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Zooplankton and the circulation pattern of the water masses in the adriatic
Netherlands oTournal of Sea Research 7i: 112-121 (1973) 7th European Symposium on Marine Biology
ZOOPLANKTON AND THE CIRCULATION PATTERN OF THE WATER MASSES IN THE ADRIATIC by T. V U ~ E T I ~ (Institute of Oceanography and Fisheries, Split, Yugoslavia) CONTENTS I. II. III. IV. V. VI.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Oceanographic characteristics . . . . . . . . . . . . . . . . . . . Zooplankton distribution . . . . . . . . . . . . . . . . . . . . . Plankton distribution in relation to hydrography . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . .
112 112 115 118
120 120
I. I N T R O D U C T I O N An understanding of the various zoogeographical aspects of the Adriatic Sea is only possible when the interrelations between the animal communities and the water masses are taken into consideration. In this paper the main hydrographic characteristics of the Adriatic will be discussed from the works by BULJA~ (1953, 1957, 1964, 1965, 1968, 1969), ZORE-ARMANDA (1963, 1966, 1968, 1969a, 1969b) and possible mechanisms controlling the distribution of animal plankton populations in this area will be considered. II. O C E A N O G R A P H I C C H A R A C T E R I S T I C S The Adriatic is a relatively shallow land-locked sea, forming part of the Mediterranean basin. The average depth of the north Adriatic is 60 m; the J a b u k a pit is 200 m. The Palagru~a sill (150 m) separates this area from the deep south Adriatic (maximum depth 1200 m). Vertically the Adriatic waters can be devided into a surface layer (0 to 40 m), an intermediate layer (40 to 150 m in the central Adriatic and 400 to 500 m in the south Adriatic) and a bottom layer, each characterized by the presence of specific water masses (ZoRF.-ARMANDA, 1963). The main directions of the water flow are determined by density gradients; differences in barometric pressure and evaporation with the corresponding vertical water movements. The most important seasonal changes occur in the surface layer
ZOOPLANKTON IN THE ADRIATIC
113
where, due to a density gradient between the north and the south Adriatic, a longitudinal flow prevails causing an outgoing water movement from the Adriatic to the Mediterranean in summer, and an incoming flow from the Mediterranean into the Adriatic in winter. In general the Adriatic currents have a low velocity (approximately 20 cm/sec) and the flow along the west coast is considerably faster than along the east coast (ZoRE-ARMANDA, 1968). In winter the surface incoming flow is faster along the east coast, in summer the outgoing flow along the west coast. However, in spring and autumn 2 separate gyres are created and an area of intensive mixing is formed over the Palagru~a sill, where a transverse flow is generated causing divergence and upwelling in spring (which continues during summer) and convergence and water sinking in autumn (continuing in winter). The intermediary layer of the Adriatic water is characterized by the inflow of the Mediterranean water into the Adriatic (the Intermediary East Mediterranean Salt Water). As a result the inflow of water from the Mediterranean via the intermediary layer and the outflow from the surface form a continuous circulation, especially in the summer period. The intensity of the exchange with the Mediterranean varies with differences in barometric pressure (ZoRE-ARMANDA, 1969) over the Mediterranean area. Increases in salinity, changes in temperature distribution and a number of other changes occur as the result of a more intensive inflow of the Mediterranean water during years of intensified flow (BuLJAN, 1953). Bottom water is formed in the north Adriatic in the years with high inflow of Mediterranean water during winter. It flows from there into the J a b u k a pit and occasionally over the sill into the south Adriatic. In the south bottom water is formed in situ in other years. Via the Strait of Otranto the south Adriatic water takes part in the formation of the bottom water of the eastern Mediterranean (ZoRE-ARMANDA, 1973). The strongest water exchange in the Adriatic occurs in the region of the Palagru~.a sill and the Strait of Otranto; in between are the following areas: the south Adriatic, the J a b u k a pit, and the north Adriatic, where various types of water can be formed. ZORE-ARMANDA (1963) has given the characteristics of 4 water types (Fig. 1). The North Adriatic water type (N) is formed in the north Adriatic during the more saline winters when strong vertical mixing occurs due to the high salinity and the temperature decrease in the surface layers. The Central Adriatic water (C) is formed in the J a b u k a pit during hyposaline winters. The South Adriatic water (S) is formed during hyposaline winters and it is permanently present in the depths of the south Adriatic. ZORE-ARMANDA (1963) thinks that this water type of
114
T. VU~ETI6
high density is formed by the cooling of the incoming Intermediary Mediterranean water ( M ) - - t h e most saline water of the Mediterran e a n - a n d mixing with the north Adriatic water (Fig. 1). Summarizing, besides seasonal differences, long-term fluctuations have also been recorded (BULJAN,1953; ZORE-ARMANDA, 1964). Hence it is possible to distinguish 2 conditions in the hydrography of the Adriatic: the hypersaline condition (Fig. l b) and the less saline condition (Fig. l a). Hypersalinity is the result of the changes which are effected by a strong inflow or advection of warmer and salter Mediterranean water towards the north, especially in winter, through the surface and intermediary layers (Fig. l b). At the same time, in the bottom layer of the north Adriatic the dense cold relatively less salt water (N) sinks and fills the J a b n k a pit from where it flows into the south Adriatic over the Palagru2a sill. In summer, in the intermediary layer, Mediterranean water enters the J a b u k a pit, and on the surface the less warm and less salt (in relation to years with less influence of Mediterranean water) north Adriatic water spreads southwards sometimes reaching the Palagru~.a sill (BULJAN, 1966).
Winter
Spring
Summer
Autumn
12o0
"°°5 °°° II 1200 ~
Illl~ ~Jl~l~i Ii v T=/'C
Sa1=3~5%* ~t~29,s2
~
T='2"C
Sa1=38,2%,
~t~z909
~T=13"C ~T='4"C" 5al--38,6"/,, ~ 5a1=38,7%, 6t=2920 ~~t~29os
Fig. l. Seasonal aspects of the hydrographical conditions in the Adriatic; N, (3 and S indicate North, Central and South Adriatic Water, M Mediterranean Water, and blank areas winter and summer surface waters; arrows indicate current directions. a. Weak influence of the Mediterranean water; b. strong influence of the Mediterranean water. (Modified after ZORE-ARMANDA, 1969).
