ADRIATIC PELAGIC PRODUCTION 65

SCI. MAR., 60 (Supl. 2): 65-77 SCIENTIA MARINA 1996

THE EUROPEAN ANCHOVY AND ITS ENVIRONMENT, I. PALOMERA and P. RUBIÉS (eds.)

Pelagic production and biomass in the Adriatic Sea*

SERENA FONDA UMANI

Department of Biology, University of Trieste, Laboratory of Marine Biology, Strada Costiera 336, 34010 Trieste, Italy.

SUMMARY: On the basis of the available data on plankton production and biomass in the Adriatic Sea, three different areasmay be delineated: a - Open waters of the central and southern Adriatic, characterized by an “oceanic” community. Prima-ry production rates are low. Microphytoplankton and especially diatoms are abundant; microzooplankton is dominated bytintinnids; herbivorous copepods are dominant throughout the year and carnivorous species are also important. The plank-ton communities show high diversity and great stability. A “top down” control appeared to regulate the energy fluxes. b -Open waters of the northern basin, characterized by neritic associations with moderate primary production rates and bio-mass. The species originating from the southern Adriatic and the Mediterranean are present throughout the year. A markedwest-to-east gradient of production and biomass is present during the stratified period. The models of phytoplankton con-trol appear to shift from “bottom up” to “top down” control, following the western-eastern gradient and changing seasonal-ly. c - A coastal zone characterized by a neritic community of low diversity and high primary production and biomass.Nanoplankton primary production generally prevails. This production appear to be used mainly by ciliates other than tintin-nids during the stratified period. Nanoplankton also represents an important food source for the abundant filter feeders. Dur-ing the persistence of the frontal system, separating coastal from off shore waters, terrigenous inputs stimulate high prima-ry production which is confined to inshore areas. Zooplankton grazing is not sufficient to control the produced biomass andtransfer to offshore is restricted, resulting in eutrophication

Key words: Primary production, plankton biomass and distribution, Adriatic Sea.

RESUMEN: PRODUCCIÓN PRIMARIA Y BIOMASA EN EL ADRIÁTICO. – En base a los datos disponibles de producción planctó-nica y biomasa del Adriático, pueden delimitarse tres áreas distintas: a - Aguas de mar abierto del Adriático central y meri-dional, caracterizadas por una comunidad “oceánica”. Las tasas de producción primaria son bajas. El microfitoplancton yespecialmente las diatomeas, son abundantes; el microzooplancton está dominado por los tintínidos; los copépodos herbí-voros dominan a lo largo de todo el año y las especies carnívoras tienen también importancia. Las comunidades planctóni-cas muestran una diversidad alta y una gran estabilidad. Un control de “top down” parece regular los flujos de energía. b -Aguas de mar abierto de la cuenca septentrional, caracterizadas por asociaciones neríticas con tasas de producción primaray biomasa moderadas. Las especies de origen sud-adriático y mediterráneo están presentes todo el año. Durante el períodode estratificación existe un marcado gradiente oeste-este de produccción y biomasa. Los modelos de control del fitoplanc-ton parecen cambiar de “bottom up” a “top down”, siguiendo el gradiente oeste-este y cambiando estacionalmente. c - Unazona costera caracterizada por una comunidad nerítica de baja diversidad y elevadas producción primaria y biomasa. Gene-ralmente predomina la producción primaria del nanoplancton. Dicha producción parece ser utilizada principalmente porciliados no tintínidos durante el período de estratificación. El nanoplancton representa asimismo una fuente importante dealimento para los abundantes filtradores. Mientras persiste el sistema frontal, que separa las aguas costeras de las oceánicas,los aportes terrígenos estimulan una elevada producción primaria, confinada a las aguas litorales. Su consumición por partedel zooplancton no alcanza a controlar la biomasa producida y la transferencia a alta mar es limitada, dando como resulta-do la eutroficación. (Traducido por los Editores).

Palabras clave: Producción primaria, biomasa y distribución planctónicas, Adriático.

*Received July 16, 1995. Accepted August 26, 1996.

INTRODUCTION

The Adriatic Sea is a semi-enclosed and elon-gated basin, stretching roughly SE to NW for 800km from the Straits of Otranto to the Gulf ofVenice (Fig.1). The elongate shape and the pres-ence of the great Dalmatian Archipelago create anextremely long coastline. The eastern coasts arehigh, rocky, and articulated. The western (Italian)coasts generally are sandy, flat and alluvional withthe exception of the Gargano Peninsula and theApulian coast in the southern part. Extensivelagoons characterized the northern part and theareas of Po River Delta. Based on the bathymetryand different oceanographic properties it is possi-ble to distinguish three distinct areas: North, Midand South Adriatic. The North Adriatic is the leastdeep (max. depth 100 m) of the three basins; itsbottom topography and sediment composition arestrongly influenced by lower sea level during thePleistocene (Brambati, 1992). Along the Italianshore the sea bottom is characterized by peliticsediments of terrigenous supplies. Sand, left overfrom Holocenic transgression, predominates in thecentral and southern part of the shelf.

