The overall pattern (Fig. 11.5) could be:(^1) A predator–prey oscillation: the protozoa eat down the phytoplankton and
excrete ammonium, then decrease themselves because they become food-
limited, releasing phytoplankton to grow, which eventually raises protozoan
stocks that then limit the cycle at its top, and the oscillation repeats.
2 Variation could be driven by intermittence in the iron supply; soluble iron
inputs, as dust (probably falling in rain squalls) or from vertical mixing,
would enhance phytoplankton growth rates, pulling their stocks up, later to
be grazed down as the grazers increase on their improved diet. Light-
transmission profiles gathered by ARGO floats rising twice daily in the
vicinity of Station P (Bishop et al. 2002) indicated a strong increase in
particulate carbon during a two-week period after passage overhead of a
dust storm originating in the Gobi Desert and observed by satellite. During
the ∼12 days after the dust cloud had passed, grazing fell behind, leaving
∼10% of the previous day’s net gain at each dawn to generate the overall
increase. While the coincidence with dust delivery was impressive, there
was also mixed-layer shoaling during the stock increase and deepening
during its decrease, suggesting the next hypothesis. Another example of a
dust-driven phytoplankton outburst in the subarctic Pacific is detailed by
Hamme et al. (2010; see Chapter 16).
3 Mixed-layer shoaling would allow more consistent illumination of
phytoplankton, increasing their growth, while storms would dilute
phytoplankton in a deeper mixing layer. Thus alternation of calmer and
stormier weather could drive the cycling.
4 Similarly, a rare period of open skies could enhance phytoplankton growth