ZOOPLANKTON
IN THE
115
ADRIATIC
III. Z O O P L A N K T O N D I S T R I B U T I O N The main distribution pattern of the water masses in the Adriatic form the framework in which various characteristics of the geographical aspects of the Adriatic zooplankton should be placed. The scheme in Fig. 2 of the main hydrographic aspects makes an easier orientation possible. The distribution of plankton organisms and the biological seasons are regulated by these aspects. LEDER (l 914) used the plankton material collected by the "Najade" expedition (1911-1914)--whose objective was to obtain a preliminary view of the Adriatic fauna and flora--to demonstrate the geographical zonation and its dynamics, and to determine by means of the plankton distribution the direction and intensity of currents and the water masses. VU~ETId (1970: Table I) already pointed out that in the north Adriatic the autochthonous and dominant plankton has a neritic character. It consists of a considerably smaller number of the holoplanktonic species, which are also smaller in size than the plankton of the open sea. As great differences exist between the winter and summer temperatures (4 to 28 ° C) and salinities (31 to 38 ~oo S) the organisms are eurythermic and euryhaline (Fig. 9). As a rule these organisms have short life cycles with several generations per year (Copepoda). N sua. I ~ g °
Winter int.~ bott.
C ~
~ ~
Spring i \ ~
S
M
~
~
14"
~ ~
~
14"
13°
~
~ t 4 "
Summer
Autumn ta"
~
~
zg
Fig.2. Schematic seasonal water movements in the North (N), Central (C) a n d South (S) Adriatic, under strong influence of the Mediterranean (M) ; Mediterranean water in solid arrows. Temperatures in °C at north and south end of the water layers.
116
T. v u 6 E w I d TABLE
I
Dominant neritic copepods in the Adriatic and the months of their maximal abundancy. North Adriatic 1965 (HuaE et al., 1969)
Species
Acartia clausi Paracalanus parvus Centropages typicus Ctenocalanus vanus Temora stylifera
Months
V, VII, X I I
Central Adriatic 1969-1970 (VudETId & REGNER, 1973)
South Adriatic 1971 (VuKANId,1971)
Centropages typicus Centropageskroyeri Acartia clausi Ctenocalanus vanus Centropages kroyeri Clausocalanusarcuicornis Temora stylifera Clausocalanus furcatus Paracalanus parvus Paracalanusparvus Clausocalanus arcuicornis Acartia clausi III, VIII, IX III, VII
They occur in dense populations, especially in summer, so that the biomass (dry weight) of the total zooplankton is high. Of the Copepoda which occur here in the highest numbers--just as the Cladocera--only 67 species can be found here against 216 species at present known for the entire Adriatic and 145 known for the south Adriatic (HugE, 1969). Among the 14 dominant species, quantitatively the most important are of small size as: Acartia clausi, Paracalanus parvus, Centropages typicus, Ctenocalanus vanus, Temora stylifera (Table I). For the species of small size CnAMI"ALBERT& GAUDY (1972) say that "principalement pour les temp6ratures sup6rieures les esp6ces de taille r6duite sont plus aptes que les grandes formes a r6guler leur m6tabolisme". Well represented are also the following species : Clausocalanus arcu#ornis, C.furcatus, Temo-
ra long#ornis, Calanus helgolandicus, Oithona plumifera, Oncaea media, O. mediterranea, Oithona helgolandica, Corycaeus giesbrechti. The community of the deep south Adriatic has all the elements of the oceanic plankton, and is considered diametrically opposite to the north Adriatic zooplanktonic community (Vu~EwId, 1970: Table I). As already mentioned here the water masses consist of the warm and saline south Adriatic water (S) and of the still salter Mediterranean water (M). The typical deep water plankton fauna of the south Adriatic influences the composition of the zooplankton in the central (C) and north Adriatic (N). Here the fauna is rich in groups and species with m a n y autochthonous elements (Table I). These organisms have a considerably longer life cycle and a smaller n u m b e r of generations a year and are more stenothermic and stenohaline than those in the north. The differences in salinity (38.6 to 38.7 ~o S) and temperature (11 to 14 ° C) are considerably smaller in the layers where the greatest nurn-
ZOOPLANKTON
IN THE
ADRIATIC
117
bers of the plankton species live. It is important to mention that in the south Adriatic community large size open sea copepods are particularly well represented, as Pleuromamma gracilis, P. abdominalis, Euchaeta hebes, E. acuta, Haloptilus longicornis; exclusively species of smaller sizes are:
Mormonilla minor, Monacilla typica, Oncaea ornata, Temoropia mayumbaensis. Characteristic are also the chaetognaths Sagitta decipiens and Krohnitta subtilis, the groups of the Ostracoda and the Euphausiacea, the larger deep sea Decapoda, and bathypelagic fishes (Vu6ETId, 1970: Table I). After the latest findings by HURE (1969c) in the bottom water (600 to 1000 m) of the south Adriatic the predominant copepods are Oncaea ornata, Temoropia mayumbaensis and Monacilla typica. In the layer of the intermediate water (200 to 600 m) Pleuromamma gracilis, Oithona setigera and Haloptilus longicornis are predominant, and in the surface layer Ctenocalanus vanus, Oithona plumifera, Clausocalanusfurcatus and C. arcuicorhis. These typically south Adriatic open sea species occur in the north in greater numbers during the years of high inflow of Mediterranean water (Vu(~ETId, 1970) and in the upwelling zones (~MELJEVA,1964). It is rather interesting that in 1967 and 1968 densest populations of Temoropia mayumbaensis were found in the layer of 600 to 1000 m (HuRE, 1969a), and in 1969 in the layer from 400 to 600 rn. The analysis of the hydrographical data (ZoRE-ARMANDA,personal communication) has shown that the year 1969 was outstandingly hypersaline (38.9 ~o S), while the previous years 1967 and 1968 were less saline. Most typical for Adriatic bottom water are probably Oncaea ornata and Monacilla typica while Mormonilla minor is representative for the Intermediary Mediterranean water of 300 to 400 m where its populations are densest. It was also found (HuRE, 1969) that in the Tyrrheanian sea Mormonilla minor and Eucalanus monachus occur in considerably greater densities than in the Adriatic. In the Jabuka pit belonging to the central part of the Adriatic the plankton shows a varied structure. In the surface layer it is neritic, in the waters below 50 m it has the elements of the deep south Adriatic. Probably during the years of high Mediterranean inflow the area is repopulated by young stages and perhaps also by adults. Also the bathypelagic fishes and other elements of the south Adriatic fauna are restored especially during years of high Mediterranean inflow. Along the edge of the Jabuka pit very recently (KA~Id, 1973) it has been registered DSL, undoubtedly caused mostly by euphausids. In the Jabuka pit the abundant Euchaeta (E. hebes) and Calanus (C. helgolandicus) layers are well known since long ago (LEDER, 1914). It seems that Euchaeta hebes thrives in this part of the Adriatic and also on the shelf of the whole area as far as Palagru~a at a depth of 60 to 200 m (GAMULIN, 1971).