Its biological characteristics are influenced bythe morphology, meteorology and hydrodynamics ofthe area, and in particular by the supply of the Poriver (Fonda Umani et al., 1992, Franco andMichelato, 1992). The water mass distributionundergoes remarkable seasonal modification due totwo main factors: the wide fluctuation of the surfaceheat fluxes and the large volume of fresh waterflowing into the shallow basin (Fonda Umani et al.,1992). During winter, when the total heat budget isnegative (Hendershott and Rizzoli, 1976), the coldwaters diluted by the western riverine inflow remainconfined in a coastal belt and are separated from theoffshore waters by a frontal system and flow south-ward (Franco, 1973; 1986). In the offshore area thewaters are highly saline, being advected from thesouthern basins and they are actively mixed by wind- driven surface cooling and mechanical stirring(Fonda Umani et al., 1992). During summer, afterthe generation of a thermocline and the injection ofPo river waters in the offshore area, heating anddilution processes generate a highly stratified watercolumn in which there are three layers separated bystrong density gradients (Fonda Umani et al., 1992).

The Mid and South Adriatic are deeper basins(max. depth 270 m and 1200 m respectively); theyare separated by the Pelagosa sill (160 m). Anothersill (800 m) situated in the Channel of Otranto sepa-rates the South Adriatic from the Ionian Sea. Themorphology of the central and southern basins hasbeen conditioned by deposition processes whichoccurred during periods of marine regression. Thebasin bottom is mainly composed of pelitic sedi-ments which have the typical mineralogical charac-teristics of their province: Dalmatian, Central Apen-nine or South - Augitic- Albanian (Brambati, 1992).

The two basins are characterized by low rates ofprimary production and a general oligotrophic con-dition (Buljan, 1964). They receive the supply ofboth northern, dense winter waters and of cold andhighly salty waters originating during winter in theeastern Mediterranean. Therefore in both basinsthree layers can be distinguished along the watercolumn: a superficial layer of low density, influ-enced, along the western side, by the indirect effectsof riverine dilution, an intermediate one with densewaters of Ionian origin and the deep one with densewater generated in winter (Buljan and Zore-Arman-da, 1976).

The distribution of dissolved nutrients is influ-enced by hydrodynamic features of the basin andshows marked seasonal differences (Tables 1 and 2).

66 S. FONDA UMANI

Fig. 1. – The Adriatic Sea: bathymetry, high and low coasts andsurface currents (from Brambati, 1992).

During the winter mixing period in the western belt,which is separated by the frontal system, nutrientconcentration can be 3 - 10 times higher than inopen waters (Franco and Michelato, 1992). Duringsummer stratification, the increase of nutrients in thesurface layer from river inputs is followed by a situ-ation of depletion due to phytoplankton utilizationand progressive transport via faecal pellets and sink-ing of particulate matter to the deeper layers.

In the Northern Adriatic, particulate organic car-bon (POC) and particulate organic nitrogen (PON)concentrations appear related mostly to the phyto-plankton crop: the mean surface phytoplankton car-bon is about 20% of total POC. During summerstratification POC range from 75 to 1701 µg l-1 andPON from 8.9 to 303 µg l-1 showing a decreasingpattern from surface to bottom layers and from eastto west (Gilmartin and Revelante, 1991). In the Gulf

of Trieste mean POC concentration ranged from610.9 at the surface to 444.9 µg l-1 at bottom layer.The mean C/N atomic ratio at the surface was 16.4and at the bottom 8.5, indicating the prevalence ofallochthonous riverine POC at the surface andautochthonous plankton at the bottom (Salvi et al.,in press). Recently, Wassmann et al. (in press) cal-culated the vertical flux of organic carbon in theGulf of Trieste: it ranged from 800 mg C m-2d-1 inJune to 200 mg C m-2d-1 in September. Phytoplank-ton carbon comprised respectively 17 - 29% and 40- 87% of the suspended POC, implying that thedetritus fraction was lower in September.

Lastly, Giordani et al., (1992) calculated by mea-suring in situ benthic fluxes that for the Adriatic asa whole about 85% of the carbon, 60% of the phos-phorus, 40% of the fixed nitrogen, and 85% of thesilica are recycled into the water column.

ADRIATIC PELAGIC PRODUCTION 67

TABLE 1. – Mean values and confidence intervals (parentheses) of five water masses in the northern Adriatic basin in September, 1987(from Franco and Michelato, 1992). The regions are: A’ - river plume waters; A - coastal waters; B’ - surface waters of the proper basin;

B - intermediate water; and C - deep layer. Nutrient concentrations are expressed as µM dm-3. Si is silicon in SiO4.

