winter mixing (that often extends to >200 m) by Synechococcus that for a few months
increase to 2–3 × 10^12 m−2 (∼0.2 g C). Eukaryotic picoplankton are fewer in numbers
but about equal in biomass to the combined cyanobacteria, particularly in the late
winter “bloom”. At this location, however, the periods of greatest cyanobacterial
dominance are those of reduced primary productivity, and small eukaryotes account
for a majority of annual organic matter production. Worden and Binder (2003) used
flow cytometry in March at BATS to examine the numbers of Prochlorococcus cells
from 50 m depth in different cell-cycle stages. DNA replication (S-phase) occurred
from late afternoon through evening. Cells with double DNA complements peaked at
midnight, followed by doubling of abundance from 20 to 40 × 10^3 ml−1. Grazers
returned counts to daytime levels by mid-morning. Similar diel cycling of somewhat
lower cell numbers was observed for Synchococcus. Growth rates of both species
from parallel dilution experiments mostly were just less than one doubling per day.
(^) Campbell et al. (1994) performed a comparable flow cytometry study at HOT (22°
45′N, 158° 00′W), finding much stronger and seasonally consistent dominance of
Prochlorococcus over Synechococcus. They also compared the abundances of
autotrophic and heterotrophic bacteria (Table 11.4). Liu et al. (1997) used flow
cytometry to examine the Prochlorococcus cell cycle at HOT, using the diel shifts in
cycle phasing to estimate growth rates. As in the Sargasso, cell replication was almost
entirely in the middle of the night and implied that rates near the surface were less
than roughly one doubling per day, less below 80 m. Cell numbers peaked around
dawn and were at a minimum around sunset, sustaining approximate daily balance of
production and consumption.
Table 11.4 Average percentages (Oct, Dec, Jan, Feb, Mar, Apr) of bacteria and algal
particles at HOT for the surface mixing layer (0–70 m), the deep chlorophyll
maximum (DCM), and integrated carbon biomass. Seasonal variability is modest and
0–70 m abundances are approximately constant, usually tapering to half or less by 120
m, except for March when they taper from 150 m.
(After Campbell et al. 1994.)
Primary productivity in the NPSG at HOT does not have a winter or spring peak,
mostly because there is no deep mixing event most winters that would fuel a bloom
with nutrients. The 20-year averages of monthly HOT data (Fig. 11.24) show a mid-