118
T. V U ~ E T I d IV. P L A N K T O N
DISTRIBUTION HYDROGRAPHY
IN R E L A T I O N
TO
The dynamics of the Adriatic make clear w h y the faunal elements of the deep south Adriatic occur farther to the north during years when the Mediterranean intermediary water reaches to the very surface of the south Adriatic. Then the strong incoming currents push these elements northwards, at the same time spreading the hydrographical conditions northwards that promote the development and growth of the inhabiting populations. It is especially in the hypersaline winters that these species of the central and south Adriatic penetrate to the north. HURE (1969) found in the north Adriatic material collected in
v¢ ,4
"
A.~tumn ~Surface
i
~
_ _
/
/
'°°t_ ~
~lntermediate
~
~
-
,Bottom
~--'~-7~/
S/ /
-Z Fig.3. Water currents (arrows), dominant spring to summer wind direction (broken arrows) and months with maximal zooplankton dry weight (framed) in the Central Adriatic (east coast). the year 1965 with high Mediterranean inflow 15 copepod species of the south, and GmRARDELLI (1965) found Sagitta decipiens. For the same reason also in the neritic region of the central Adriatic an increased number of species was found that year (RECNER & VU~ETId, 1973). The species of the deep south waters remain present in the north as long as the characteristics of the water remain unchanged. The deepsea south Adriatic species cannot survive at high summer temperatures (26 ° C) and the low winter ones (4 ° C), and there is no deep water for
ZOOPLANKTON
IN THE
ADRIATIC
119
them to shelter. By vertical migration they sink to the bottom where the north Adriatic bottom water carries them back into theJabuka pit and further towards the south. The neritic species of the north Adriatic can be found transported far south by the surface waters which in summer flow from north to south and from the coast towards the open sea. To relate certain hydrographical and biological changes a block diagram (Fig. 3) has been drawn for the area of the central Adriatic where long-term monthly observations were conducted at permanent stations (BuLJAN et al., 1969). In winter incoming currents towards the north exist in the surface and in the intermediary layers, and also mixed southern bottom water reaches the surface of this area by convectional mixing. After PLAHIN (1971) this convection is subpolar in the north Adriatic and subtropic in the south Adriatic. In this way nutrients can enter the area from the south Adriatic (and possibly also from the Jabuka pit or from the north). This occurs certainly more strongly during years with increased Mediterranean inflow at the time of strong advection of the intermediary water (especially in winter) and also during the upwelling periods (spring and summer, BULJAN, 1965). Certainly the period of phytoplankton production depends on it (PUCHER-PETKOVId,1970). In the open central Adriatic the zooplankton (dry weight) shows a distinct spring maximum in March and April. At the same time maximal nitrate concentrations have been found (STOJANOSKI,1973), which points maybe to upwelling. Upwelling may be caused later in the spring-summer period by dominant coastward wind directions (the maestral). The spring maximum of the zooplankton is characterized by the typical deep sea species; the maximal population densities at the beginning and by the end of summer are attained by the typical neritic species. The maximal quantities of zooplankton (dry weight) occur firstly in the open sea (Fig. 3) and then farther towards the coast, this coincides with the trophic migration direction of the sardine (Mu~INId, 1973). The geographical distribution of the zooplankton is connected to the different conditions for feeding and reproduction in different biotopes (water masses) ; and the dynamics of the hydrography of the Adriatic have conditioned the varied zoogeographical aspects. The most characteristic ones are the differences between the north Adriatic, the south Adriatic, and separately that of the transitional region of the central Adriatic (the Jabuka pit and Palagru~a sill).
