Hydrographic and productivity conditions in the Bay of Mali Ston

Report on the work made during the dam construction of the Trebišnjica hydroelectric power plant

Abstract

        The purpose of research undertaken in the Bay of Mali Ston in the course of 1963 and 1964 was to determine the hydrographical and some biological conditions in that area.

         Additional observations were made in 1967 and 1968, as soon as the dam construction for the Trebišnjica Hydroelectric Power Plant has been com­pleted. It had to be found whether noticeable changes will occur in the waters of the Bay of Mali Ston as a consequence of the mentioned construc­tion, and how far the life in that water basin will be affected.

        The behaviour of a series of parameters was observed for that purpose, such as temperature of the sea water, salinity, oxygen content, quantity of phosphate (free phosphate and total phosphorus), sea water transparency, phytoplankton and zooplankton abundance.

        The primary production in the Bay waters has been estimated and the balance of salt calculated.

        During the first cycle of research the checking of the mentioned factors was performed monthly, while a bimonthly checking took place during the second cycle.

        Observations were made at the following three localities: Usko (situated in the strait separating the Bay of Mali Ston from the Neretva Sound), Bistri­na and Kuta (both of them deeply land-locked). The positions of these sta­tions are shown in Fig. 1.

        As regards temperature conditions found in the course of 1963/64 the peculiar situation pravailing at the Bistrina station was quite surprising as the annual amplitudes of temperature fluctuation recorded at that station were unsuspectedly lower than those found at the stations Usko and Kuta. Such a situation is otherwise characteristic of the less enclosed sea water bodies. It has been found that submarine freshwater springs and other fresh­water influx cause to increase the winter minimum values of the sea water temperature at the Bistrina station.

         Another characteristic of the mentioned stations consists in the fact that-owing to their waters being shallow-the thermocline practically does not appear in the warmer seasons. The appearance of the so-called temperature inversion, in the course of the coldest season of the year, is quite significant.

        The salinity investigations have shown that the highest percentage of freshwater in the Bay occurs in the Bistrina station, although the freshwater inflow from the mainland is felt in the whole area.

         The annual amplitudes of salinity fluctuation are here particularly pro­nounced. The stratification of salinity values can here frequently be observed, which is not the case with temperature values.

         The amplitudes of oxygen fluctuation (O₂ml/l) and oxygen saturation (O₂%) in the area situated between the Usko station and the head of the Bay, reveal an abundant presence of marine flora and, at the same time, a pro­nounced circulation of water in the Bay.

        According to our expectation, the waters at the two inner stations have shown higher contents of both free phosphate and total phosporus than the water at the Usko station.

        The fluctuation of water transparency was rather low, the lowest occuring at the Bistrina station.

        The 1967/68 investigations, involving the same factors, revealed the occurance of similar changes in the Bay. This fact points to a constant, although periodically reduced, influence of freshwater.

        The annual amplitude of temperature fluctuation at the Bistrina station has lately shown somewhat higher values, resembling to those found at Usko. This is considered to be one of the consequences following the changes in the influx of freshwater from the hinterland.

        As far as salinity values at the Bistrina station are concerned, the diffe­rence between the maximum and minimum obviously reduced during the latest period of investigations, was still the highest in the whole area. This fact also reveals a continued presence of submarine and surface springs in the Bay, only affecting its water to a somewhat lesser extent.

        The fluctuation of oxygen content and saturation with oxygen slightly declined in comparison with the results from 1963/64, the difference, however, not being too great.

         The data concerning free phosphates and total phosphorus were generally higher in 1967/68. The observation was made that these two values appeared in reversed positions at the Kuta and Bistrina stations, being rather low in the bottom layer of the former and equally so in the surface layer of the latter.

         During the 1967/68 cycle of investigations, the water transparency was found to be generally lower which is considered to be a consequence of the increased presence of biological components in the Bay. The Bistrina station still shows the lowest fluctuation of this factor although it was somewhat higher when compared with the results of the 1963/64 investigations.

