Hydrographic Characteristics and Plankton Structure of the Southeastern Part of the Southern Adriatic Sea

Vera Vukanić1, Dušan Vukanić2, Nebojša V. Živić3, Tatjana Jakšić3, Stevan Čanak1

 

1 State University of Novi Pazar, Department of Biomedical sciences, Vuka Karadzića, Novi Pazar, Serbia; E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

2 Institute of Marine Biology Kotor, University Montenegro, Dobrota bb, Kotor, Montenegro

3 University of Priština, Kosovska Mitrovica, Faculty of Science and Mathematics, Lole Ribara 29, 38220, Kosovska Mitrovica, Serbia. E-mail: This e-mail address is being protected from spambots. You need JavaScript enabled to view it

 

Abstract

Some data on long term seasonal investigation of hydrography and plankton are presented in this paper. The most important fact is that the whole area is exposed to the impact of the fresh water from numerous sources from the coast and submarine springs in, and the impact of rivers Bojana and Drim. The exchange of the intermediate water-masses between the Adriatic and Ionian basins is also of great importance. Basic ecological data, as well as characteristic phytoplankton and zooplankton groups and species are described in this paper.

Keywords: Boka Kotorska Bay, open waters, Southern Adriatic, hydrography, phytoplankton, zooplankton.

Introduction

The complex community of plankton consists of very small living organisms of significant ecologic and biologic importance. Research on them and their environment, as well as the interaction of biocenosis-biotope relationships are of special importance. Research on hydrographic conditions in correlation with the primary (phytoplankton) and secondary (zooplankton) production is always a complex task requiring many answers (Kovalev et al., 2003).

The Boka Kotorska Bay is composed of several smaller broad bays and is the only part of the Southern Adriatic with jagged coastline. It is influenced by specific hydrological and climatic conditions that are unique in Europe. The surrounding karstic region is arid during the summer season, and abundant in rainfall during the autumn and winter periods which causes the specific physical-chemical and environmental properties of this bay.

The Boka Kotorska Bay spread across a wide plateau on the continental shelf with depths ranging from 100 m to 200 m and has no characteristic island groups such as those which can be found in the Middle and Northern Adriatic. For hydrological properties and plankton community structure of the coastal and offshore waters of the Southeastern region of the Southern Adriatic, it is important to know that this area is under the influence of freshwater inflows from numerous springs and submarine sources in the Boka Kotorska Bay, the regime of the Bojana River and neighbouring Albanian rivers, as well as the inter-basin changes in intermediate water masses from the Ionian Sea to the Adriatic.

This paper presents some data on changes in phytoplankton population dynamics for two fractions: microplankton (cells bigger than 20 µm) and nanoplankton (cell less than 20 µm). The relative share of individual groups in the total population of the microplankton community and the most numerous species (more than 104 cells per dm3) have been presented.

Special attention in the studies of zooplankton was given to compare the data obtained in previous similar studies in the Adriatic Sea. In the general analysis of the obtained data on the spatial-time fluctuations, special attention was paid to the composition, dynamics of species or entire population groups as such.

 

Materials and Methods

Zooplankton was sampled at ten stations in 2010. The plankton nets used were of Nansen type, either surface area of the opening 1/4 m2 - length 2.5 m, or surface area of the opening 1m2 - length 3.5m. The mesh sizes where 150µm and 200µm. Vertical samples were taken from 10m to 0m at the shallow stations of each bay. The net was pulled horizontally under the rope angle of 25º, by gradual method of expedition "Hvar", so the rope was shortened on each 10 minutes within a total of 30 minutes, and at a ship speed of 2 nautical miles per hour (NM/h). The collected zooplankton material was fixated on the ship, in 2.5% formaldehyde-sea water. The qualitative and quantitative analyses were done in the laboratory from the representative sample of 1/25 of total catch, under the stereomicroscope and binocular lens. After that, the whole catch was carefully analysed in order to record any rare species. The adult individuals of Copepoda were determined by species, while nauplii and juvenile Copepoda were represented together as copepodites. The quantity was presented as a number of individuals under m-2 below sea surface (ind.m-2). The population dynamics were studied for two fractions: microplankton (cells >20μm) and nanoplankton (cell <20 μm). Samples were taken at three representative stations in Boka Kotorska Bay from three depth levels. The number of nanoplankton cells was determined by counting 10-40 selected visual fields on the bottom of the chamber at a magnification of 320 times. Micro- plankton organisms are classified into five higher taxonomic categories: Bacillariphyceae, Dinophyceae, Haptophyceae, Dictyophiceae and Euglenophyceae. Temperature, salinity, pH, oxygen saturation and percent oxygen saturation were measured in situ with a probe Multiline P-4, at 0.5m, 2m, 5m and 10m in the shallow part of the Bay near the coast, and at 0.5 m, 10 m, 15 m, 20 m and 30 m of depth at central deep stations. The transparency of the sea was measured with a Secchi disk, 30 cm in diameter, white in colour. The colour of the sea was measured according to Forell from I to XXI.