120
T. V U C E T I C
V. S U M M A R Y A n a t t e m p t has been m a d e to present the general distribution p a t t e r n o f the A d r i a t i c w a t e r masses as a m e c h a n i s m for m a i n t e n a n c e of the g e o g r a p h i c a l distribution of the p l a n k t o n c o m m u n i t y (zooplankton) in the case of the Adriatic Sea. Based on the available d a t a an effort has been m a d e to integrate z o o p l a n k t o n distribution d a t a with the d y n a m i c h y d r o g r a p h i c a l processes that take place in the e n v i r o n m e n t a n d to show some of the physical m e c h a n i s m s which control distributional p h e n o m e n a of zoop l a n k t o n populations. VI. R E F E R E N C E S BULJAI%M., 1953. Fluctuations of salinity in the Adriatic.--Invj. Inst. Oceanogr. Ribarst. 2 (2): 1-64. - - - - , 1957. Fluctuation of temperature in the waters of the open Adriatic.--Acta adriat. 8 (7): 1-26. - - - - , 1964. Some results of a few years hydrographic investigations of the sea near Maslinica.--Pomorski Zborn. 2: 907-923. - - - - , 1965. Anomalies of temperature and chlorinity of sea water on the station Ston6ica (Middle Adriatic).--Pomorski Zborn. 3: 949-995. --, 1968. Fluctuation on oceanographic properties of the Central Adriatic in the period 1962-1967.--Pomorski Zborn. 6: 845-864. BULJAN,M.,J. KARLOVAC,T. PUCHER-PETKOVI6,T. VUGETI(~• M. ZORE-ARMANDA, 1969. Oceanographic conditions in the Middle Adriatic area.--Thalassia jugosl. 5: 27-34. CHAMPALBERT, G. & R. GAUDY, 1972. I~tude de la respiration chez des cop6podes de niveaux bathym6triques vari6s dans la r6gion sud marocaine et canarienne. --Mar. Biol. 12 (2): 159-169. GAMULIN,T., 1948. Prilog poznavanju zooplanktona srednjedalmatinskog oto6nog podru~ja.-- Acta adriat. 3 (7) : 1-38. - - , 1971. Comparaisons entre le zooplancton de la bales de Naples et celui de l'Adriatique meridionale pros Dubrovnik.--Rapp. P.-v. R6un. Commn int. Explor. Scient. Mer Mediterr. 20 (3): 379-383. GHIRARDELLI,E. & M. SPECCHI, 1965. Chaetognathes et Cladoc~res du golfe de Trieste (Recherches prdliminaires).--Rapp. P.-v. Rdun. Commn int. Explor. Scient. Mer Mediterr. 18 (2): 403-407. HURE,J. & B. SCOTTOD1 CARLO, 1968. Comparazione tra lo zooplancton del Golfo di Napoli e dell' Adriatico meridionale presso Dubrovnik. I. Copepoda.-Pubbl. Staz. zool. Napoli 36 (1): 21-102. - - - - , 1969a. Ripartizione quantitativa e distribuzione verticale dei Copepodi pelagici di profondita su una stazione nel Mar Tirreno ed una nell' Adriatico Meridionale.--Pubbl. Staz. zool. Napoli 37: 51-83. --, 1969b. Copepodi pelagici dell' Adriatico settentrionale nel periodo gennaiodieembre 1965.--Pubbl. Staz. zool. Napoli 37 (2): 173-195. - - - - , 1969c. Diurnal vertical migration of some deep water copepods in the Southern Adriatic (East Mediterranean).--Pubbl. Staz. zool. Napoli 37: 581-598. Khfild, I., 1973. Preliminary observation of DSL in Adriatic. Note Institut za Oceanografiju i Ribarstvo, Split (in press).
Z O O P L A N K T O N IN THE A D R I A T I C
121
LEDErt, H., 1915. l~bersicht der Ergebnisse der biologischen Beobachtungen auf der 10ten "Najade" Kreuzung.--Berichte fiber die Terminfahrten 8-12. Mu2Ir~I6, R., 1973. Migrations of adult sardines in the Central Adriatic.--Neth. J. Sea Res. 7: 19-30. PUCH~R-PzTKOW6, T., 1971. Recherches sur la production primaire et la densit~ des populations du phytoplancton en Adriatique moyenne (1962-1967).--Rapp. P.-v. R6un Commn int. Explor. Scient. Mer Mediterr. 20 (3): 339-343. PLAnm, A., 1971. Formirovanie karakteristik dubinskih vod Sredozemnovo mora u uvjetima rozvoja konvektnih pereme~enia.--Okeanologija 4: 623-628. RZGNER, D. & T. VUSETI6, 1973. Dugoro6ne fluktuacije kopepoda u obalnom podru~ju srednjeg Jadrana-Ka~telanski zaljev, Split.--Acta adriat. (in press). SRm~L~VA, A. A., 1964. Nouvelles esp~ces de Cop6podes dans l a m e r Adriatique et les traits caract~ristiques de leur distribution en relation avec des conditions hydrologiques.--Okeanologija 4 (6): 1066-1072. STOJANOSKI, L. 1973. Dinamika pojavljivanja du~ikovih soli u vodama srednjeg i ju~.nog J a d r a n a u toku 1970-1971 godine i njihov odnos prema organskoj produkciji. Mg. thesis, Zagreb. VU6~TX6, T., 1961. Sur la r6partition des Chaetognathes en Adriatique et leur utilisation comme indlcateurs biologiques des conditions hydrographiques.-Rapp. P.-v. R6un. Commn int. Explor. Scient. Mer Mediterr. 16 (2) : 111-I 16. --, 1963, Upotreba zooplanktoskih organizama kao indikatora razli6itih vodenih masa ili tipova vode. Zooplanktonic species as biological indicators of certain water masses.--Hidrogr. Godi~n. 1962: 73-80. --, 1969. Distribution ofSagitta decipiens (Fowler) and identifications of Mediterranean water masses circulation.--Bull. Inst. oc6anogra. Monaco 69 (1398). , 1970. Les principales masses d'eau en Adriatique et leur influence sur les communaut6s p61agiques.--Journfes Etud. planctonol., C.I.E.S.M. Monaco 105-114. VUSETId, T. & D. R~ONER, 1973. Les fluctuations des Cop6podes p61agiques dans la r6gion de Split.--Rapp. P.-v. R6un. Commn int. Explor. Scient. Mer Mediterr. 21 (8): 485-487. VUKAm6, D., 1971. Copepods of Boka Kotorska Bay.--Stud. Marina 5: 3-35. ZOR~-ARMANDA,M., 1963. Les masses d'eau de l a m e r Adriatique.--Aeta adriat. 10 (3): 1-93. --, 1966. The system of currents found at a control station in the middle Adriatic. - - A c t a adriat. 10 (11): 1-20. , 1968. The system of currents in the Adriatic Sea.--Stud. Rev. gen. Fish. Coun. Mediterr. 34: 1-48. , 1969a. Water exchange between the Adriatic and the eastern Mediterranean. - - D e e p Sea Res. 16: 171-178. --, 1969b. Temperature relations in the Adriatic sea.--Acta adriat. 13 (5) : 1-50. --, 1973. Formation of eastern Mediterranean deep water in the Adriatic.--Coll. Int. : Processes of formation of oceanic deep waters in particular in the western Mediterranean, Paris 1972 (in press).
