Development of the diatom-Phaeocystis spring bloom in the Dutch coastal zone of the North Sea: the silicon depletion versus the daily irradiance threshold hypothesis |
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Authors: | Peperzak L; Colijn F; Gieskes W W C; Peeters J C H |
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Institution: | National Institute for Coastal and Marine Management/RIKZ PO Box 8039, NL-4330 EA Middelburg, The Netherlands
1Forschungs- und Technologie Zentrum Westkste, Hafentrn D 25761 Bsum, Germany
2Department of Marine Biology, University of Groningen PO Box 14, NL-9 750 AA Haren, The Netherlands |
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Abstract: | The Dutch coastal zone of the North Sea is characterized byhigh nutrient inputs and low water column irradiance due tohigh concentrations of suspended matter. The vernal phytoplanktonblooms are dominated by diatoms and the flagellate Phaeocystis(Haptophyta). Two hypotheses that predict the timing of thePhaeocystzs bloom were tested with field data collected in 1992.The first one, stating that the Phaeocystis bloom starts whensilicon has been depleted by a preceding diatom bloom, couldnot be bracketed with observations. The second hypothesis, predictingthat Phaeocysus blooms under nitrogen- and phosphorus-repleteconditions after a daily irradiance threshold has been passedcould be supported by field observations. Furthermore, it isshown that the Dutch coastal zone is not continuously homogeneouslymixed. It is argued that this is an important factor in thelife cycle of Phaeocystis and in foam formation on nearby shores.Grazing by the ciliate Strombidium sulcatum and heterotrophicgymnodinoid flagellates accounted for 92% of Phaeocystis cellloss after the bloom. |
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