 

Results and Discussion

Temperature: The Adriatic is a warm sea. Water from its deepest layers has a relatively high temperature and is always warmer than 11-12°C. Surface layer heats by solar heating during the warm period of the year, up to and over 20°C. From the surface to the depth the temperature slowly decreases in the beginning and then in the zone of a few tens of meters begins to decline steeply. This is the level of temperature rise or the thermocline. The temperature continues to drop slowly from thermocline deeper to the bottom. The surface of the sea cools during the fall and winter. This causes the vertical movement of the water masses and as a result, the water is mixed to achieve a uniform temperature from the surface to the bottom. This condition is called isothermal. The isothermal condition is established initially at a high temperature (at about 18-19°C in the southern Adriatic), but because of the winter cooling becomes lower. On the open sea in the middle and southern part of the Adriatic, temperatures do not drop below 11°C. Near the coast and in the northern shallow Adriatic, the process of cooling is much more powerful. The isotherm in the Adriatic Sea starts from the coast to the open sea and from north to south. The isotherm along the coast begins in October or November, and sometimes at the end of September and lasts almost until the end of March, and the temperature of the entire column drops another 4-5°C. The maximum temperature mean value is 15.20°C (at a depth from 0 m to 10 m), and minimum is 12.90°C (from 1000 m to 1050 m).

The temperature gradient is only 2.30°C. It is common that the Southern Adriatic is warmer from the north and middle but only during the winter. The open sea is then warmer than the coastal region. The difference between the temperature of the Northern and Southern Adriatic is 8°C in the winter, but it is only about 2°C in the summer season.

Salinity: The Adriatic Sea is of high salinity, about 38.30 ‰, slightly lower than the eastern Mediterranean (around 39 ‰), but higher than the salinity in the western Mediterranean (37 ‰).

It was found that in the open Southern Adriatic the mean value of salinity ranged from 38,48 ‰ - 38,60 ‰ for several years. Fluctuations and a decline in salinity of sea water are much larger in the coastal area. The main factor for increased salinity of the Adriatic water depends on the water intensity through the Strait of Otranto of the Mediterranean water into the Adriatic. Another factor contributing to a decline in salinity levels depends on freshwater flow regimes. Water is regularly saltier in the Southern Adriatic than in the north. Distribution of salinity at depth fluctuates during the year. The vertical gradient of salinity is most pronounced in the period from April to August, almost at the same time as the onset of the thermocline. Besides the annual, there is perennial salinity fluctuation in the Adriatic Sea. It is caused by periodic inflow of significantly saltier Mediterranean water into the Adriatic. This phenomenon of salinization of the Adriatic is called the "Adriatic ingression" (Buljan, 1953, 1964; Buljan & Zore-Armanda, 1971) and it happens every 9-10 years. During these so-called "ingressions years" in the Southern Adriatic, the salinity achieves 39 ‰. Our data from the winter season of 2012 match the saltier water from the Mediterranean to the profile of the Strait of Otranto (P-2, P-3, and P-4). The values in the intermediate layer ranged from 38.84 ‰ to 38.96 ‰, in the surface layer from 38.60 ‰ to 38.65 ‰ and in the the bottom layer from 38.81 ‰ to 38.87 ‰. The minimum value of the surface layers was 38.51 ‰, P-11, and the maximum 38.86 ‰, P-14. Station P-11 was more influenced by the offshore sea.