ZOOPLANKTON AND THE CIRCULATION PATTERN OF THE WATER MASSES IN THE ADRIATIC by T. V U ~ E T I ~ (Institute of Oceanography and Fisheries, Split, Yugoslavia) CONTENTS I. II. III. IV. V. VI.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . Oceanographic characteristics . . . . . . . . . . . . . . . . . . . Zooplankton distribution . . . . . . . . . . . . . . . . . . . . . Plankton distribution in relation to hydrography . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . .
112 112 115 118
120 120
I. I N T R O D U C T I O N An understanding of the various zoogeographical aspects of the Adriatic Sea is only possible when the interrelations between the animal communities and the water masses are taken into consideration. In this paper the main hydrographic characteristics of the Adriatic will be discussed from the works by BULJA~ (1953, 1957, 1964, 1965, 1968, 1969), ZORE-ARMANDA (1963, 1966, 1968, 1969a, 1969b) and possible mechanisms controlling the distribution of animal plankton populations in this area will be considered. II. O C E A N O G R A P H I C C H A R A C T E R I S T I C S The Adriatic is a relatively shallow land-locked sea, forming part of the Mediterranean basin. The average depth of the north Adriatic is 60 m; the J a b u k a pit is 200 m. The Palagru~a sill (150 m) separates this area from the deep south Adriatic (maximum depth 1200 m). Vertically the Adriatic waters can be devided into a surface layer (0 to 40 m), an intermediate layer (40 to 150 m in the central Adriatic and 400 to 500 m in the south Adriatic) and a bottom layer, each characterized by the presence of specific water masses (ZoRF.-ARMANDA, 1963). The main directions of the water flow are determined by density gradients; differences in barometric pressure and evaporation with the corresponding vertical water movements. The most important seasonal changes occur in the surface layer
ZOOPLANKTON IN THE ADRIATIC
113
where, due to a density gradient between the north and the south Adriatic, a longitudinal flow prevails causing an outgoing water movement from the Adriatic to the Mediterranean in summer, and an incoming flow from the Mediterranean into the Adriatic in winter. In general the Adriatic currents have a low velocity (approximately 20 cm/sec) and the flow along the west coast is considerably faster than along the east coast (ZoRE-ARMANDA, 1968). In winter the surface incoming flow is faster along the east coast, in summer the outgoing flow along the west coast. However, in spring and autumn 2 separate gyres are created and an area of intensive mixing is formed over the Palagru~a sill, where a transverse flow is generated causing divergence and upwelling in spring (which continues during summer) and convergence and water sinking in autumn (continuing in winter). The intermediary layer of the Adriatic water is characterized by the inflow of the Mediterranean water into the Adriatic (the Intermediary East Mediterranean Salt Water). As a result the inflow of water from the Mediterranean via the intermediary layer and the outflow from the surface form a continuous circulation, especially in the summer period. The intensity of the exchange with the Mediterranean varies with differences in barometric pressure (ZoRE-ARMANDA, 1969) over the Mediterranean area. Increases in salinity, changes in temperature distribution and a number of other changes occur as the result of a more intensive inflow of the Mediterranean water during years of intensified flow (BuLJAN, 1953). Bottom water is formed in the north Adriatic in the years with high inflow of Mediterranean water during winter. It flows from there into the J a b u k a pit and occasionally over the sill into the south Adriatic. In the south bottom water is formed in situ in other years. Via the Strait of Otranto the south Adriatic water takes part in the formation of the bottom water of the eastern Mediterranean (ZoRE-ARMANDA, 1973). The strongest water exchange in the Adriatic occurs in the region of the Palagru~.a sill and the Strait of Otranto; in between are the following areas: the south Adriatic, the J a b u k a pit, and the north Adriatic, where various types of water can be formed. ZORE-ARMANDA (1963) has given the characteristics of 4 water types (Fig. 1). The North Adriatic water type (N) is formed in the north Adriatic during the more saline winters when strong vertical mixing occurs due to the high salinity and the temperature decrease in the surface layers. The Central Adriatic water (C) is formed in the J a b u k a pit during hyposaline winters. The South Adriatic water (S) is formed during hyposaline winters and it is permanently present in the depths of the south Adriatic. ZORE-ARMANDA (1963) thinks that this water type of
114
T. VU~ETI6
high density is formed by the cooling of the incoming Intermediary Mediterranean water ( M ) - - t h e most saline water of the Mediterran e a n - a n d mixing with the north Adriatic water (Fig. 1). Summarizing, besides seasonal differences, long-term fluctuations have also been recorded (BULJAN,1953; ZORE-ARMANDA, 1964). Hence it is possible to distinguish 2 conditions in the hydrography of the Adriatic: the hypersaline condition (Fig. l b) and the less saline condition (Fig. l a). Hypersalinity is the result of the changes which are effected by a strong inflow or advection of warmer and salter Mediterranean water towards the north, especially in winter, through the surface and intermediary layers (Fig. l b). At the same time, in the bottom layer of the north Adriatic the dense cold relatively less salt water (N) sinks and fills the J a b n k a pit from where it flows into the south Adriatic over the Palagru2a sill. In summer, in the intermediary layer, Mediterranean water enters the J a b u k a pit, and on the surface the less warm and less salt (in relation to years with less influence of Mediterranean water) north Adriatic water spreads southwards sometimes reaching the Palagru~.a sill (BULJAN, 1966).
Winter
Spring
Summer
Autumn
12o0
"°°5 °°° II 1200 ~
Illl~ ~Jl~l~i Ii v T=/'C
Sa1=3~5%* ~t~29,s2
~
T='2"C
Sa1=38,2%,
~t~z909
~T=13"C ~T='4"C" 5al--38,6"/,, ~ 5a1=38,7%, 6t=2920 ~~t~29os
Fig. l. Seasonal aspects of the hydrographical conditions in the Adriatic; N, (3 and S indicate North, Central and South Adriatic Water, M Mediterranean Water, and blank areas winter and summer surface waters; arrows indicate current directions. a. Weak influence of the Mediterranean water; b. strong influence of the Mediterranean water. (Modified after ZORE-ARMANDA, 1969).
