spring), the deeper the critical depth. Increasing algal-growth rates near the surface
increase the vertical integral of production, driving the critical depth down. Spring
blooms do not occur in this ocean until significant stratification sets in above the
levels calculated. Roughly, the theory works.
Alternative Scenarios
(^) Boss et al. (2008) deployed an ARGO-type CTD float, augmented with a light-
scattering sensor (particle density) and a fluorometer, which remained in the vicinity
of 50°N, 47°W (subarctic waters east of Newfoundland and south of Greenland) for
over two years. Most of the time it resided at 1000 m, making profiles to the surface
every five days and recording variables each 50 m up to 400 m, then at closer
intervals on upward. This provided a good record of the relation between mixed-layer
depth (defined as the first depth with density – σt – greater by 0.125 kg m−3 than the
sea surface) and both chlorophyll and beam scattering. All profiling was near
midnight to get the diel maxima of chlorophyll estimates. Comparisons of chlorophyll
levels (calibrated before launch in a laboratory) to pixels recorded in nearby satellite
passes were within expected uncertainties. Spring blooms indicated by both particle
scattering and chlorophyll (Plate 11.1) peaked close to June 1 each year, and levels
above 1 μg liter−1 lasted ∼40 days in 2005, ∼30 days in 2006. The actual increases
from the winter low <0.2 μg liter−1 were gradual and started very early: from 11
December of 2004 for the 2005 bloom, and from ∼21 February in 2006. The decrease
after the bloom was approximately the mirror image of the long increase.
(^) Throughout the two years, chlorophyll was very evenly mixed down to the point
defined by Δσt = 0.125 and almost absent below that level (Plate 11.1). Clearly,
mixing depth is critical, but it was never great enough to eliminate all phytoplankton
even in the middle of winter. Moreover, the winter–spring increase developed
gradually at first, following quite exactly the slow shoaling of mixing. Then, as the
chlorophyll concentration approached 1 μg liter−1, there was a month-long doubling
or tripling when the mixing depth leveled off above 50–75 m. A shadow of particle
scattering below Δσt = 0.125 (Fig. 6 in Boss et al. 2008) occurred during the spring
bloom. A likely interpretation is that light and nutrients in winter are always sufficient
for some algae, almost certainly pico- and nanoplankton, to sustain and increase their
stocks. Dilution of protozoan grazers and slowing of their activity by cooling are
likely also important. Then the dramatic spring increase occurs when illumination is
at its annual maximum and mixing is constrained to a shallow enough layer to allow
rapid diatom growth. The diatoms then rain out as they become nutrient limited, and
there are greater losses to grazing by mesozooplankton and possibly also by
dinoflagellates.
(^) Behrenfeld (2010) has confirmed this pattern for the central subarctic Atlantic from