Temp. Sal. η AOU N-NH3 N-NO2 N-NO3 P-PO4 Si-SiO4 Chl.a(ºC) (PSU) (kg m-3) (cm3 dm-3) (µM dm-3) (µg dm-3)

A’ 23.11 24.59 16.03 -0.62 1.39 0.96 26.97 0.22 23.78 9.47(0.60) (4.76) (3.53) (0. 42) (0.98) (0.42) (18.23) (0.19) (13.87) (3.08)

A 24.47 33.96 22.72 -0.44 0.76 0.17 2.65 0.08 3.30 2.56(0.11) (0.20) (0.14) (0.09) (0.14) (0.03) (0.67) (0.02) (0.53) (0.67)

B’ 24.63 36.14 24.32 -0.24 0.48 0.03 0.13 0.07 1.58 0.63(0.7) (0.09) (0.08) (0.04) (0.11) (0.01) (0.04) (0.01) (0.18) (0.06)

B 21.53 37.89 26.54 -0.13 1.29 0.07 0.24 0.13 4.73 0.85(0.62) (0.08) (0.22) (0.48) (1.06) (0.04) (0.17) (0.06) (2.35) (0.22)

C 15.96 38.28 28.27 0.63 1.54 0.08 0.58 0.30 10.15 1.53(0.62) (0.03) (0.15) (0.39) (0.46) (0.03) (0.18) (0.20) (2.26) (0.38)

TABLE 2. – Mean values and confidence intervals (parentheses) of five water masses in the northern Adriatic basin in February, 1987 (fromFranco and Michelato, 1992). The regions are: A’ - river plume waters; A - coastal waters; B - proper basin dense waters; and C - proper

basin waters. Nutrient concentrations are expressed as µM dm-3. Si is silicon in SiO4.

Temp. Sal. η AOU N-NH3 N-NO2 N-NO3 P-PO4 Si-SiO4 Chl.a(ºC) (PSU) (kg m-3) (cm3 dm-3) (µM dm-3) (µg dm-3)

A’ 5.29 32.78 25.88 0.02 17.83 0.82 33.81 0.60 16.03 4.43(0.84) (2.26) (1.73) (0.09) (2.97) (0.30) (2.40) (0.36) (8.72)

A 5.21 36.73 29.02 0.04 1.52 0.49 10.56 0.18 6.78 1.42(0.40) (0.28) (0.22) (0.06) (0.56) (0.06) (3.14) (0.07) (0.53) (0.80)

B 6.92 38.00 29.79 -0.02 0.44 0.21 1.04 0.18 4.06 1.62(0.13) (0.05) (0.03) (0.06) (0.11) (0.04) (0.28) (0.05) (0.23) (0.30)

C 7.77 38.10 29.75 0.03 0.48 0.36 0.71 0.18 4.36 1.59(0.07) (0.03) (0.02) (0.04) (0.21) (0.07) (0.14) (0.03) (0.22) (0.22)

PRIMARY PRODUCTION ANDPHYTOPLANKTON BIOMASS

The rivers flowing into the northern Adriatic arethe major sources of external nutrient input, espe-cially so during stratified periods (Franco, 1973;Degobbis and Gilmartin, 1990). Water massexchange between the northern region and theremainder of the essentially oligotrophic Adriatic, aswell as the major influence of Ionian water in thesouth, have a great influence on the productivity andstanding crops of different sub-areas. A terrigenoussupply of nutrients in some semi-enclosed bays andchannels of the eastern coast and all along the west-ern coast via run-off, influences productivity in arelatively narrow coastal belt. Consequently, bio-mass and production rates are spatially very vari-able. Buljan (1964) estimated the productivity of theAdriatic Sea on the basis of its hydrographic proper-ties and suggested four productivity zones: openwaters of the central and southern Adriatic with lowproduction; the shallow northern Adriatic includinga narrow coastal belt along the western coast andcharacterized by permanent high production; thearea of moderate production occupying easterncoastal waters; and the limited zones with high pro-duction under strong coastal influences (lagoons andembayments) along the eastern and also westernshores.

Northern Adriatic

The northern Adriatic has been recognized formany years as a region of high marine production atseveral trophic levels from phytoplankton to fish. Aregion of high but variable phytoplankton biomassand production was quantified off the delta of theRiver Po and related to the spreading of its plume(Franco, 1973; Gilmartin and Revelante, 1981), anda marked west to east gradient of the standing cropand production was observed (Smodlaka and Reve-lante, 1983). In Table 3 the mean values for chloro-phyll a as well as maximum and minimum valuesfor primary production (measured by 14C method)are reported for the eastern and western part (FondaUmani et al., 1992).

