25° to 35°W using nine years of SeaWiFS data. Specifically for 45° to 50°N (Fig.
11.14), the surface chlorophyll is minimal (∼0.2 μg liter−1) close to January 1, well
before the maximum mixed-layer deepening, then immediately begins a gradual
increase amounting to a doubling. There follows a sharp acceleration of stock increase
on a date ranging from mid-March to late May (Fig. 11.14), the classical spring
bloom. This strong acceleration occurs during the steepest ascent of the mixing-layer
depth (approximated by Behrenfeld from a wind-data assimilating model of mixing)
and rapid increase of illumination (PAR, estimated by SeaWiFS). These are exactly
the conditions proposed by critical depth theory for promotion of diatom blooms. The
interesting feature revealed from both the ARGO floats and satellite analysis is the
slower increase of algal stocks from the winter solstice to about the spring equinox.
Very likely the modification of critical depth theory required by this will be found to
be a very large winter reduction in the community “respiration”, primarily grazing on
pico- and nanoplankton. The euphotic-zone protozoan community must be greatly
diluted by vertical mixing and its feeding and reproductive potential must be reduced
by cold winter temperatures. This interpretation is akin to what Behrenfeld refers to as
a “dilution-recoupling hypothesis”. However, he has a substantially different
interpretation, for which we refer you to his paper. The approximation that community
catabolism of algal production is constant seasonally and with depth was forced on
Gran and Braarud and Sverdrup by lack of information. It was never satisfying, and
these new observations call for its adequate quantification.
Fig. 11.14 Above: SeaWiFS OC4v4 chlorophyll estimates averaged weekly for a box
bounded by 45°N and 50°N and by 25°W and 35°W. Below: the same data enlarged
for 1999 to 2002 to emphasize the sharp changes in slope between slow increase from
about the solstice to rapid increase in early spring.
(After Behrenfeld 2010.)