JOURNAL OF PLANKTON RESEARCH
VOLUME 32 j
NUMBER 4 j
PAGES 441–456 j 2010
Fig. 1. Scheme of the experimental chamber (plan view).
10.00 to 16.00, to exclude the diurnal rhythm factor. We used small fluorescent UVB lamps UF-19 (Russian analog to PHILIPS 9 Watt Short Compact Fluorescent UVB Narrowband PL-S9W/01) and red and yellow LED lamps as a light source in the experiment. The UV light is scattered in the very upper water layers of 1–2 m, and the red and yellow light penetrates deeper down to 20 m depth (Ahlquist, 1965, cited under Jerlov, 1976), so we assume these two wavelengths may indi- cate the upper and lower boundaries of the photic layer for the plankton. The light intensity was regulated by a dimmer and set up accordingly to illuminometer values.
The light patch of 15 mm diameter and of about 30 lux intensity (similar to 5 m depth at midday) was created by an iris aperture and was applied evenly to the left and the right part of the experimental chamber, 10 replicates each, to exclude any factor of chamber pos- ition. Animals, using a new sub-sample each time, were gently placed via Pasteur pipette in the center of the experimental chamber with open ’doors’. The duration of the experiment was 10 min, then the ’doors’ were closed, animals were taken from each part (’light’, ’center’ and ’dark’) separately by Pasteur pipette and counted under a dissection microscope (magnification
40). A minimum of 20 replications with new animals
each time was performed for each experimental series.
significant and therefore they were combined (red and yellow). Primary data were normalized via arc tg root transformation (Sokal and Rohlf, 1995). These values were used later in ANOVA to analyze the effects of such factors as acclimation duration, light wavelength, sampling depth and food conditions. If the animals dis- tributed evenly in the experimental chamber and no significant differences were found in their relative abun- dance in the chamber parts, we assumed that no photo- taxis occurred. All the zooplankton sampling and thus experiments for each season were conducted within a 7–10 days period. The datasets were combined on each species for certain season, water layer occupied, light wavelength and acclimation period and food/light acclimation pattern. Animals were divided by (i) trophic characteristic into ’herbivorous’ and ’non-herbivorous’ and (ii) biogeographical distribution as arctic, boreal and eurybiont species (Table I). The designation ’herbi- vorous’ in this study includes those animals with algae in their diet, so this group united herbivorous, as well as predominantly herbivorous and omnivorous. Others were treated as non-herbivorous. Heterogeneity of the variances was tested with the Levene test.
R E S U LT S
Vertical distribution of zooplankton species: temperature preferences
The Student t-test was used to assess the significance of the differences between means. The differences between the animal responses to red and yellow light were not
The distribution of the species in the photic layer differs from season to season (Fig. 2). We treated the spring and autumn equinoxes and the polar day as the season of both cold photic and aphotic layers according to the
Downloaded from http://plankt.oxfordjournals.org/ by guest on February 11, 2015