Density: It plays an important role in the formation and movement of water masses and is important in the ecological relationships with plankton communities. It depends on the T-S factor and decreases with temperature increases and mixing with fresh water. Density causes vertical turbulence and this movement is called the thermohaline convection. There is a layer of thermal rise at a depth of 20 m to 70 m below the sea surface, where the water density increases rapidly with depth - pycnocline. When an organism or organic waste sinks from the sea surface to the area of pycnocline it slows or even stops. The density values of ocean waters fluctuate from 21 to 27.5. Surface water in the Adriatic Sea during the winter often has a high density, greater than 29 (Buljan & Zore-Armanda, 1971). According to our data, values range from 29.6 to 29.39. Densities lower than 29 were found only at depths of 0 m, 10 m and 20 m, and the values for deep sea were above 29.

Dissolved oxygen (O2): The level of dissolved oxygen in sea water depends on several factors, but mostly on temperature and salinity. In the Adriatic Sea values range from 5 ml/l to 6 ml/l. According to our data values ranged in the open Southern Adriatic from 4.08 ml/l to 5.55 ml/l, and saturation percentage from 71.40% to 98.50%. In the layers from 0 m to 20 m the percentage of saturation always exceeded 90%, then decreased with the the depth, and to the bottom has increased to a depth of about 1000 m. This indicates that the Southern Adriatic is well aerated and that there are no areas of stagnation anywhere in the water column.

The concentration of hydrogen ions (pH): In seawater, the pH ranges from pH 7.5 to 8.6. According our data pH did not fall below 8.5 per 1000 m of depth nor did the value go above 8.67 to 20 m up to 50 m of depth.

Nutritive salts: marine dissolved organic phosphorus (orthophosphates), nitrogen (ammonium salts, nitrates and nitrites) and silicon (orthosilicic acid) have a fundamental role in primary organic production - and are given the common name - nutritive salts. The process of photosynthesis is the main mechanism of nutritive salts assimilation in the sea.

The systematic measuring of nutrient concentrations in the open waters of the Adriatic began during 1950. The collected data showed that the greater part of the Adriatic's concentration of nutritive salts in the surface layer is low. This is especially emphasized for the concentration of phosphorus, which is often present in concentrations near quantification limits (0.02 mmol/m3), and certainly lower than 0.1 mmol/m3. The concentration of total nitrogen and inorganic reactive silicates are generally lower than 2, respectively 2.5 mmol/m3 (Degobis, 1988).

It is important to note that the content of nutrients in Adriatic seawater is lower comparing to the ocean water, leading to lowerorganic production in the Adriatic.

Phosphates in the southern Adriatic varied during the month of March from 0.01 mmol/m3 (P-2 to 10 meters and a P-4 to 0 meters) to 0.57mmol/m3 (P-2 to 20 meters). We registered a value of 0.53 mmol/m3 at stations P-11 at 150 meters and P-14 at 200 meters 0.30mmol/m3.

All the other values did not exceed 0.18mmol/m3. The mean value of phosphate content for the entire southern Adriatic was 0.09mmol/m3, while the mean values at different stations ranged from 0.13 mmol/m3 up to 0.15 mmol/m3.

Total phosphorus ranged from 0.01 mmol/m3 (P-7 on 1000 m) up to 0.72 mmol/m3 (P-3 on 300 m), and measured 0.19 mmol/m3 for the entire region.

Nitrogen in the sea exists in the form of nitrate, nitrite and ammonium. In addition, it is available in the form of elemental nitrogen, and organic compounds. These compounds are the bearers of the most important processes in the living cell. Phytoplankton can absorb each of the three types of dissolved inorganic nitrogen, which must previously be reduced into ammonium salts that are more easily incorporated into amino acids than the nitrate and nitrite, with the consumption of additional energy (Raymont, 1980). It has been observed that some estuary flagellates can only use ammonium salts as their nitrogen source. When in the same concentrations, nitrate is easier to assimilate than nitrite from higher oxidative conditions, (Eppley & Coasworth, 1968). Nitrite absorption is half as fast as ammonium salt intake (Conway, 1977).

Nitrates have oscillated within the range of 0.44 mmol/m3 (P-14 on 1 m) to 6.85 mmol/m3 (P-2 on 200 m). Values higher than 3 mmol/m3 were recorded at all stations at a depth of 100 m and deeper, in addition to station P-13, where this value is recorded only at a depth of 1000 m. The mean value of nitrate content for the entire Southern Adriatic is 2.15 mmol/m3, while the mean values at individual stations range from 1.53 mmol/m3 (P-9) to 3.20 mmol/m3 (P-4).

