Zooplankton investigations in the sea water lakes »Malo Jezero« and »Veliko Jezero« on the island of Mljet (1952-1953)
Abstract
The investigations upon which the present study is based were an integral part of a complex research work aimed at recording the prevailing conditions in the area of the two sea water lakes on the Island of Mljet in order to find out the degree of their productivity.
The samples obtained by vertical hauls with the Hensen net at two permanent stations, Malo Jezero and Veliko Jezero, during the time span from February 1952 to February 1953, were used for the quantitative and qualitative analyses of zooplankton. To make a comparison possible, additional investigations were made at a station situated at the entrance from the offing (Gonoturska).
In order to obtain as accurate data as possible, measurements were made of both the »wet« (semi-dried) and dried zooplankton, as well as of the volume of sediment, and the counting of total zooplankton was also performed.
(1) No pronounced variations of the quantitative data result from the value curves of the above measurements for the station Malo Jezero. It has been found that this was due to a rather regular zooplankton composition, made up mainly of copepods whose volume and water content do not manifest great variations.
Owing to pronounced variations of the zooplankton composition, the quantitative data differed considerably at the station Veliko Jezero. It has been established that the high values of zooplankton volume and »wet« weight are due to the occurrence of adult Sagitta which contain a high percentage of water, while the high values of zooplankton dry weight and the low values of volume are caused by the abundance of mollusca larvae.
At the station Gonoturska, considerable differences appeared in May between the number of organisms on the one hand, and the volume of sediment, the »wet« and the dry weight on the other hand, due to the numerous occurrence of copepodite stages.
(2) By comparing the dry weight values referring to zooplankton biomass at the stations Malo Jezero, Gonoturska and Veliko Jezero, the ratio 1: 3: 17,7 has been obtained.
(3) It has been established that the high values of zooplankton biomass, found for the station Veliko Jezero, stand markedly out among all the values known so far for the Adriatic.
(4) It is evident from the monthly averages of zooplankton dry weight that the maxima at both Malo Jezero and Veliko Jezero have a regular occurrence during the summer months, i.e. when the surface temperature culminates, while the minima are found during the winter. At the outer station Gonoturska, on the contrary, the maximum dry weight values were noticed by the end of winter and during the early spring, while the minimum ones took place in autumn.
(5) Another result of the investigations is the observation that the Copepoda group is by far the most numerous group in the investigated area, occurring in percentages ranging between 27 and 97% at the station Veliko Jezero, and between 94 and 97% at the station Malo Jezero. - It has also been found that the species Acartia clausi (50,88%) ranks first in abundance at the station Malo Jezero, with Centropages kröyeri (31,04%) following next, while the most numerous species at the station Veliko Jezero is Pseudocalanus elongatus (43,3%), followed by Calanus helgolandicus (14,5%) and Paracalanus parvus (11,4%).
(6) In connection with the hydrographic and topographic conditions prevailing at the station Malo Jezero (which, being a shallow basin, is subject to great annual temperature and salinity fluctuations, beside having a H2S zone which lies at a depth of 19-20 m) it has been established that but a small number of eurythermal and euryhaline species occurred in the area, such as copepods Acartia clausi, Paracalanus parvus, Centropages kröyeri, Oitona nana, and chaetognath Sagitta setosa. A few specimens of the species Calanus helgolandicus were noticed at the station Malo Jezero, but only in August and September 1954. A more pronounced airing of the lake water and a decrease of the H2S zone preceded that same year, in July and August. Being considerably deeper than the station Malo Jezero, the station Veliko Jezero had low temperatures in the bottom layers during the summer months. Conditions like these seem to favour the development of the species Pseudocalanus elongatus and Calanus helgolandicus which were present in higher numbers, particularly during the summer months. As to Pseudocalanus, it has been observed that this species regularly keeps to cooler bottom layers, not participating in any daily vertical migrations as it is the case with Calanus helgolandicus. The zooplankton samples taken at the outer station Gonoturska differed from the samples taken in the Mljet lakes, having the characteristics of the zooplankton of the open Adriatic. It has been found that the influence of the open Adriatic on the station Veliko Jezero is considerably diminished owing to the shallow Soline Bay which hinders the entering of the open sea organisms, particularly of those prevailing in the deeper layers.
(7) The qualitative anlyses in this paper have shown the composition of species encountered in the course of our investigations, and their occurrence during the year of investigations.
