(^) Extinction of light with depth means that at some depth, fairly close to the surface,
irradiance will be the primary factor limiting productivity. That is true everywhere in
the oceans. When it is said that phytoplankton growth is nutrient-limited, reference is
to the upper water column or euphotic zone. The photosynthetic compensation depth
varies with surface irradiance and the absorption coefficient (k), and is often assumed
to be the depth at which irradiance is 1% of the surface value, maximally 70 m at
lowest k(−0.067). However, subtropical gyres with minimal k have a strongly shade-
adapted flora in the deeper euphotic zone forming a “deep chlorophyll maximum”
(Fig. 3.9) around 100 m (∼0.1% of the surface irradiance) in which net photosynthesis
is still positive. This very widespread feature of the subtropical and tropical oceans is
due to large amounts of chlorophyll per cell resulting from shade acclimation,
although changes in species composition also contribute to some of the differences.
Fig. 3.9 A vertical chlorophyll section from 49°N to 33°S in the Atlantic Ocean.
North of 45°N and south of 40° the chlorophyll maximum is in the upper 50 m. Deep
chlorophyll maxima occur in the subtropical gyres between 100 and 175 m depth.
Surface outcropping and a shallow (50 m) subsurface maximum occur within the
equatorial band.
(^) (After Serret et al. 2006.)
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