winter low of about 30 mmol C m−2 day−1 (integrated to 100 m), and a May to August
high around 54 mmol C m−2 day−1. In mass units the annual mean is ∼514 mg C m−2
day−1; annual total ∼190 g C m−2. The effect of the shallow summer mixed layer,
maintaining the phytoplankton stock high in the euphotic zone, is likely more
important to the summer peak of production than the maximum in irradiance. In
addition to production by pico- and nanophytoplankton, there are also in many years
subsurface “blooms” of diatoms during which chlorophyll rises to ∼0.15 μg liter−1
(Venrick 1974; Dore et al. 2008). The diatoms belong to the genera Rhizosolenia,
Hemiaulus, and Mastogloia, all of which harbor symbiotic, nitrogen-fixing
cyanophytes called Richelia. As Dore et al. document, these subsurface blooms occur
principally in cyclonic eddies, which generate upward mixing of nutrients from the
vicinity of the nutricline, including the silicic acid to synthesize diatom frustules. Just
this brief pulse of diatom production and symbiont nitrogen fixation adds
substantially to the annual total of new production in the region and presumably to the
draw-down of nutrients.
Fig. 11.24 Monthly averages (with range bars and quartile boxes) at Stn. ALOHA of
mixed-layer depth, depth-integrated (0–100 m) [nitrate + nitrite], incident flux of
photosynthetically available radiation (PAR) and depth-integrated (0–100 m) primary
production.
(From Church et al. 2009.)
In the Sargasso Sea (Fig. 11.25) a modest pulse of nitrate usually reaches the upper