Modelling the seasonal succession of Emiliania huxleyi and other phytoplankton in the Bering Sea

Merico, Agostino (2003) Modelling the seasonal succession of Emiliania huxleyi and other phytoplankton in the Bering Sea. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Several years of physical and biological anomalies have affected the Bering Sea shelf ecosystem since 1997. Such anomalies reached their peak in a striking visual phenomenon: the first appearance in the area of bright waters caused by massive blooms of the cocolithophore Emiliania huxleyi (E huxleyi). This study provides an insight into the mechanisms of phytoplankton succession in the south-eastern part of the shelf during such years and addresses the causes of E. huxleyi success by means of a ½-dimensional time-dependent ecosystem model, field data and satellite-derived information. A number of potential hypotheses are identified based on field observations conducted in the area and on previous knowledge of E. huxleyi general ecology. The key hypotheses are then considered as causative factors and explored with the model. The model also includes carbon chemistry routines in order to investigate the relatives between phytoplankton and the carbonate system.

Archived satellite imagery (from 1978 to 1997), examined with the aim of establishing the history of the presence of E. huxleyi in the Bering Sea, revealed that a small bloom was already present in 1996, a precursor of the big blooms which occurred the following years. No blooms were detected before 1996.

The modelling study suggests that E. huxleyi blooms were initiated in 1997 by a shallow mixed layer depth in conjunction with a lack of photoinhibition in this species. A top-down control by microzooplankton selectively grazing phytoplankton other than E. huxleyi appears to be responsible for the unusual long persistence of the blooms (from three to four months). Compelling evidence are also provided that can potentially explain the typical diatom-coccolithophore succession sequence in terms of calcite saturation state (a variable recently shown to be crucial for the production of calcium carbonate by all marine calcifying organisms). Therefore, a simple ecological mechanical is proposed: "microzooplankton grazing responds to frustule silicification and coccosphere calcification".

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