In the northernmost part, the Gulf of Trieste, thechlorophyll a biomass is moderate with a meanvalue around 1 µg dm-3 in the eastern section andaround 1.6 µg dm-3 in the western section (Olivottiet al, 1986), while annual productivity varies from42 to 53 g C m-2 y-1 (Faganeli et al., 1981; Monti,

1986). During 1986/87 the temporal trend of phyto-plankton biomass (arithmetical mean of carbon con-tent values at four depths) was described by Catalet-to et al., (1993): it ranged between 24.37 and 500.40mg C m-3, with a maximum in September due todiatoms which generally contribute more than otherfractions (namely nanoplankton and dinoflagellates)to the total amount (Fig. 2).

Near shore areas of the central andsouthern Adriatic

The chlorophyll a concentration of the easternpart of the central and southern Adriatic is generallybelow 1 µg dm-3 (Pucher-Petkovic and Zore-Arman-da, 1973). However, a steady increase has beenobserved in the last 15 years in some near shoreareas like Kastela Bay (Marasovic et al., 1988) themean annual values exceeding 1.3 µg dm-3 recently.Very high chl a values are found along the westerncoast of the Italian region of Emilia-Romagna,where close to the coast the annual mean chl a isover 10 µg dm-3, falling below 8 µg dm-3 about 2 Kmoffshore (Anon., 1984) (Table 4).

68 S. FONDA UMANI

TABLE 3. – Chlorophyll a mean values and maximum and minimumof primary production (measured by 14C method) in western and

eastern North Adriatic

North Adriatic Western Eastern

chlorophyll a 2.87 µg.dm-3 0.9 µg.dm-3

max. 30 µg. C dm-3h-1 10 µg. C dm-3 h-1

production min. <1 µg. C dm-3h-1 <1 µg. C dm-3 h-1

Fig. 2. – Carbon content annual pattern of the three main phyto-plankton fractions (dinoflagellates [dinofl.], nanoplankton[nanopl.], diatoms) in the Gulf of Trieste during the period

1986/1987 (from Cataletto et al., 1993).

The first measurements of the rate of primaryproduction in the Adriatic, reported for the easterncoast of the central Adriatic, were < 1 µg C dm-3 h-1

(Cviic, 1962). Thereafter the area has been continu-ously surveyed and values have been below 8 µg Cdm-3 h-1 (Pucher-Petkovic, 1966) (Table 4). Recent-ly, estimates of primary production were found to behigher (Pucher-Petkovic and Marasovic, 1988): theannual primary production rising from about 159 toabout 200 g C m-2 y-1 in the Bay of Kastela and fromabout 60 to 90 g C m-2 y-1 in the eastern coastal area,which is under the influence of open waters.

Central and southern Adriatic open waters

The offshore waters of the central and southernAdriatic also show an east to west gradient in bothchl a and primary production values during winter(Faganeli et al., 1989). However, the coastal influ-ence extends further into the open sea along thewestern coast, especially in the area off Puglia.Generally chl a values are consistently below 0.5µg dm-3 and rates of primary production lower than1 µg C dm-3 h-1 (Table 2); however, data are tooscarce to allow generalizations. In April 1990 dis-tribution of phytoplankton biomass as carbon con-tent was described in a central area: integrated val-ues on a 0 - 50 m depth interval range between 3.7and 285.6 µg C m-2 with maxima in the northern-most coastal area. Vertical distribution shows thehighest values in the surface layer due to the preva-lence of dinoflagellates (namely Prorocentrumgenus) (unpublished data).

PHYTOPLANKTON COMPOSITION ANDHARMFUL BLOOMS

The main characteristics of phytoplankton popu-lations in the Adriatic Sea can be summarized as fol-lows (Fonda Umani et al., 1992):

1 - a nanoplankton fraction, that is numericallydominant throughout the whole Adriatic Sea andmore abundant in the northern part and along thecoasts.

2 - the most important group of microphyto-plankton is diatoms, which vary temporally and spa-tially with intermittent riverine inputs and generallyshow two maxima (spring and autumn) throughoutthe water column and are more evident in coastalwaters. Among them Nitzschia seriata, Skeletonemacostatum, Thalassiosira, Cyclotella spp. and smaller

species of the genus Chaetoceros prevail in thenorthern and coastal part, while in the central andsouthern area the number of species increases andthe biomass diminishes. The total number of diatomspecies in the southern area ranges up to 144 (84Centricae and 60 Pennate) (Vilicic, 1983).

3 - dinoflagellates are characteristic of the sum-mer period, but their number never reaches high val-ues, with the exception of “red tide” phenomena.

4 - coccolithophorides are associated with Ionianinput waters and are more abundant in the south,during the winter period when the meridional intru-sion is more important. They are also present in thenorthern part.

In the last 30 years two different trends of harm-ful blooms can be recognized. Since the beginningof the sixties and until 1987 the most important phe-nomena which affected water quality were “redtides”, mostly due to monospecific dinoflagellateblooms (Fonda Umani et al., 1992). In the last 8years large mucous aggregates related to exudatehyperproduction by diatoms invaded the wholenorthern basin on three occasions, causing enor-mous problems to tourism and fisheries. The “maresporco” phenomenon was recognized to be equal tothose which appeared in the past century and report-ed in the literature (Fonda Umani et al., 1989). Dur-ing their final phase both these phenomena can leadto an increase in oxygen demand at the bottom layeraggravating the widespread situations of hypoxiawhich, mostly in the northern basin, characterizedthe end of the stratified period (Stachowitsch, 1984;Aleffi et al., 1992).

