JOURNAL OF PLANKTON RESEARCH
VOLUME 32 j
NUMBER 4 j
PAGES 441–456 j 2010
(illumination minimum) and secondl , near midday (illumination maximum). The dynamics of food con- sumption by these crustaceans also have two maxima associated with morning (8.30) and evening (20.20) hours, when the proportion of feeding individuals in the population in the surface water layer reaches the maximum value of 85% (Vakatov and Martynova, submitted for publication). It is also possible to divide the animals by biogeographical groups into Arctic
glacialis, P. minutus, M. longa, O. borealis) and boreal
longicornis, C. hamatus, Acartia spp., E. nordmanni).
Oithona similis is a eurybiont species (Prygunkova, 1974). The light responses of predominantly herbivorous and omnivorous copepods and cladocerans, such as C. glacialis (May), Acartia spp., E. nordmanni and young copepodites of C. hamatus and T. longicornis agree with the patterns of the vertical distribution of their popu- lations (Prygunkova, 1974) and their potential food in the White Sea. These species tend to inhabit the upper water layers with sufficient food availability. Feeding rates of T. longicornis, C. hamatus, Acartia spp., inhabiting the surface (photic) layer in the White Sea during the whole summer, depend significantly on the illumination level (Martynova, 2005). For example, in Acartia spp., this parameter is significantly higher at high illumina- tion. The significant differences in the light responses between hungry and fed animals indicate that light is a signal of food availability for these species. As it was shown earlier, hungry animals increase their positive reaction to light, which agrees with data on hungry and non-hungry Daphnia behavior (Stutzman, 2000). The hungry animals tend to respond strongly to light than more fed. Thus, RY light could provide a signal of food availability for animals, inhabiting the upper (photic) water layer, especially hungry. In addition, the water layer with illumination optimal for boreal species E. nordmanni and Acartia spp. (the most intensely feeding in the light) may also be optimal with respect to temp- erature. It is known that the White Sea is characterized by pronounced vertical water stratification (Babkov, 1998). The upper 15–20 m layer inhabited by boreal crustaceans has the highest temperature in summer. Furthermore, the rate of feeding in all crustaceans of the superfamily Centropagoidea is significantly higher at þ15/þ168S than at þ108S (Martynova, 2005), usually observed in the uppermost 0–5 m layer in July– August, which is the reproductive period in these species (Prygunkova, 1974). This temperature depen- dence was also shown for Daphnia longispina (boreal species). The range of vertical diurnal migrations was higher at lower water temperatures, close to those criti- cal for normal development of this species (Young and Watt, 1996). Moreover, daphnids migrate to the upper
layers not only for feeding, but also to experience optimal temperature for ovarian development even when the upper water layer is low in seston (Winder et al., 2003). Low temperatures could also slow the light behavioral response in several limnoplankton species (Persaud and Williamson, 2005).
Two groups, arctic and boreal, significantly depend on the temperature. From this perspective, positive phototaxis of boreal (’warm water’) species (Acartia spp., C. hamatus, T. longicornis, E. nordmanni) must be essential for their survival in the harsh White Sea environment (Martynova et al., 2009). On the other hand, Hansson et al. (Hansson et al., 2007) showed that nearly continu- ous daylight at high latitudes relaxes the diel migratory behavior in zooplankton making it independent of the predation risk. At lower latitudes, however, such nearly continuous daylight leads to pronounced diel rhythms in migration.
Zooplankton may show local behavioral adaptations in their circadian rhythm. They are also able to assess potential benefits of diel migration and completely sup- press diel migration at constant daylight irrespective of the predator risk (Hansson et al., 2007). However, the behavior of mature C. hamatus (boreal species) remains difficult to understand. These copepodites are charac- terized by negative phototaxis that corresponds well with data, indicating that they feed intensely in darkness (Martynova, 2005). The maximum ingestion rate for a related crustacean species Centropages typicus was noted in twilight and at night (Saiz et al., 1992; Calbet et al., 1999). In contrast, adult individuals of a different crus- tacean, T. longicornis divided into two groups with oppo- site patterns of phototaxis in the autumn period, when the changes in night and day light regime are pro- nounced. Most of them were still characterized by a negative light response, which is supported by the data indicating that feeding was more active in darkness (Martynova, 2005). Furthermore, mature C. hamatus and T. longicornis inhabit the upper 0–10 m water layer in the White Sea. During the da , they prefer the 5–10 m depth layer, whereas at night, migrate to the surface (0– 5 m) (Kutcheva, personal communication). We explain this pattern by dividing the population of these two species into two groups depending on the developmen- tal stage which would help to reduce intraspecific food competition. The separation of predominantly herbivor- ous Arctic P. minutus into two groups with different light responses throughout the year is difficult to explain. However, the vertical distribution of this species in the White Sea has two maxima, especially in summer (Martynova and Kutcheva, unpublished results): first, in upper water layers (10–25 m) and the second, below the thermocline (60–100 m). Also, P. minutus has
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