ZOOPLANKTON
IN THE
115
ADRIATIC
III. Z O O P L A N K T O N D I S T R I B U T I O N The main distribution pattern of the water masses in the Adriatic form the framework in which various characteristics of the geographical aspects of the Adriatic zooplankton should be placed. The scheme in Fig. 2 of the main hydrographic aspects makes an easier orientation possible. The distribution of plankton organisms and the biological seasons are regulated by these aspects. LEDER (l 914) used the plankton material collected by the "Najade" expedition (1911-1914)--whose objective was to obtain a preliminary view of the Adriatic fauna and flora--to demonstrate the geographical zonation and its dynamics, and to determine by means of the plankton distribution the direction and intensity of currents and the water masses. VU~ETId (1970: Table I) already pointed out that in the north Adriatic the autochthonous and dominant plankton has a neritic character. It consists of a considerably smaller number of the holoplanktonic species, which are also smaller in size than the plankton of the open sea. As great differences exist between the winter and summer temperatures (4 to 28 ° C) and salinities (31 to 38 ~oo S) the organisms are eurythermic and euryhaline (Fig. 9). As a rule these organisms have short life cycles with several generations per year (Copepoda). N sua. I ~ g °
Winter int.~ bott.
C ~
~ ~
Spring i \ ~
S
M
~
~
14"
~ ~
~
14"
13°
~
~ t 4 "
Summer
Autumn ta"
~
~
zg
Fig.2. Schematic seasonal water movements in the North (N), Central (C) a n d South (S) Adriatic, under strong influence of the Mediterranean (M) ; Mediterranean water in solid arrows. Temperatures in °C at north and south end of the water layers.
116
T. v u 6 E w I d TABLE
I
Dominant neritic copepods in the Adriatic and the months of their maximal abundancy. North Adriatic 1965 (HuaE et al., 1969)
Species
Acartia clausi Paracalanus parvus Centropages typicus Ctenocalanus vanus Temora stylifera
Months
V, VII, X I I
Central Adriatic 1969-1970 (VudETId & REGNER, 1973)
South Adriatic 1971 (VuKANId,1971)
Centropages typicus Centropageskroyeri Acartia clausi Ctenocalanus vanus Centropages kroyeri Clausocalanusarcuicornis Temora stylifera Clausocalanus furcatus Paracalanus parvus Paracalanusparvus Clausocalanus arcuicornis Acartia clausi III, VIII, IX III, VII
They occur in dense populations, especially in summer, so that the biomass (dry weight) of the total zooplankton is high. Of the Copepoda which occur here in the highest numbers--just as the Cladocera--only 67 species can be found here against 216 species at present known for the entire Adriatic and 145 known for the south Adriatic (HugE, 1969). Among the 14 dominant species, quantitatively the most important are of small size as: Acartia clausi, Paracalanus parvus, Centropages typicus, Ctenocalanus vanus, Temora stylifera (Table I). For the species of small size CnAMI"ALBERT& GAUDY (1972) say that "principalement pour les temp6ratures sup6rieures les esp6ces de taille r6duite sont plus aptes que les grandes formes a r6guler leur m6tabolisme". Well represented are also the following species : Clausocalanus arcu#ornis, C.furcatus, Temo-
ra long#ornis, Calanus helgolandicus, Oithona plumifera, Oncaea media, O. mediterranea, Oithona helgolandica, Corycaeus giesbrechti. The community of the deep south Adriatic has all the elements of the oceanic plankton, and is considered diametrically opposite to the north Adriatic zooplanktonic community (Vu~EwId, 1970: Table I). As already mentioned here the water masses consist of the warm and saline south Adriatic water (S) and of the still salter Mediterranean water (M). The typical deep water plankton fauna of the south Adriatic influences the composition of the zooplankton in the central (C) and north Adriatic (N). Here the fauna is rich in groups and species with m a n y autochthonous elements (Table I). These organisms have a considerably longer life cycle and a smaller n u m b e r of generations a year and are more stenothermic and stenohaline than those in the north. The differences in salinity (38.6 to 38.7 ~o S) and temperature (11 to 14 ° C) are considerably smaller in the layers where the greatest nurn-
ZOOPLANKTON
IN THE
ADRIATIC
117
bers of the plankton species live. It is important to mention that in the south Adriatic community large size open sea copepods are particularly well represented, as Pleuromamma gracilis, P. abdominalis, Euchaeta hebes, E. acuta, Haloptilus longicornis; exclusively species of smaller sizes are:
Mormonilla minor, Monacilla typica, Oncaea ornata, Temoropia mayumbaensis. Characteristic are also the chaetognaths Sagitta decipiens and Krohnitta subtilis, the groups of the Ostracoda and the Euphausiacea, the larger deep sea Decapoda, and bathypelagic fishes (Vu6ETId, 1970: Table I). After the latest findings by HURE (1969c) in the bottom water (600 to 1000 m) of the south Adriatic the predominant copepods are Oncaea ornata, Temoropia mayumbaensis and Monacilla typica. In the layer of the intermediate water (200 to 600 m) Pleuromamma gracilis, Oithona setigera and Haloptilus longicornis are predominant, and in the surface layer Ctenocalanus vanus, Oithona plumifera, Clausocalanusfurcatus and C. arcuicorhis. These typically south Adriatic open sea species occur in the north in greater numbers during the years of high inflow of Mediterranean water (Vu(~ETId, 1970) and in the upwelling zones (~MELJEVA,1964). It is rather interesting that in 1967 and 1968 densest populations of Temoropia mayumbaensis were found in the layer of 600 to 1000 m (HuRE, 1969a), and in 1969 in the layer from 400 to 600 rn. The analysis of the hydrographical data (ZoRE-ARMANDA,personal communication) has shown that the year 1969 was outstandingly hypersaline (38.9 ~o S), while the previous years 1967 and 1968 were less saline. Most typical for Adriatic bottom water are probably Oncaea ornata and Monacilla typica while Mormonilla minor is representative for the Intermediary Mediterranean water of 300 to 400 m where its populations are densest. It was also found (HuRE, 1969) that in the Tyrrheanian sea Mormonilla minor and Eucalanus monachus occur in considerably greater densities than in the Adriatic. In the Jabuka pit belonging to the central part of the Adriatic the plankton shows a varied structure. In the surface layer it is neritic, in the waters below 50 m it has the elements of the deep south Adriatic. Probably during the years of high Mediterranean inflow the area is repopulated by young stages and perhaps also by adults. Also the bathypelagic fishes and other elements of the south Adriatic fauna are restored especially during years of high Mediterranean inflow. Along the edge of the Jabuka pit very recently (KA~Id, 1973) it has been registered DSL, undoubtedly caused mostly by euphausids. In the Jabuka pit the abundant Euchaeta (E. hebes) and Calanus (C. helgolandicus) layers are well known since long ago (LEDER, 1914). It seems that Euchaeta hebes thrives in this part of the Adriatic and also on the shelf of the whole area as far as Palagru~a at a depth of 60 to 200 m (GAMULIN, 1971).
