Actual phytoplankton growth rates are not strongly affected by nutrient limitation; the
very small cells (Prochlorococcus, Synechococcus, and picoeukaryotes), are those
most capable of obtaining sufficient nutrients from low concentrations. The growth
rates range from one to two divisions per day. Banse argues convincingly that loss
rates to cell sinking and vertical mixing can amount at most to a few percent of the
growth rates, and therefore the apparent steady states which generally pertain must be
attributed to cell death, most of it from grazing, although viral lysis is also important.
Almost all actual herbivory (eating of phytoplankton) is by protozoans, simply
because the dominant phytoplankton are so small. The array of protozoans includes
heterotrophic nanoflagellates, larger ciliates, and heterotrophic dinoflagellates, just as
in oligotrophic high-latitude habitats. The exact proportions of different groups and
species are not well characterized. Copepods, larvaceans, and other mesozooplankton
particle feeders are, then, in a strict sense carnivorous, feeding at the third and higher
trophic levels, even though most of them are smaller than their high-latitude relatives
in the same taxonomic families.
Fig. 11.31 Comparison of phytoplankton growth rates as a function of temperature in
several oceanic, oligotrophic systems. Different symbol shapes from different
workers. Plain symbols are from nitrate-depleted areas, mostly the NPSG. Circled
symbols are from HNLC areas; those below 15°C from the subarctic Pacific. Growth
rates obtained in various ways (see original reference); all rates exceed the
phytoplankton stock increase rates. Numbers indicate multiple samples.
(^) (Simplified from Banse 1995.)
By and large, the mesozooplankton of the NPSG are continuously active, without a
seasonal rest phase. Life cycles are not annual or twice-yearly as in subpolar habitats,
but are a few weeks or a month in surface layers. Development of reproductive
cohorts is not readily followed in these systems. At HOT, collections to 160 m with