Vertical distribution of zooplankton in the bay Veliko jezero on the island Mljet
Abstract
It appears from the investigations of the daily vertical distribution of total zooplankton during the summer season that certain organisms regularly kept near the surface (Paracalanus parvus, Centropages Kröyeri, Evadne spinifera, Evadne tergestina, Muggiaea kochi, and decapod larvae) while others remained in the deeper layers near the bottom (Pseudocalanus elongatus, Calanus helgolandicus, Oicopleura dioica and lamellibranch-larvae). Some others, however, were observed in all the layers (Oithona nana, Sagitta setosa, and gastropod larvae).
A diurnal vertical migration of some species has been established on the basis of occurrence and different concentrations of zooplankton organisms in various layers in the course of 24 hours. A more intensive migration has been observed during the warmer months (May, June, July) when a termal stratification of the sea water prevails and when the light is more intense.
In January and November, however, when the homothermal state prevails and when the light is less intense, those species occur in all the layers from surface to bottom by day and by night.
It has been observed with Calanus helgolandicus that this species does not pass the 13-15° C temperature limit during its night-time migration towards the surface at the time of the maximum temperature of sea water in August.
An even lower temperature limit (about 13° C) has been established in the case of Pseudocalanus elongatus which species seems to be stenothermal.
No regularity in the vertical distribution of Paracalanus parvus in connection with temperature chages of the sea water has been observed.
No correlation has been established between the vertical distribution of zooplankton organisms and fluctuation of salinity in the sea water.
