Phytoplankton blooms are strongly impacted by microzooplankton grazing in coastal North Pacific waters |
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Authors: | S L Strom M A Brainard J L Holmes M B Olson |
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Institution: | (1) Shannon Point Marine Center, Western Washington University, 1900 Shannon Point Rd., Anacortes, WA 98221, USA e-mail: stroms@cc.wwu.edu, US |
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Abstract: | Phytoplankton growth and microzooplankton grazing were measured in two productive coastal regions of the North Pacific: northern
Puget Sound and the coastal Gulf of Alaska. Rates of phytoplankton growth (range: 0.09–2.69 day−1) and microzooplankton grazing (range: 0.00–2.10 day−1) varied seasonally, with lowest values in late fall and winter, and highest values in spring and summer. Chlorophyll concentrations
also varied widely (0.19–13.65 μg l−1). Large (>8 μm) phytoplankton cells consistently dominated phytoplankton communities under bloom conditions, contributing
on average 65% of total chlorophyll biomass when chlorophyll exceeded 2 μg l−1. Microzooplankton grazing was an important loss process affecting phytoplankton, with grazing rates equivalent to nearly
two-thirds (64%) of growth rates on average. Both small and large phytoplankton cells were consumed, with the ratio of grazing
to growth (g:μ) for the two size classes averaging 0.80 and 0.42, respectively. Perhaps surprisingly, the coupling between microzooplankton
grazing and phytoplankton growth was tighter during phytoplankton blooms than during low biomass periods, with g:μ averaging 0.78 during blooms and 0.49 at other times. This tight coupling may be a result of the high potential growth
and ingestion rates of protist grazers, some of which feed on bloom-forming diatoms and other large phytoplankton. Large ciliates
and Gyrodinium-like dinoflagellates contributed substantially to microzooplankton biomass at diatom bloom stations in the Gulf of Alaska,
and microzooplankton biomass overall was strongly correlated with >8 μm chlorophyll concentrations. Because grazing tended
to be proportionally greater when phytoplankton biomass was high, the absolute amount of chlorophyll consumed by microzooplankton
was often substantial. In nearly two-thirds of the experiments (14/23), more chlorophyll was ingested by microzooplankton
than was available for all other biological and physical loss processes combined. Microzooplankton were important intermediaries
in the transfer of primary production to higher trophic levels in these coastal marine food webs.
Received: 12 November 1999 / Accepted: 4 October 2000 |
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