Red tide phenomena are related to fresh waterinputs rich in nutrients, to fast heating of the sur-face layer at the beginning of summer, and graz-ing inefficiency (Sellner and Fonda Umani, in

ADRIATIC PELAGIC PRODUCTION 69

TABLE 4. – Minimum and maximum of chlorophyll a and primaryproduction (measured by 14C method) in the western and easternpart of near shore areas of Mid and South Adriatic Sea, mean val-ues of chlorophyll a and primary production in open waters of Mid

and Sotuh Adriatic Sea.

Near shore areas of the central and southern Adriaticwestern eastern

chlorophyll a 10 - 8 µg. dm-3 1 - 1.3 µg. dm-3

primary production <1 - 8 µg. C dm-3 h -1

Open waters

clorophyll a = < 0.5 µg. dm-3

primary production <1 µg. C dm-3 h -1

press). Some times the red tides affected thewhole Northern Adriatic, as in September - Octo-ber of 1984 (Artegiani et al., 1985), but theyappear mostly in confined areas, especially in anarrow coastal belt along Emilia Romagna (Table5), where the frontal system persists also duringsummer months (Franco and Michelato, 1992).Here, after early spring diatom increases, dinofla-gellate blooms generally commenced in March -April. The principal contributors were Noctilucamiliaris and Glenodinium lenticula, followed byearly summer blooms of Prorocentrum micans, P.scutellum, Scrippsiella trochoidea and G. foli-aceum (Sellner and Fonda Umani, in press). Insummer - fall, Gyrodinium corii (Viviani, 1983),Gonyaulax polyedra (Montanari and Rinaldi,1983) and Katodinium rotundatum prevailed(Boni, 1983; Boni et al., 1983). In most cases, theblooms were heterogeneously distributed alongthe Emilia Romagna coast, decreasing in intensi-ty from north to south.

Other regions of the basin and coastal bays aretypified by “red tides” (Table 6). The whole Gulf ofTrieste was infested by a bloom of Peridiniumovum in August 1973 (Bussani, 1974.) In June1977 a Noctiluca miliaris bloom was registered insome area of the northern Adriatic (Cassinari et al.,1979). A Noctiluca miliaris bloom appeared againin the summer of 1980 in several north Adriaticcoastal areas (Bianchi et al., 1982; Fonda Umani etal., 1983; Malej, 1983). In the seventies bloomsdue to Gonyaulax polyedra, Gymnodinium adriati-cus and Noctiluca miliaris were reported by Maret-ic et al. (1978) in the harbour of Pula. In Septem-ber 1977 a Gonyaulax polyedra bloom appeared inthe Gulf of Trieste (Fonda Umani, 1985). In June1981 a certain discoloration due to Exuviella mari-na was recorded in the harbour of Trieste. In Sep-tember 1982 a brown line due to Gonyaulax polye-dra was observed 2 miles off the sea shore in theGulf of Trieste. At the end of May 1983, in themost inshore areas of the Gulf of Trieste, dis-coloured waters due to Scrippsiella faeroense (tro-choidea) occurred (Fonda Umani and Honsell,1984). Coloured strips of 2 Km length due toScrippsiella trochoidea were registered along thecoasts of the Gulf of Trieste at the end of Septem-ber 1987 (Cabrini et al., 1990).

Dinoflagellate blooms in the western region ofthe northern Adriatic have not induced any toxici-ty in consumers, even if some of the bloom-form-ing taxa (e.g. Prorocentrum lima, Gonyaulax poly-edra) have been previously identified in toxin pro-duction. Recurring mass mortalities of bottomcommunities are mainly attributable to anoxiainduced by mineralization of phytoplankton pro-duction common to the stratified, shallow coastalarea (Orel et al., 1993). There are reports, howev-er, of toxin accumulation in Northern Adriatic

70 S. FONDA UMANI

Table 5. – Year, period, and species involved in “red tides” alongthe Emilia Romagna coast.