Nitrites have ranged from 0.02 mg/m3 (P-3 300 m; P-4 at 650 m; P-8 at 300 m; P-13 at 1,000 m and 1.050 m) and 0.25 mmol/m3 (P-7 to 50 m; P-13 to 200 m). Mean values at stations ranged from 0.06 mmol/m3 (P-2, P-6, P-8) to 0.11 mmol/m3 (P-9), and measured 0.08 mmol/m3 for the whole Southern Adriatic area.

Ammonium salt ranged from 0.02 mmol/m3 (P-3 to 300 m) and 2.88 mmol/m3 (P-11 at 150 m). At station P-3 at 10 m, and P-13 at 50 m, the value was 1.04 mmol/m3 and 1.16 mmol/m3 and all the rest were lower than 1 mmol/m3. Mean values of ammonium salt for the entire area was 0.28 mmol/m3 and by stations, from 0.14 mmol/m3 (P-7) to 0.79 mmol/m3 (P-11).

Silicon participates in the processes of assimilation and regeneration were in organic form (mainly orthosilicic acid and amorphous silica dioxide). Orthosilicic acid is the main form of dissolved inorganic silica in seawater. Its main role is the growth of diatoms and the regeneration of the salt formed from dead organism shells. The regeneration of nutritive salts refers to the release of soluble inorganic forms from organisms in the water or from disintegration of their remains.

Silicates have ranged from 0.50 mmol/m3 (P-13 at 10 m) to 9.05 mmol/m3 (P-4 at 500 m). Mean values at stations ranged from 2.22 mmol/m3 (P-9) to 3.85 mmol/m3 (P-4) and measured 2.99 mmol/m3 for the whole area.

 

Hydrographic Characteristics of Boka Kotorska Bay

The waters of the Boka Kotorska Bay have very pronounced seasonal temperature fluctuations. The surface layers have low winter temperatures and perennial research shows minimum temperatures in January and February, and peak temperatures in July and August.

In Boka Kotorska Bay, the minimal surface temperature is reported to be in January (9.60°C), while the mean minimum temperature for the entire period was estimated to be 10.75°C in February. The maximum temperature was recorded in August (25.80°C), while the maximum temperature mean value for the entire period was 25.27°C in August. Thermocline begins to occur at a depth of 0-15m in May (temperature gradients ranged from 3.52°C - 6.75°C), and extends to August (7.80°C - 8.60°C). At a depth of 15m – 30 m differences are small, 0.5°C to 2.25°C. Later in the fall and winter, gradually, a uniform temperature is established throughout the water column, so that the difference in temperature between the layers is not significant, especially in deep water (0.1°C - 0.4°C in December).

The salinity in the Bay of Kotor varies from month to month in the surface layers, ranging from 3.13 ‰ to 35.81 ‰. The lowest value of salinity was recorded in January. The salinity of the a central layers varied in the range of 38.86 ‰ (I) to 38.66 ‰ (V) and on the bottom from 34, 52‰ (X) to 39,02‰ (IV I VII).

Dissolved oxygen ranged from 4.33 to 10.48 ml/l to 10,48ml/l (III). This maximum corresponds to a saturation of 152.99%, and at the V, VI and VII station saturation exceeded 100%. The other values indicate that saturation did not go below 73.78%. All levels were sufficiently oxygenated.

The concentration of hydrogen ions (pH) ranged from 8.02 to 8.60.

Nitrite values ranged from 0.00 mmol/m3 to 0.07 mmol/m3, nitrate and 1.08 mmol/m3 to 67.55 mmol/m3, phosphates from 0.00 mmol/m3 to 0.92 mmol/m3 and silicates from 0.00 to 92.25 mmol/m3.

In the Bay of Tivat temperatures ranged from 11.40°C to 25.40°C. The lowest temperature was recorded in March, and the highest in July. The maximum temperature of the surface layers was recorded in July and August 23.95°C to 25.40°C, and the minimum in March 11.40°C to 11.40°C.

