Dickey’s convincing model of the interactions among available irradiance,
enhancement of absorbance by phytoplankton pigments, and mixing (a function of
measured wind speed) suggests that the effect of pigment on stratification is at most a
one-day acceleration of the bloom.
Mesozooplankton
(^) Like the subarctic Pacific, the dominant mesozooplankton are copepods, particularly
species of Calanus and Pseudocalanus. Among larger copepods, Calanus
helgolandicus is dominant to the east and just north of the Gulf Stream, Calanus
finmarchicus in a northeasterly expanding stripe from Cape Cod crossing in the
vicinity of Iceland then into the Norwegian and Barents Seas, and Calanus glacialis
more abundant approaching and under the arctic ice. All of those share the family trait
of resting in late copepodite stages (C4 and C5). Calanus finmarchicus matures at
different dates according to subregion (Plate 8.2): January in the Gulf of Maine,
February in the Norwegian Sea and March in the Irminger Sea (Planque et al. 1997).
In each case, the timing anticipates the spring bloom by weeks to months, so that
females are matured and actively spawning when the spring bloom gets started.
(^) Unlike the subarctic Pacific Neocalanus species, Calanus females must eat to
spawn. Niehoff et al. (1999) showed for the Norwegian Sea that pre-bloom spawning
produces copepodites that use the bloom to grow – the main stock production of the
year. Females are actually much less numerous during the bloom than before,
although spawning at maximal rates, and copepodites from eggs produced during the
bloom must depend after the bloom upon microheterotrophs that constitute suitably
sized food in that interval. In all regions except the Irminger Sea, diapause of C.
finmarchicus at depth resumes in June or July. Irminger Sea stocks only rest after
about October. Over most of the range, except for the northern Norwegian Sea, there
are two generations – the first maturing immediately without rest (at least without
prolonged rest) to produce the second. Many of the smaller copepods are sac
spawners, Oithona and Pseudocalanus in particular. The relative importance of large
and small copepods in grazing is not well quantified. The abundant epipelagic North
Atlantic euphausiids belong to only a few species: Euphausia krohni (southern
subarctic and farther south), Meganyctiphanes norvegica (particularly abundant in
shelf waters), Thysanoessa inermis and Thysanoessa longicaudata (both abundant in
more northern reaches above Iceland).The dominant chaetognath, Parasagitta
elegans, is also found in the North Pacific.
Upper Trophic Levels
(^) Small schooling fish of the subarctic Atlantic include several distinct populations of
Atlantic herring (Clupea harrengus), which migrate to nearshore spawning areas in