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KEYWORDS: Copepods; UVB; Visible light; Life cycle; Feeding - page 7 / 16

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D. M. MARTYNOVA AND A. V. GORDEEVA

j

LIGHT-DEPENDENT BEHAVIOR OF ABUNDANT ZOOPLANKTON SPECIES

Fig. 2. The vertical distribution pattern of the species and temperature profiles. The pie charts indicate the contribution of each biogeographical group to total species numbers.

thermocline was observed in this period and the temp- erature in the whole water column was only several degrees above 0 Celsius (autumn equinox) or even below (the spring equinox and polar day). The summer period (August) was characterized by a well-pronounced thermocline, and thus we treated this as a period with warm photic and cold aphotic layers. Vertical species distribution significantly differs depending on the temp- erature of the water layer (Fig. 2). The minor impact of boreal species in the period of cold photic layer is the result of their decreasing presence in the autumn.

Responses of zooplankton species to red and yellow (560–680 nm, RY) light

significantly by the duration or light conditions of acclimation (Table I).

The eurybiont cosmopolitan species O. similis, inhab- iting the whole water column in the White Sea, dis- played a clear, positive response to light, which was more pronounced after prolonged acclimation of these animals in darkness in May and August (Fig. 3b and Table I). The specimens, sampled in the periods of spring and autumn equinox, were also positively photo- tactic, but were indifferent to any of the acclimation factors applied. Mature copepodites, sampled in the aphotic layer in August, showed no significant intensifi- cation of the phototactic reaction after acclimation in both light and dark conditions (Table I).

Various acclimation designs did not affect the phototac- tic reaction. Zooplankton response either stayed unchanged or become more intense. Thus, we further refer to either significant intensification or no effect of acclimation on light response.

Eurybiont species Polychaeta larvae representing different biogeographical groups were combined since they displayed no signifi- cant differences in light response [ANOVA: F(1, 60) ¼ 0.53; P ¼ 0.45]. Polychaete larvae were abundant only in the spring (May) in the photic layer soon after the ice

melting and all

showed

positive

phototaxis

(Fig. 3a).

However,

their

light

response

was not

affected

Boreal species Positive phototaxis was characteristic of the clado- ceran E. nordmanni, inhabiting the surface layer during the summer. Meantime, there were no signifi- cant effects of the duration or light conditions of acclimation on the responses of the animals to light (Table I), as was observed for Polychaeta larvae. Keeping animals without food also did not intensify the light response.

Neritic copepod species (Acartia spp., T. longicornis, C. hamatus), which also dominated in the photic layer in summer time, had a different response pattern to RY light. Pre-mature developmental stages of Acartia spp. tended to increase their positive light responses after

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