Quantitative ecology investigations of the zooplankton during the fertilization experiments in the bay Veliko jezero (I. Mljet)
Abstract
Based on the rich material collected during the complex oceanographic investigations from 1951 to 1955 in the region of the bay of Mljet ecologic examinations of zooplankton were carried out. The present work brings the results of these examinations during the experiment of fertilization in the bay Veliko jezero.
Examinations have shown that the bay of Mljet is a unique biotope for ecologic examinations of plankton communities due to its topographic and hydrographic conditions. This deep bay, in one of its parts (Veliko jezero) preserves very low temperatures even in the summer period. The inflow and outflow currents are of surface character because the entrance is very narrow and shallow and, as the bay does not greatly exchange water with the open sea, it could serve as a large natural aquarim. In the basin of the Veliko jezero the temperature varies from 8.2 to 27.0° C, the salinity being from 30.0 to 37.2% c. Due to these changes there is some oscillation in the sea-water density so that the surface layers density varies from 22.7 to 28.3 σt, The sea-water transparency changes considerably each season, the annual differences might be from 10.5 to 21 m. In the bay Malo jezero the periodical occurrence of H2S was established with the result that at that time it was much poorer in species than the Veliko jezero. It was thus found that in 1954 the Malo jezero was aerated i. e. without H2S, while at the same time considerable quantities of copepod C. helgolandicus, which had not been present before in this part of the lake, were detected.
As the main qualitative characteristic of the zooplankton community of this biotope, it has been established that the zooplankton is represented with few species, but with a high density of population. The main biomass consists of the copepods which have been represented throughout the year from 27.9-94.6%, followed by the mollusc larvae from 1.1-64.5%, chaetognaths 0.4-9.2%, copelats 0-8.1%, and at last by other groups in smaller quantities.
Among the copepods per biomass first is Calanus helgolandicus, followed by Paracalanus parvus and Pseudocalanus elongatus. It has been found that the population density of the copepod C. helgolandicus in the Veliko jezero has reached the highest values up to now recorded in the Adriatic.
Examining the total zooplankton data (dry weight) it has been established that the Veliko jezero, according to the high standing-crop, comes first in relation to the other areas in the Adriatic.
The seasonal variations in the zooplankton production show that the maximal quantities always appear during the warm period (June, July, August), while in the open Adriatic usually much earlier, in March and April.
The relation between the zooplankton crop fluctuation and the ecologic factors has been examined. The relation between the zooplankton and the bacteria has shown that the zooplankton increase appears at the same time with (1952) or somewhat earlier (1951) than the bacterial increase. The relation between the zooplankton and the phytoplankton is rather similar, i. e. in 1952 and 1953 the zooplankton increase started after the phytoplankton did, except in 1954. Then the main vernal phytoplankton maximum lasted for a short time so that it could not have been registered, but the later phytoplankton increase was the result of the fertilization (P u c h e r - P e t k o v i ć, 1960).
Analyzing the fluctuation of the zooplankton changes it has been stated that with the temperature increase, the zooplankton increases as well. This is the result of, besides the phytoplankton production and the holozooplankton reproduction, a higher presence of merozooplankton during the warmer part of the year. According to the findings it seems that, due to the temperature increase of the surface layers, there might be a horizontal shifting of the zooplankton towards the bay’s depression (station Vrbovačka) in which there are rather stable low temperatures even during the summer months. This might result in a certain biomass increase or the zooplankton density in this part of the bay.
The transparency changes of the sea-water were not the result of the phytoplankton production increase only (P u c h e r - P e t k o v i ć, 1960), but also of the more intensive zooplankton increase. The low transparency was recorded with the low values of zooplankton but with the appearance of the copelats Oicopleura dioica in greater quantity.
It was found that the standing crop of the zooplankton in 1951, i. e. before the fertilization, and the one in 1954, during the fertilization, differed in 8.8 mg per m3 or cca 42% which is also one indirect proof of the primary production increase in the Veliko jezero after the fertilization besides those previously established by B u l j a n (1957), M o r o v i ć (1958), and P u c h e r - P e t k o v i ć (1960).
Besides this, the increase in carnivora in 1953 was established, especially in the pelagic fish and larvae which, due to good feeding conditions, had a low mortality (B u l j a n, 1957; V u č e t i ć, 1957). This is thought to have contributed to the balance of the zooplankton production, i. e. it has prevented a higher accumulating of the zooplankton standing crop, which is, according to C u s h i n g (1959), the sign of a balanced system or production cycle.
