grazer stocks are very low in winter and early spring, the theory works well because
grazing then is a minor part of “community” respiration. This is true in the mid-
latitude North Atlantic and elsewhere close to coasts, so the theory works well for the
“classical” seasonal cycle problem. In other regions, the mixing depth remains
important, but it sometimes works in a somewhat different fashion (e.g. see Nelson &
Smith 1991). Seasonal mixing effects on phytoplankton stocks are different over
shallow shelves, since the bottom can act as a lower limit to vertical mixing, a limit
always above the critical depth. A study by van Haren et al. (1998) provides a high-
resolution time-series of chlorophyll and other relevant data from a site at 45 m depth
in the central North Sea. Chlorophyll increased from the mid-winter level of 0.5 to 3–
6 mg m−3 starting in mid-February, as soon as photosynthetically active radiation
(PAR) exceeded 6.5 μmol photons m−2 s−1. That is approximately the physiological
compensation (PS − R ≥ 0) intensity in those waters (Tett 1990). Water-column
stratification was only established long after the bloom became nutrient limited and
diminished. Such early-season blooms are reported for many shallow, coastal sites,
including Georges Bank and Narragansett Bay.
Fig. 11.9 Data for 1949 from the weathership at Station “M” (66°N, 28°E), showing
the relationship between the approximate critical depth (shading between approximate
k values of 0.075 and 0.10) and mixing depth. Phytoplankton counts increased in
April–May, when critical depth exceeded the mixing depth.
(^) (After Sverdrup 1953.)