(^) The interactions of these lower trophic levels result in a still unexplained variation
in phytoplankton stocks (Fig. 11.5). While the variation is confined within narrower
limits than in coastal temperate or North Atlantic waters, it slowly but recurrently
wanders between about 0.15 and 0.6 μg liter−1. There are also daily cycles, partly
day–night fluorescence variation, and partly stock increase in the light, and decrease
due to grazing in the dark (Bishop et al. 1999). The longer oscillations result from the
small daily differentials, which can be either positive or negative. Those are smaller
than the 10–40-day variations, suggesting that the latter correspond to real variation in
phytoplankton standing stock. Moreover, at the >10 day periods, there are inverse
variations of upper-water-column ammonium concentration: ammonium is taken up
as stocks increase and is regenerated as stocks decrease (Miller et al. 1991a & b).
Presumably, protist grazers increase during the regeneration phase and their grazing
overtakes and reverses phytoplankton increase, returning their fixed nitrogen to the
water column.
Fig. 11.5 Short-term cycling of phytoplankton biomass (measured as chlorophyll
standing stock) near 50°N, 145°W.
(^) (Plotted from moored-fluorometer data graciously provided by Phillip Boyd.)
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