin-containing cryptophytes after the nutrient enrichments, and peaked with >0.6 d−1 on Day 3, indicating that >45% of the standing stock of each phytoplankton group was removed per day. Both the growth and mortality rates of alloxanthin-containing cryptophytes were relatively high (>1 and >0.5 d−1, respectively) until the bloom, suggesting that a homeostatic mechanism might exist between predators and their prey. Overall, microzooplankton grazing showed a rapid response to the increase in phytoplankton abundance after the nutrient enrichments, and affected the magnitude of the bloom significantly. High grazing activity of microzooplankton contributed to an increase in the abundance of heterotrophic dinoflagellates with 7-24 μm in cell size, the fraction of large-sized (>10 μm) chlorophyll a, and stimulated the growth of larger-sized ciliates after the bloom. |
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Keywords: | Bloom Mesocosm Microzooplankton Phytoplankton Pigments |
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