The salinity in the surface layers varied from 16.49 ‰ to 37.81 ‰. The lowest salinity was recorded in March, while the minimum mean value was 25.12 ‰.

At a depth of 15m salinity reached values as from the open sea, values of central layers ranged from 33.84 ‰ to 38 ‰, and benthic layers ranged from 35.46 ‰ to 39.25 ‰.

Oxygen content varied in the range of 4.78 ml/ l to 8.68ml/l.

The pH values ranged from 8.07 to 8.46. Nitrate values ranged from 0.00 mmol/m3 to 0.07 mmol/m3, nitrite 0.93 mmol/m3 to 59.93 mmol/m3, phosphate from 0.00 mmol/m3 to 0.90 mmol/m3 and silicate 0.00 mmol/m3 to 38.75 mmol/m3.

In the Bay of Herceg Novi the minimum temperature was recorded in the surface layer in February (11.0°C), and the minimum mean value was estimated to be 11.65°C in the same month. The maximum temperature was 25.00°C (July) and the mean value was 23.90°C (August). Temperature fluctuates by depth during the year. The first four and the last two months of the year are almost without a gradient, until the thermocline starts to establish at the beginning of May. This strengthened during the summer until September, when it started to recede by lowering isotherms from 15°C all the way to the bottom in December.

The salinity of this area on the surface ranged from 23.28 ‰ (January) to 38.06 ‰ (September). That same year and the very same month the highest values were recorded in Bay of Kotor (35.81 ‰) and the Bay of Tivat (37.81 ‰). Mean values in surface layers ranged from 29.41 ‰ to 37.00 ‰. The salinity gradient at the surface increased from Herceg Novi to the Bay of Kotor. Oxygen content was within the range of 4.67 ml/l to 7.80 ml/l (March) and the concentration of hydrogen ions (pH) in the range of 7.95 to 8.44. Nitrate values ranged from 0.00 mmol/m3 to 0.71 mmol/m3, nitrate from 1.04 mmol/m3 to 56.13 mmol/m3, phosphate, from 0.00 mmol/m3 to 0.72 mmol/m3, silicate from 0.00 mmol/m3 to 7.10 mmol/m3.

 

Phytoplankton

The phytoplankton community is dominated by representatives of the Bacillariophyta class with an average of over 70%. The dominance of this group is the result of prevailing species: Leptocylindrus minimus, Skeletonema costatum, Chaetoceros affinis in the Bay of Kotor, Skeletonema costatum and Chaetoceros compressus in the Bay of Tivat, and significantly lower Pseudo-nitzschia delicatissima in Bay of Herceg Novi. The density values of certain species exceeded 105 cells/dm3 and 106 cells/dm3. This value never exceeded 104 cells/dm3 in the Bay of Herceg Novi, despite the strong influence of eutrophication in the Boka Kotorska Bay. Lower values of population density in the Bay of Herceg Novi were caused by the influence of the open sea on phytocoenosis.

A high population density of the Chaetoceros compressus species (Bacillariophyta) in the Bay of Tivat was recorded with more than 105 cells/dm3, and it was not previously found in such large numbers.

Most species belong to forms of the tempered-Atlantic type. Often the appearance of dense microplankton populations with more than 105 cells/dm3 and 106 cells/dm3 in the Bay of Kotor can be explained by the closeness of the Bay influenced by the process of eutrophication.

In the Bay of Tivat values were lower due to the greater influence of the open sea, regardless of anthropogenic impacts that were more pronounced. Microplankton population densities in the Bay of Herceg Novi were lower, 104 cells/dm3.

Increased values of microplankton population density in the summer, when it is expected to stagnate, as previously observed (Vuksanović et al., 1999), occur as a result of the eutrophication process throughout the year and especially during the summer months. This is especially noticeable in the bays of Kotor and Tivat.

Changes in nanoplankton population density were significantly noticeable because of small fractions. In particular, this applies to bays of Kotor and Herceg Novi. In the Bay of Herceg Novi maximum values were two times greater than in the Bay of Tivat. This is the result of the competing relationship between micro and nanoplankton. In areas where the eutrophication process is more pronounced microplankton population density was higher (the bays of Kotor and Tivat). Conversely, in Bay of Herceg Novi the eutrophication process was decreased due to the impact of the open sea.

