The nutricline is also the level of a doubling to tripling of chlorophyll concentration,
the deep chlorophyll maximum (DCM, Fig. 3.9, Fig. 11.23a). This is observed in
virtually all subtropical and tropical oceanic regions. The significantly increased
nutrient availability just above the photosynthetic compensation depth enables
phytoplankton to synthesize more chlorophyll (which requires iron and uses much
fixed nitrogen) to compensate for the low irradiance: they “spread their antennae” to
gather the waning supply of photons. Principally, it is a shift in the amount of
chlorophyll per cell, not an increase in the cell counts. On the other hand, Venrick
(1982) has shown for the “CLIMAX” station (30°N, 155°W) that at least the diatom
component almost completely changes in species composition at a fairly sharp
boundary just above the DCM. The flora is not simply shade-acclimated individuals
of the phytoplankton above; it is a different, shade-adapted community. There are also
increases in the cell sizes of cyanobacteria (DuRand et al. 2002). The contribution of
the DCM to total production is significant, but the bulk of integrated production
occurs only a short distance below the sea surface (Fig. 11.23b).
Primary Producers
(^) In all subtropical gyres the dominant phytoplankton in numbers and biomass are
picoplankton. DuRand et al. (2002) used flow cytometry at BATS (31°40′N,
64°10′W) to characterize their seasonal cycling. Two genera of cyanobacteria,
Prochlorococcus, and Synechococcus, alternate in numerical dominance. Summer to
early winter numbers of Prochlorococcus integrated to 200 m (most cells above 125
m) are on the order of 10^13 m−2 (∼0.5 g organic carbon), partly replaced during mid-