118
T. V U ~ E T I d IV. P L A N K T O N
DISTRIBUTION HYDROGRAPHY
IN R E L A T I O N
TO
The dynamics of the Adriatic make clear w h y the faunal elements of the deep south Adriatic occur farther to the north during years when the Mediterranean intermediary water reaches to the very surface of the south Adriatic. Then the strong incoming currents push these elements northwards, at the same time spreading the hydrographical conditions northwards that promote the development and growth of the inhabiting populations. It is especially in the hypersaline winters that these species of the central and south Adriatic penetrate to the north. HURE (1969) found in the north Adriatic material collected in
v¢ ,4
"
A.~tumn ~Surface
i
~
_ _
/
/
'°°t_ ~
~lntermediate
~
~
-
,Bottom
~--'~-7~/
S/ /
-Z Fig.3. Water currents (arrows), dominant spring to summer wind direction (broken arrows) and months with maximal zooplankton dry weight (framed) in the Central Adriatic (east coast). the year 1965 with high Mediterranean inflow 15 copepod species of the south, and GmRARDELLI (1965) found Sagitta decipiens. For the same reason also in the neritic region of the central Adriatic an increased number of species was found that year (RECNER & VU~ETId, 1973). The species of the deep south waters remain present in the north as long as the characteristics of the water remain unchanged. The deepsea south Adriatic species cannot survive at high summer temperatures (26 ° C) and the low winter ones (4 ° C), and there is no deep water for
ZOOPLANKTON
IN THE
ADRIATIC
119
them to shelter. By vertical migration they sink to the bottom where the north Adriatic bottom water carries them back into theJabuka pit and further towards the south. The neritic species of the north Adriatic can be found transported far south by the surface waters which in summer flow from north to south and from the coast towards the open sea. To relate certain hydrographical and biological changes a block diagram (Fig. 3) has been drawn for the area of the central Adriatic where long-term monthly observations were conducted at permanent stations (BuLJAN et al., 1969). In winter incoming currents towards the north exist in the surface and in the intermediary layers, and also mixed southern bottom water reaches the surface of this area by convectional mixing. After PLAHIN (1971) this convection is subpolar in the north Adriatic and subtropic in the south Adriatic. In this way nutrients can enter the area from the south Adriatic (and possibly also from the Jabuka pit or from the north). This occurs certainly more strongly during years with increased Mediterranean inflow at the time of strong advection of the intermediary water (especially in winter) and also during the upwelling periods (spring and summer, BULJAN, 1965). Certainly the period of phytoplankton production depends on it (PUCHER-PETKOVId,1970). In the open central Adriatic the zooplankton (dry weight) shows a distinct spring maximum in March and April. At the same time maximal nitrate concentrations have been found (STOJANOSKI,1973), which points maybe to upwelling. Upwelling may be caused later in the spring-summer period by dominant coastward wind directions (the maestral). The spring maximum of the zooplankton is characterized by the typical deep sea species; the maximal population densities at the beginning and by the end of summer are attained by the typical neritic species. The maximal quantities of zooplankton (dry weight) occur firstly in the open sea (Fig. 3) and then farther towards the coast, this coincides with the trophic migration direction of the sardine (Mu~INId, 1973). The geographical distribution of the zooplankton is connected to the different conditions for feeding and reproduction in different biotopes (water masses) ; and the dynamics of the hydrography of the Adriatic have conditioned the varied zoogeographical aspects. The most characteristic ones are the differences between the north Adriatic, the south Adriatic, and separately that of the transitional region of the central Adriatic (the Jabuka pit and Palagru~a sill).