          For the Usko station the salinity balance for 1963/64 has been calculated; it offered the initial data and orientation concerning the fluctuation of sali­nity values in the Bay of Mali Ston, and the Neretva Sound. Any increase in salinity occuring in a layer at the Usko station, represents an inflow of sea water into the Bay of Mali Ston, while on the contrary, any decrease of salinity values shows and outflow of freshwater from the Bay. The negative balance of salinity in 1963/64 was established for the Mali Ston Bay. A 14 metres high column of sea water, having a surface of 1 m² , expended to the surrounding waters 64.15 kg of salt, while receiving from them only 54,90 kg during the said period of time. The total turnover of salts was 4.95 times higher at the Usko station in 1963/64 than that occuring at the Stončica station in the open Middle Adriatic in 1967.

         The size of organic production was calculated for the year 1963/64 by the P=∑ΔO₂—O’₂ method (BULJAN, 1968, 1969). An abundant production characte­rized the colder season. Meanwhile the consumption of organic assimilation was higher in the warmer season. The net production at the Usko station amounted to 55% of the 1963/64 total production. The total annual production per 1 m³ at the Usko station amounted to 1.15 g C, corresponding to the amount obtained by the same method, applied to the open part of the Middle Adriatic and, approximately to the data obtained at this station by means of the ¹⁴C method (1.56 g C/m²/year).

        The fundamental characteristics of phytoplankton in the waters of the Mali Ston Bay have been determined.

        All the stations showed the same seasonal rhythm of phytoplankton growth. 

        Spring (June) and autumn (September-October) are the phytoplankton blooming seasons. The minimum density of phytoplankton populations takes place from January through March.

         The highest extent of fluctuation of phytoplankton quantities was obser­ved at the Bistrina station.

        Among the stations located in the Bay, the two inner ones (Bistrina and Kuta), showed a higher abundance in phytoplankton than the Usko station. The same result was obtained by measuring the primary production with the application of the ¹⁴C method.

        By comparing the phytoplankton quantities recorded in the course of both series of investigations, the 1967 and 1968 periods were observed to be richer in phytoplankton than the 1963 and 1964 ones. The recorded quantities of phytoplankton at the inner stations were roughly twice as large in 1967/68 than in 1963/64, while at the Usko station they were 4.7 times larger. We may concluded that the above-mentioned increase corresponds to the general increase of production in the Adriatic basin during the 1967/68 period.

         No significant differences were observed in the occurrence of phytoplank­ton in various layers of the Bay. At the Bistrina station, and, to a lesser extent at the Kuta station too, the surface layer was somewhat richer in phytoplankton than the lower layers in 1963 and 1964. In 1967 and 1968, meanwhile, the maximum values were recorded in deeper layers, which no doubt was in connection with the changing vertical extension of P-PO₄ in the mentioned years.

        We may conclude that the increased density of phytoplankton populations in 1967 and 1968 affected the transparency of waters in the Bay.

         The composition of phytoplankton is quite typical for the inshore area of the eastern Adriatic.

          The paper also deals with the principal representatives of the phytoplank­ton biomass.

          The qualitative composition of zooplankton in the waters of the Mali Ston possesses the main characteristics of shallow enclosed waters, i.e. a limited number of zooplankton groups and species, as also an abundance of mero­plankton. This phenomenon was particularly pronounced at the Kuta station. Only four zooplankton groups were represented there in cosiderable quan­tities.

        The waters of the Bistrina station showed a somewhat larger amount of species, especially Copepods indicating that these waters were neritic in a lesser degree than the waters at the Kuta station.

         With regard to zooplankton composition, the Usko station showed a quite different one if compared with the inner stations of the Bay. A large number of species was recorded, corresponding to the situation existing in the Middle Dalmatian island region.

        The frequent occurence of meroplankton is an additional evidence indi­cating the identical presence of neritic waters in the areas of the Mali Ston Bay stations.

        The occurrence in the Bay of a number of stenotermic and stenohalinic species during the winter season and in early spring, particularly at the Kuta station, was attributed to the considerable influence of the Neretva Sound on the one hand and to the higher presence of freshwater at the Bistrina station on the other.

        As far as zooplankton quantities (annual average values) are concerned, the Usko station was the richest in zooplankton, while the Bistrina station is at the bottom of the scale, which fact seems to be closely related with the degree of salinity found in the investigated waters.

        The maximum annual values of the zooplankton biomass were recorded at the Kuta station in December, at the Bistrina one in March, and at the Usko one in April. The minimum zooplankton biomass was recorded at the Kuta and Usko station in September, and in the Bistrina one in November.