D. M. MARTYNOVA AND A. V. GORDEEVA
LIGHT-DEPENDENT BEHAVIOR OF ABUNDANT ZOOPLANKTON SPECIES
(Burenkov et al., 2004). Diel patterns of vertical distri- bution of certain copepod species have been described previously (Bogorov, 1946; Pertzova, 1997), but these authors did not arrive at the same conclusions. All the data on vertical distribution and diel vertical migrations of calanoids inhabiting the White Sea are predomi- nantly published in Russian and are extremely outdated. There are no data on the effects of light on the behav- ior of White Sea zooplankton, and the driving factors of the diel vertical migrations are still open. This study is the first to distinguish experimentally the differences in the behavioral responses to specific light wavelengths for major White Sea zooplankton species in relation to their feeding, vertical distribution, biogeography and life cycle. Our main approach involved the choice between darkness and light (UV or visible, red and yellow) of the intensity close to natural. Thus, it was possible to test the hypothesis that these wavelengths indicate the layer of optimal food availability especially for herbivorous and migrating animals.
Fifty-two experiments were performed to investigate the light-dependent behavior of the abundant zooplankton species in different seasons during 2005–2006: (i) the spring equinox, 12/12 h day–night light pattern (ice coverage, end of March); and three times through the ice free season; (ii) the polar da , 22/2 h day–night
(late May–June), (iii) 17/7 h August), (iv) the fall equinox,
day – night
day – night
(beginning of October). The day length was defined as a period between the sunrise and sunset according to the ephemeris. Zooplankton sampling was performed close to the Cape Kartesh (Kandalaksha Ba , the White Sea, 66820.2 N; 33838.9 E, 65 m depth) and in the innermost part of Chupa Bay (Kandalaksha Ba , the White Sea, 66820.0 N; 33837.7 E, 30 m depth). Zooplankton was sampled by means of a Juday net (37 cm diameter, 100 mm mesh) using standard tech- niques (Harris et al., 2000). The water column was divided into two sampling layers: (i) photic, 0–15 m, (ii)
aphotic, 15 m to (Burenkov et al.,
bottom, according to 2004). The animals
Burenkov et al. were collected
during the day (about maximum light intensity), tical distribution) is more
midday in the period of because the light impact (ver- pronounced during the day-
the vicinity of zooplankton sampling sites. Light inten- sity measurements were performed simultaneously in the water column at 0, 5, 10 and 15 m depth by means of illuminometer U-116 (Russian analog to FX-200 Illuminometer). We have chosen the light intensity of 5 m depth (about 30 lux) as experimental, because it constituted about 10% of light intensity for the red light observed at the surface (Jerlov, 1976), and varied as 300 to 600 lux. Sampled zooplankton were gently diluted with seawater, placed in a dark container and immedi- ately transported to the shore laboratory.
Acclimation of zooplankton
Live animals were sorted by species and developmental stages under the dissecting microscope (total magnifi- cation 40) and then acclimated to either total dark- ness or visible ’scattered sunlight’ (UV and IR radiation reduced, 30 lux intensit , LED white lamps), during 24, 48, 72 or 96 h in temperature conditions similar to natural. The animals were also kept under two different food conditions: (i) natural seawater was collected according to the zooplankton sampling by 5-l Niskin bottle close to surface and (ii) filtered seawater (GF/C fiber glass filters, 1 mm). We tested if the length of the light/food acclimation period might increase/decrease the light response of the animals and also tried to find the ’critical’ time period for such increase/decrease. The acclimation of animals and further experiments were conducted at the temperature similar to natural (the temperature of zooplankton sampling depth) to avoid temperature-induced stress. The light-dependent behavior of 10 zooplankton species was investigated (eight species of Copepoda (Metridia longa, Calanus glacia- lis, Pseudocalanus minutus, Oithona similis, Oncaea borealis,
emora longicornis, Centropages hamatus, Acartia spp.), one Cladocera species (Evadne nordmanni) and Polychaeta larvae. The number of specimens used in each exper- iment ranged from 28 to 500 individuals and was size-dependent.
The experimental chamber was made of transparent plastic, 50 250 50 (W:L:H) mm, the central part (50 50 mm) could be isolated by means of sliding ’doors’ (Fig. 1). The experimental chamber was filled with natural seawater of specific temperature. The depth of the water layer was 5 to 7 mm, and as the animals ranged in body length from 0.1 to 2.2 mm, ver-
light (see review, measurements were with 0.18C accuracy
Mauchline, 1998). Temperature performed with a MIDAS CTD through the 0–15 m water layer at
gravitation-induced movements were excluded. All the experiments were performed during the daytime from
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