The process of eutrophication in the bays of Kotor and Tivat was more strongly expressed and colonial microplankton forms dominated, such as: Skeletonema costatum, Pseudo-nitzschia delicatissima, Leptocylindrus minimus, etc. These Bacillarophyceae species are characteristic for areas with pronounced eutrophication.

 

fig01
Figure 1: The percentage of microplankton groups in the investigated station in the Boka Kotorska Bay during 2009.

 

Zooplankton

The most important group of zooplankton- lower crustaceans – Copepods was covered in data analysis of the waters in the in Boka Kotorska Bay. Previous data showed that the Boka Kotorska's water is characterized by monotony in the these animals composition in the plankton. This composition changes longitudinally from a shallow part of the Boka Kotorska Bay towards the open sea. This phenomenon has been observed throughout the Adriatic Sea, from north to south, where the deep Southern Adriatic has oceanic characteristics. Large number of species towards the open sea does not mean an increase in the total quantity of the group. The zooplankton community biomass grows in the opposite direction towards the shallow part of the Bay. This has been caused by a significant increase in the number of typical estuarine-neritic species and juvenile forms of the same (Vukanić, 1971).

Recent research conducted on the characteristics of zooplankton communities in coastal and offshore waters in the Southern Adriatic, especially for quantitative most important group of zooplankton were more recent and dated from second half of the twentieth century. Data on quantitative and qualitative distribution of zooplankton in the Southern Adriatic are contained in the works of: Gamulin, 1938; Hoenigman, 1955; Hure, 1973, 1979; Hure & Scotto di Carlo, 1963, 1970; Vukanić,D., 1971, 1974, 1977, 1979, 2000, Vukanic,V., 2008, 2012., Molinero et al. 2009).

The percentage share of Copepoda in the open sea ranges from 82.20% to 84.72%. Important share belongs to the group Appendicularia (Copelata) after copepods, from 3.42% to 6.28%. Other groups of zooplankton organisms according to the percentage of their participation were: Siphonopohorae (2.79% - 2.80%), Chaetognatha (2.43% - 2.49%), Ostracoda (1.58% - 2.56%), Cladocera (1.46% - 2.05%), Pteropoda (0.63% - 1.1%), and with less than 1% of participation groups: Medusozoa, Polychaeta, Euphausiacea, Amphipoda, Desmomiaria and Cyclomiaria.

 

fig02
Figure 2: Percentage composition of zooplankton in studied area during 2010.

 

Zooplankton community, as nutritional base for pelagic and some benthic fish, according to years of research, has been divided to (Vučetič, 1971):

North Adriatic Community (Biocenosis I) stretches from the Istrian Peninsula to Italy or biotope of the Northern Adriatic water masses. A small number of holo-planktonic species from the south can be found there. Meroplankton were more abundant, particularly with crustaceans and benthic Mollusca larvae. Real neritic plankton is present there. The biological characteristics of this biocenosis are that this species is of multi-generational with large numbers appearing mainly in the summer. The situation is similar in the Boka Kotorska Bay and the River Bojana estuary (Vukanić, 1971, 1974, 1979). In this part of the Adriatic organisms are eurythermal (12°C to 26°C) and euryhaline (31‰ and 37‰). Here the sea is the coldest in the winter and hottest in the summer in the entire Adriatic. Of the approximately 200 known species of copepods in the Adriatic Sea, 51 species are present in the north and 63 species in the Boka Kotorska Bay.

The South Community (Biocenosis II) is diametrically opposed and lives in the intermediate water and the Southern Mediterranean deep water. The common characteristic of these salty and warm waters is the large number of species (Copepoda = 136).

These are two the most diverse complexes of zooplanktonic species in the Adriatic.

Biocenosis III is the mixing zone of the communities in transit areas biota: the Strait of Otranto, Jabuka Pit, Palagruza Sill. The expedition "Najade" recorded the presence of Calanidae and Euchaeta layers at Jabuka Pit.

The fishing zone in the Southern Adriatic extends parallel to the coast 5-6 km away from the mainland. The surface of this zone is 456 km2. The present data are based on the distribution of zooplankton groups, especially Copepoda, Siphonophorae and Chaetognatha, on which fish primarily feed and are undoubtedly significant indicators of zooplankton abundance and the metabolism of the zooplankton community.