Year Month Species

1968 October/November P. depressum1969 May P. depressum1975 September C. karstenii 1976 July P. micans

from August to December G. corii1977 July G. polyedra, G. corii

August P. micansOctober G. corii

1978 March N. miliarisAugust/September G. polyedraSeptember/October G. corii

1979. June G. foliaceumAugust/September G. polyedraOctober/November G. corii

1980 March N. miliaris, K. rotundatum,G. lenticula

June P. micans1981 March G. lenticula

June K. rotundatumAugust/September/October Gymnodinium sp.October K. rotundatum

1982 April G. lenticulaJune P. scutellumJuly/August G. polyedraAugust G. tamarensisSeptember S. trochoidea

1983 September Gymnodinium sp.1984 August/September G. polyedra, Gymnodinium,

MassarthiaOctober/November Gymnodinium

1985 July/August G. polyedra1986 August S. trochoidea1987 October/November Gymnodinium1990 March/April N. miliaris

August G. polyedra, S. trochoidea,Cochlodinium

1991 July S. trochoidea

TABLE 6. – Year, period, species and area involved in “red tides” inthe Northern Adriatic Sea.

Year Month Species Area

1973 August P. ovum Gulf of Trieste 1977 June N. miliaris Gulf of Trieste1968 dinoflagellates Pula harbour 1978 September G. polyedra Gulf of Trieste 1980 June N. miliaris North Adriatic 1981 June E. marina Trieste harbour 1982 September G. polyedra Gulf of Trieste 1983 May S. faeroense Gulf of Trieste

September G.polyedra Gulf of Trieste1984 October Gymnodinium sp. North Adriatic 1987 September S. trochoidea Gulf of Trieste

waters. In June, 1989, diarrhetic shellfish poison-ing (DSP) was reported along the Emilia Romagnacoast due to the consumption of mussels and toxi-city was assigned to Dinophysis fortii and D. sac-culus (Boni et al., 1992). Abundances less than 200cells l. of species belonging to this genus can causetoxicity (Marcaillou Le Baut and Lassus, 1985).Very low densities of Dinophysis, and in particularD. acuminata, D. caudata, D. fortii, D. rotundata,D. sacculus, appear capable of inducing toxicity infiltering bivalves in the Gulf of Trieste (Della Log-gia et al., 1993).

“Mare sporco” phenomena were reported in thenorthern basin since 1729 with a regularity ofabout 10 - 20 years (Fonda Umani et al., 1989) butthey did not appear from 1930 to 1988. In the sum-mers of 1988, 1989 and 1991 they invaded thewhole northern Adriatic, generally appearing earli-er in eastern regions (Degobbis et al., 1995). Theyare due to hyperproduction of polysaccharidic exu-dates by a few species of diatoms (such as Cylin-drotheca closterium, Skeletonema costatum andChaetoceros spp.) which prevail in late spring inambient waters and inside the aggregates also insummer (Revelante and Gilmartin, 1991; Degobbiset al., 1995). It seems that a high N/P ratio in a sit-uation of nutrient depletion favours such kind ofhyperproduction (Lewin, 1955; Jones and Stewart,1969; Myklestad, 1977; Monti et al., 1995). Thiscondition is achieved in surface waters after freshwater inputs, particularly when the inputs follow apulsing rhythm. Degobbis et al. (1995) recognizedin a particular ten-year pattern of the Po river dis-charge one of the most important causes of thisphenomenon. In particular, during the thirties andin the period from the fifties up to the end of theseventies discharge maxima and minima werealternating each 1 - 2 years. In contrast, the twen-ties, forties and eighties were characterized by alonger period of decreased freshwater input (1921- 1924, 1942 - 1946, and 1979 - 1982 respectively)followed by a period of higher discharge (1926,1948 - 1949, and 1983 - 1986 respectively) withpulses of daily discharge rate exceeding 5000 m3/s(up to 9000 m3/s). Then a year of lower and anoth-er (1928, 1951, 1988) of higher dischargeoccurred, but more distributed during spring, withlower pulses (daily discharge rates up to 5000m3/s). This peculiar discharge pattern can selectspecies composition and modify relationships infood webs, resulting in a high rate of exudate pro-duction and significant accumulation.

DISTRIBUTION OF ZOOPLANKTONBIOMASS AND PRODUCTION

The standing stock of zooplankton in the Adriat-ic Sea has generally been measured using verticalhauls from above bottom in the shallow northernAdriatic and near shore areas and from 50 m to sur-face in other regions. Little is known about the zoo-plankton biomass of the deep southern Adriaticbasin. Benovic et al. (1984) indicated that zones ofgenerally high standing crops were located in thenorthern Adriatic including the Gulf of Trieste, withaverage dry weight of over 14 mg m-3 and an ash-free dry weight (AFDW) of about 12 mg m- 3. Theestimates of zooplankton biomass in near shorewaters along the eastern coast, with the exception ofsome bays like Kastela (Vucetic, 1977), gave meanvalues below 10 mg m-3 dry weight and below 8 mgm-3 AFDW. Significantly higher values were record-ed along the western coast (Fig. 3) (Fonda Umani etal., 1982), except during lower river outflows, whenmaximal values were located in the eastern part ofthe basin (Fonda Umani et al., 1994). Generallyhigher values are linked with areas characterized byincreased phytoplankton biomass and production(areas influenced by the River Po plume and the SWregion approaching the Straits of Otranto). Duringwinter Faganeli et al. (1989) found a high zooplank-ton standing stock in the region above the Jabukatrench. In April of 1990 high values of biomass (≈12 mg m-3 AFDW) were observed in the SouthAdriatic trench (unpublished data). During the samecruise particularly high values of biomass, calculat-ed on m-2 basis were observed in the south trenchand in mid Adriatic, ranging from 4.8 mg m-2, in thesouthernmost station, to 1029 mg m-2 in a centralstation near the Jabuka trench (approximately 0.096mg m-3 and 7.36 mg m-3 respectively).

