JOURNAL OF PLANKTON RESEARCH
VOLUME 32 j
NUMBER 4 j
PAGES 441–456 j 2010
Fig. 4. The vertical distribution of the species, % of total species number, of different food, temperature and light preferences in the photic and aphotic layers: (a) all the species; (b) only herbivorous. Phototaxis (light response) pattern: þ, positive; 2, negative; 0, indifferent.
species. Almost all boreal herbivorous species present in the photic layer during the warm period have positive phototaxis (Fig. 4b). However, this pattern changes slightly for the Arctic herbivores, which could include up to 40% indifferent to and light avoiding.
The UV effect on the light response
Three species were tested, Acartia spp. (boreal, omnivor- ous), P. minutus (arctic, predominantly herbivorous) and O. borealis (arctic, with unknown food preferences). Acartia spp. (photic layer) and P. minutus (aphotic layer) showed positive phototaxis in August (Table II). Mature
and older copepodites (CIV-CV) of P. minutus increased their positive response to the UV light after 24 h acclimation to red light. The same was observed for mature females of Acartia spp.
In March, P. minutus copepods also showed positive UV phototaxis. Furthermore, 24 h and longer periods of starvation (keeping animals in filtered water) increased their light responses. However, animals sampled from different water layers or acclimated at different light conditions did not significantly change their behavior. The interaction between the ’light’ and ’food’ factors was not significant, ANOVA: F(1, 128) ¼ 0.001; P ¼ 0.97. Oncaea borealis was indifferent to UV
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