300–400 m.
(^) As stated, the dominant phytoplankton are in the nano- and pico- size range. A
significant but never dominant part of the biomass is Synechococcus, particularly in
late summer and fall beneath the thermocline and above the halocline.
Prochlorococcus, common in even more oligotrophic waters, are nearly absent. There
are generally eukaryotic phytoplankton as major constituents, accounting for more
carbon than the picophytoplankton, but exactly which are abundant varies with every
visit to the oceanic sector. Booth et al. (1993) observed coccoid Chlorophyceae
(Chlorella and Nannochloris), Prasinophytes (Mantoniella spp.), Chrysophyceae,
small diatoms (Nitzschia cylindroformis), and Prymnesiophyceae (Phaeocyctis spp.)
as dominant forms on different dates, and dozens of other species of secondary
importance. Dominance of any particular species or group has not yet been connected
to specific conditions. All are species expected to be readily accessible to protozoan
grazers, and observations show abundant heterotrophic and mixotrophic
dinoflagellates, choanoflagellates, flagellates (e.g. Bodo cf. parvulus) and ciliates
(Strombidiidae, including Laboea and Strombidium).
(^) The mesozooplankton in the subarctic Pacific are dominated, especially from March
to July, by five species of copepod: Neocalanus plumchrus, Neocalanus flemingeri,
Neocalanus cristatus, Eucalanus bungii, and Metridia pacifica. Copepodite stages of
the first two are abundant in the layer above the seasonal thermocline; those of the
latter two are abundant below it and probably primarily scavenge fecal matter sinking
down from the surface layer. Metridia is a strong diel migrator (>200 m in older
stages); the others are not. All but Metridia have a resting stage as fifth copepodites
(C5). This stage loads with large amounts of nutriment stored as liquid wax, then
descends to depths below 400 m (to >2000 m) as a refuge from late summer warming
and predation. The different species have somewhat different reproductive schedules,
as discussed in Chapter 8. The dominant species, N. plumchrus, distributes its
spawning at depth from late summer through to early spring, apparently “spreading its
bets” across the entire season. The winning nauplii that get food to grow are those
arriving at the surface as first-feeding stages when production increase is strong due
to rising insolation and surface thermal stratification. All of these copepods feed
primarily on protozoa, and they constitute the trophic level that shows strong seasonal
cycling (Fig. 11.4). Thus, seasonality of primary production is first exhibited as
biomass accumulation two to three trophic levels away from the primary producers.
Fig. 11.4 Annual cycle of zooplankton biomass in the upper 150 m in the oceanic
subarctic Pacific. The build-up in spring is from growth, mostly of large copepods.
Decline in early summer is from descent of these copepods to depths below 500 m for
diapause.
(^) (After Fulton 1983.)