Very few data are available for microzooplanktonbiomass in the whole Adriatic Sea. The temporal pat-tern of microzooplankton biomass as Carbon contentwas described during 1986/87 solely in the Gulf ofTrieste. It ranged from 0.54 to 7.35 mg C m -3, andwas composed mainly of ciliates other than tintinnidsin spring, nauplii in summer and tintinnids in June.Microzooplankton biomass was of the same order ofmagnitude as netzooplankton and two orders lessthan that of primary producer biomass (Figs. 4 and 5)(Cataletto et al., 1993).

Generally the biomass of both fractions (micro-and net- zooplankton) decreases from the northernto southern part, and from coastal to open waters.

ADRIATIC PELAGIC PRODUCTION 71

Data on zooplankton production in the AdriaticSea are scarce, and are mostly estimated on the basisof primary production and fish. Pucher-Petkovic etal.(1971) and Karlovac et al. (1974) estimated zoo-plankton dry weight production in the whole Adriat-ic to be about 1.9x106 tons annually. An estimate ofdaily C production using the method of LeBorgne(1982) varied between 0.6 to 3.0 mg C m-2 d-1, giv-ing an annual secondary production < 1 g C m-2 y-1

in the Gulf of Trieste (Malej, 1984).

ZOOPLANKTON COMMUNITIES.

On the basis of available data, the characteristicdistribution of zooplankton was described by Hureet al. (1980), Specchi and Fonda Umani (1987),Ghirardelli et al. (1989) and Fonda Umani et al.(1992):

1 - the number of species belonging to the meso-zooplankton increases along the north-south gradi-ent (e.g. from 30 species of copepods encounteredregularly in the Gulf of Trieste (Malej, 1979; Spec-

chi and Fonda Umani, 1983) to more than 130 in thesouthern part (Hure et al., 1980); 3 species ofchaetognatha in the northern part and 9 - 10 in thesouthern part (Gamulin, 1979; Gamulin and Ghi-rardelli, 1983), 4 species of calicophora in the northand 22 in the south (Gamulin, 1979), 9 species ofappendicularian in the north and 27 in the south(Skaramuca, 1983) etc., with cladocerans an excep-tion, being better represented in the northern area(Specchi, 1970).

2 - it is possible to distinguish three communitiesof copepod fauna (Hure et al., 1980) which charac-terize the different areas of the Adriatic Sea i.e. estu-arine, coastal and oceanic.

3 - concerning mesozooplankton as a whole wecan identify four characteristic associations (FondaUmani et al., 1994) (Fig. 6):

a) a northern coastal association characterizedby strictly neritic species, low diversity, clearprevalence in summer of Penilia avirostris andduring the rest of the year of Acartia clausi whichcan spread to the southern areas inside the 20 mbathymetry line

72 S. FONDA UMANI

Fig. 3. – Geographical distribution of netzooplankton biomass (as AFDW) in the Adriatic Sea in December 1979/ January 1980 (from Fonda Umani et al., 1994).

b) a neritic central one which includes thewhole central area south of the Ravenna - Lussinotransept; it is still characterized by neritic speciesand by an increase in the percentage of Para-calanus parvus.

c) an offshore central association which caninclude in some seasons all the southern area as faras the South trench, which is characterized by a highdiversity, the presence of P. parvus and the increaseof “oceanic” species (Hure et al., 1980)

ADRIATIC PELAGIC PRODUCTION 73

Fig. 4. – Carbon content annual pattern of the three main micro-zooplankton fractions (tintinnnids, other ciliates, nauplia) inthe Gulf of Trieste during the period 1986/1987 (from

Cataletto et al., 1993).

Fig. 5. – Carbon content annual pattern of the three main netzoo-plankton trophic groups (herbivorous, filter feeder, mixed feeder,carnivorous) in the Gulf of Trieste during the period 1986/1987

(from Cataletto et al., 1993).

Fig. 6. – Geographical distribution of the four netzooplankton associations (see text) identi-fied by cluster analysis in September 1986 (from Fonda Umani et al., 1994). • coastal nerit-ic association; ∆ northern offshore association; * central offshore association, s southern

oceanic association.

d) a southern oceanic association, present only inautumn - winter, confined to the South trench, charac-terized by a high faunistic paucity, probably due to thedepth of sampling (from 50 m to surface) which doesnot include any deeper community.