 

fig03
Figure 3: Variations of Copepoda during the investigated period at Station P-1, depth 100 meters to 0 meters.

 

fig04
Figure 4: Variations of Copepoda during the investigated period at Station P-2, depth 200 meters to 0 meters.

 

Data based on vertical distribution of zooplankton started from the bottom to the surface of the sea, depths of more than 100 m showed that the large portion of the zooplankton inhabit the entire water column. Maximum concentration of dominant zooplankton groups of Copepoda was found in coastal/open Adriatic. A smaller community was identified on the open sea close to Palagruza Sill and by the east and west coast of the Strait of Otranto.

Relative richness in nutritive salts in the southeast part of the Southern Adriatic is the consequence of freshwater inflow from the Bojana and Drim rivers into the sea. These inflows bring bioelements from the land and doing so, they enrich this area. This comes from intermediary water masses form Ionian Sea, too. The proven phenomena of ingression (Buljan, 1953) has significant influence on physical-chemical characteristics in the Southern Adriatic and wider. The intensity and richness of the primary production of organic phytoplankton depends on the amount of organic phosphorus and nitrogen salts in the water. Since these quantities were above the average in the coastal part of the sea, proportionately, the richness of primary organic matter is higher than those in other northern areas.

The first link in the food chain is zooplankton, whose richness (biomass) in the sea is in relation to the density of phytoplankton. Following links in the food chain are in direct dependence on the previous and refer to the abundance of fish and other higher level community life-forms in the sea. A significant example of this is the dominant group, Copepoda, with the largest biomass of 60 000 individuals/m2 in the spring and 5000 individuals/m2 in the summer, at the mouth of the Bojana River. The abundance of zooplankton, in addition to other environmental benefits, certainly depends on large inflows from the mainland in late winter and early spring. The first and general distribution regularity of zooplankton animals and their biomass is increasing in the transversal direction from the coast towards deeper waters, as emphasized in the Boka Kotorska Bay. Time spatial oscillations of species distribution are reflected in their numbers and size of the biomass. The highest quantity of zooplankton in the Boka Kotorska Bay appears in March, and that is caused by strong neritic eurythermal and eurihalin species, their larvae and juvenile stages that are dominant community in mero-plankton. Dominant copepods are usually: Paracalanus parvus, Centropages kröyeri, Acartia clausi, Euterpina acutifrons, Oithona nana and Oncea subtilis.

There is a significant increase in nauplius larvae presence and various stages of these species in late winter and early spring. The maximum abundance occurs in mid-April. This is the time for mussel (Mytilus galloprovincialis Lamk.) spawning and numerous mussel larvae are present in the zooplankton, especially in shallow parts of the Bay.

After the spring maximum, the biomass of zooplankton decreases until the end of July, when it reappears and another lower peak occurs. Very expressive fluctuation of environmental factors in the Boka Kotorski Bay and the reduced impact of the open sea, create specific environmental benefits for the growth and development of typical estuarine-neritic fauna.

The deep Southern Adriatic represents, in some sense, an ecologically separated life area (biotope) with fairly stable physical and chemical properties as opposed to its north-western part. Some authors (Früchtl, 1920) pointed to the fact that there is a maximum of horizontal range of the species that live closest to the surface of the sea.

 

Conclusion

Based on years of zooplankton communities (biocenosis) research as feeding grounds and as an important link in biological production in the sea, it is divided according to the main biotopes into the following communities: Northern Adriatic, Southern Adriatic and transit.

The first link in the sea's food chain is zooplankton, whose richness of content (biomass) is in direct relation with the content of (density) phytoplankton. The next link in the food chain is in direct dependence on the previous and refers to the abundance of fish and other higher animal communities in the sea.

The abundance of zooplankton in the spring, in addition to other environmental benefits, depends on major inflows from the mainland into the sea, and thus increases content of inorganic phosphorus and nitrogen salts. The richness of the epipelagic production of primary (phytoplankton) and secondary (zooplankton) and the Bay waters of Montenegrin coast has been confirmed by the results of this research.

Taking into account the qualitative and quantitative composition of the plankton, the conclusion can be made that the waters above the continental shelf, closer to shore, are richer with biomass, while the offshore waters are richer in taxonomic diversity.

 

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