ocean in late winter, often February or March, when mixing reaches slightly below
200 m. There is a modest increase then in production and phytoplankton standing
stocks (Steinberg et al. 2001); much of both coming from the picoeukaryotes and
“large algae”. Probably the main difference between the Atlantic and Pacific time-
series in regard to winter mixing and production occurs because BATS is 10° farther
north than HOT, experiencing both greater winter cooling (and thus convective
mixing) and greater winter winds. The shift from Prochlorococcus to Synechococcus
during the BATS period of deep mixing corresponds, too, to a general latitudinal shift
in their relative dominance northward. It is likely that there are late winter blooms at
the poleward edges of all gyres similar to that at BATS and a gradient in seasonality
of production. The annual total production average for BATS, 155 g C m−2 yr−1, is
somewhat less than the HOT estimate of ∼190 g C m−2 yr−1. Interannual variations in
productivity occur in subtropical gyres, but with monthly time-series sampling at a
single location, the resolution and spatial coverage are inadequate to estimate
variation of annual totals reliably, although an increase of 1.5-fold from 1989 to 2004
is suggested by the HOT data (Corno et al. 2008).
Fig. 11.25 BATS time-series data showing the late winter injections of nitrate into the
euphotic zone and the responses of phytoplankton standing stock (chlorophyll-a
abundance) and primary production rate. The persistent chlorophyll maximum,
usually just above 100 m, is typical of all oceanic, tropical waters.
(^) (After Steinberg et al. 2001.)