CONCLUSIONS

Regarding available plankton data, three differ-ent areas may be delineated in the Adriatic Sea:

a - open waters of the central and southern Adri-atic, characterized by an “oceanic” community.The microphytoplankton and especially diatomsare quantitatively important. The microzooplank-ton is dominated by tintinnids, as also observed inthe northern Thyrrenian Sea (Fonda Umani andMonti, 1993). Herbivorous copepods are dominantin mesozooplankton community throughout theyear, while carnivorous species are also better rep-resented. The community of this area shows highdiversity and greater stability. A “top down” con-trol (Herndl, 1992) appeared to regulate the ener-getic fluxes.

b - open waters of the northern basin, character-ized by neritic associations with moderate biomass.The species originating from the southern Adriaticand the Mediterranean are present throughout theyear. A marked west - to - east gradient in the pro-duction and biomass was present in the stratifiedperiod (Smodlaka and Revelante, 1983). The mod-els of phytoplankton control seem to shift from “bot-tom up” to “top down” control (Herndl, 1992) fol-lowing the western - eastern gradient and changeseasonally.

c - a coastal zone characterized by a neritic com-munity of low diversity and high biomass. Duringwinter the eastern coast is influenced by inputs orig-inating from the southern Adriatic. In this areananoplankton primary production prevails. On thebasis of field observations (Cabrini et al., 1989,Cataletto et al., 1993; Turk et al., 1990) productionrelated to nanoplankton appeared to be used by cili-ates other than tintinnids during the period of strati-fication. Moreover, nanoplankton represents animportant food source for the abundant filter feed-ers. During the persistence of the frontal system ter-rigenous inputs stimulate high primary productionwhich is confined to inshore areas. Zooplanktongrazing is not sufficient to control the biomass pro-duced and transfer to offshore areas is restricted, thatresulting in eutrophication.

A characteristic of the whole basin and especial-ly of its northern part, is the high year to year vari-ability of oceanographic properties and plankton.Generally it is impossible to identify any successionbased on the behaviour of a single species as usual-ly described in the Mediterranean areas (Margalef,1958). In the Gulf of Trieste it was possible to iden-tify ecological associations (sensu Estrada et al.,1985) for phytoplankton and microzooplanktonusing cluster analysis on a long time data set (1986- 1991). The spectral analysis of time series appliedto the groups allowed the identification of three phy-toplankton groups with a clear periodicity (in spring:Skeletonema - Thalassiosira; in summer: Leptocilin-drus - Nitzschia - Rhizosolenia; in winter Dinoph-ysis - Dictyocha) and two for microzooplanktongroups (in winter: Stenosemella - Steenstrupiellaand in summer: Helicostomella - Favella (FondaUmani et al., 1995). The same kind of analysisapplied to a netzooplankton data set (1970 - 1980)highlighted the periodicity of two ecological associ-ations (late spring - summer characterized by Acar-tia clausi and Temora longicornis and autumn - win-ter characterized by Temora stylifera and Oncaea sp.(Cataletto et al., 1995).

Furthermore long term changes in the planktoncommunity have been observed: heterotrophicdinoflagellate Noctiluca miliaris, scarce before1970, has become increasingly abundant in the northAdriatic plankton, giving rise to discoloured waters.The jellyfish Pelagia noctiluca, not recorded in thenorthern Adriatic until 1976, appeared in massesbetween 1976 and 1985 (Rottini Sandrini, 1982;Rottini Sandrini et al., 1980, 1981; Malej, 1989).These two latter events seemed to have negativelyinfluenced the net zooplankton community whichshowed a marked decrease of biomass around themiddle of the seventies (Cataletto et al., 1995). Thisdecrease, jointly with an increase of nutrient con-centration (Provini et al., 1992) influenced phyto-plankton biomass which reached in those years thehighest values (Fonda Umani, 1991)

In the following period (1984 - 1995) a cleardecrease of chlorophyll concentration was observedin the Gulf of Trieste (Malej and Fonda Umani, inpress). No decrease of nutrient concentration wasregistered in the same years, but a sharp increase ofnetzooplankton biomass. This fact supports thehypothesis that in the last years, at least in the north-ernmost part of the Adriatic, phytoplankton biomasshas been more efficiently controlled by zooplanktongrazing. Furthermore, the observed increase of net-

74 S. FONDA UMANI

zooplankton biomass is more significant duringspringtime, influencing mainly the diatom springbloom, which is the most important source of organ-ic material in this environment.

Blooms of jellyfish affected as well the anchovypopulation, which decreased in those years but notso sharply as from 1986 (Regner, this volume). Theregistered increase of netzooplankton during the lastdecade could be due to the scarcity of upper con-sumers such as anchovy, which seem to be nega-tively affected by macroaggregate appearances.

ACKNOWLEDGEMENTS

Thanks are due to Professor Nival and to ananonymous reviewer for their helpful suggestions.

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