120
T. V U C E T I C
V. S U M M A R Y A n a t t e m p t has been m a d e to present the general distribution p a t t e r n o f the A d r i a t i c w a t e r masses as a m e c h a n i s m for m a i n t e n a n c e of the g e o g r a p h i c a l distribution of the p l a n k t o n c o m m u n i t y (zooplankton) in the case of the Adriatic Sea. Based on the available d a t a an effort has been m a d e to integrate z o o p l a n k t o n distribution d a t a with the d y n a m i c h y d r o g r a p h i c a l processes that take place in the e n v i r o n m e n t a n d to show some of the physical m e c h a n i s m s which control distributional p h e n o m e n a of zoop l a n k t o n populations. VI. R E F E R E N C E S BULJAI%M., 1953. Fluctuations of salinity in the Adriatic.--Invj. Inst. Oceanogr. Ribarst. 2 (2): 1-64. - - - - , 1957. Fluctuation of temperature in the waters of the open Adriatic.--Acta adriat. 8 (7): 1-26. - - - - , 1964. Some results of a few years hydrographic investigations of the sea near Maslinica.--Pomorski Zborn. 2: 907-923. - - - - , 1965. Anomalies of temperature and chlorinity of sea water on the station Ston6ica (Middle Adriatic).--Pomorski Zborn. 3: 949-995. --, 1968. Fluctuation on oceanographic properties of the Central Adriatic in the period 1962-1967.--Pomorski Zborn. 6: 845-864. BULJAN,M.,J. KARLOVAC,T. PUCHER-PETKOVI6,T. VUGETI(~• M. ZORE-ARMANDA, 1969. Oceanographic conditions in the Middle Adriatic area.--Thalassia jugosl. 5: 27-34. CHAMPALBERT, G. & R. GAUDY, 1972. I~tude de la respiration chez des cop6podes de niveaux bathym6triques vari6s dans la r6gion sud marocaine et canarienne. --Mar. Biol. 12 (2): 159-169. GAMULIN,T., 1948. Prilog poznavanju zooplanktona srednjedalmatinskog oto6nog podru~ja.-- Acta adriat. 3 (7) : 1-38. - - , 1971. Comparaisons entre le zooplancton de la bales de Naples et celui de l'Adriatique meridionale pros Dubrovnik.--Rapp. P.-v. R6un. Commn int. Explor. Scient. Mer Mediterr. 20 (3): 379-383. GHIRARDELLI,E. & M. SPECCHI, 1965. Chaetognathes et Cladoc~res du golfe de Trieste (Recherches prdliminaires).--Rapp. P.-v. Rdun. Commn int. Explor. Scient. Mer Mediterr. 18 (2): 403-407. HURE,J. & B. SCOTTOD1 CARLO, 1968. Comparazione tra lo zooplancton del Golfo di Napoli e dell' Adriatico meridionale presso Dubrovnik. I. Copepoda.-Pubbl. Staz. zool. Napoli 36 (1): 21-102. - - - - , 1969a. Ripartizione quantitativa e distribuzione verticale dei Copepodi pelagici di profondita su una stazione nel Mar Tirreno ed una nell' Adriatico Meridionale.--Pubbl. Staz. zool. Napoli 37: 51-83. --, 1969b. Copepodi pelagici dell' Adriatico settentrionale nel periodo gennaiodieembre 1965.--Pubbl. Staz. zool. Napoli 37 (2): 173-195. - - - - , 1969c. Diurnal vertical migration of some deep water copepods in the Southern Adriatic (East Mediterranean).--Pubbl. Staz. zool. Napoli 37: 581-598. Khfild, I., 1973. Preliminary observation of DSL in Adriatic. Note Institut za Oceanografiju i Ribarstvo, Split (in press).
Z O O P L A N K T O N IN THE A D R I A T I C
121
LEDErt, H., 1915. l~bersicht der Ergebnisse der biologischen Beobachtungen auf der 10ten "Najade" Kreuzung.--Berichte fiber die Terminfahrten 8-12. Mu2Ir~I6, R., 1973. Migrations of adult sardines in the Central Adriatic.--Neth. J. Sea Res. 7: 19-30. PUCH~R-PzTKOW6, T., 1971. Recherches sur la production primaire et la densit~ des populations du phytoplancton en Adriatique moyenne (1962-1967).--Rapp. P.-v. R6un Commn int. Explor. Scient. Mer Mediterr. 20 (3): 339-343. PLAnm, A., 1971. Formirovanie karakteristik dubinskih vod Sredozemnovo mora u uvjetima rozvoja konvektnih pereme~enia.--Okeanologija 4: 623-628. RZGNER, D. & T. VUSETI6, 1973. Dugoro6ne fluktuacije kopepoda u obalnom podru~ju srednjeg Jadrana-Ka~telanski zaljev, Split.--Acta adriat. (in press). SRm~L~VA, A. A., 1964. Nouvelles esp~ces de Cop6podes dans l a m e r Adriatique et les traits caract~ristiques de leur distribution en relation avec des conditions hydrologiques.--Okeanologija 4 (6): 1066-1072. STOJANOSKI, L. 1973. Dinamika pojavljivanja du~ikovih soli u vodama srednjeg i ju~.nog J a d r a n a u toku 1970-1971 godine i njihov odnos prema organskoj produkciji. Mg. thesis, Zagreb. VU6~TX6, T., 1961. Sur la r6partition des Chaetognathes en Adriatique et leur utilisation comme indlcateurs biologiques des conditions hydrographiques.-Rapp. P.-v. R6un. Commn int. Explor. Scient. Mer Mediterr. 16 (2) : 111-I 16. --, 1963, Upotreba zooplanktoskih organizama kao indikatora razli6itih vodenih masa ili tipova vode. Zooplanktonic species as biological indicators of certain water masses.--Hidrogr. Godi~n. 1962: 73-80. --, 1969. Distribution ofSagitta decipiens (Fowler) and identifications of Mediterranean water masses circulation.--Bull. Inst. oc6anogra. Monaco 69 (1398). , 1970. Les principales masses d'eau en Adriatique et leur influence sur les communaut6s p61agiques.--Journfes Etud. planctonol., C.I.E.S.M. Monaco 105-114. VUSETId, T. & D. R~ONER, 1973. Les fluctuations des Cop6podes p61agiques dans la r6gion de Split.--Rapp. P.-v. R6un. Commn int. Explor. Scient. Mer Mediterr. 21 (8): 485-487. VUKAm6, D., 1971. Copepods of Boka Kotorska Bay.--Stud. Marina 5: 3-35. ZOR~-ARMANDA,M., 1963. Les masses d'eau de l a m e r Adriatique.--Aeta adriat. 10 (3): 1-93. --, 1966. The system of currents found at a control station in the middle Adriatic. - - A c t a adriat. 10 (11): 1-20. , 1968. The system of currents in the Adriatic Sea.--Stud. Rev. gen. Fish. Coun. Mediterr. 34: 1-48. , 1969a. Water exchange between the Adriatic and the eastern Mediterranean. - - D e e p Sea Res. 16: 171-178. --, 1969b. Temperature relations in the Adriatic sea.--Acta adriat. 13 (5) : 1-50. --, 1973. Formation of eastern Mediterranean deep water in the Adriatic.--Coll. Int. : Processes of formation of oceanic deep waters in particular in the western Mediterranean, Paris 1972 (in press).