depth are from community metabolism of photosynthate. In addition, however, there
will be losses from the upper, lighted layer through vertical mixing. In general,
mixing above the first significant thermal step in the water column is rapid, for some
practical purposes instantaneous. Mixing through this thermal step is slow. Sverdrup
predicted that spring blooms would get under way at the time during heating of the
water column when the thermocline rises above the critical depth. Before that time,
mixing would make 1/P(dP/dt) negative for the euphotic zone overall. Afterward,
mixing losses would be small and stock would accumulate above the thermocline.
Fig. 11.8 Features of Sverdrup’s (1953) critical depth model for spring bloom
initiation. Gross photosynthesis decays exponentially downward in response to
progressive diminution of irradiance according to Beer’s Law (Iz = I 0 e−kz), where k is
the coefficient of extinction due to absorbance and scattering, and z is the depth). Net
photosynthesis is shifted left by the assumed depth-invariant community respiration.
Shaded areas show zones of positive (above the compensation depth) and negative
below) net photosynthesis down to the critical depth where the integrals are equal.
(^) (After Sverdrup 1953.)
Critical-depth theory works, in a general way. Sverdrup (1953) showed data (Fig.
11.9) from a weather ship stationed in the high-temperate North Atlantic (66°N, 2°E),
where phytoplankton, evaluated with nets and thus larger algae, did increase shortly
after water-column stratification. In general, blooms, where they are important, begin
shortly after the first, non-transient establishment of the seasonal thermocline. Bloom
timing varies between years according to variation in the surface heating. Where