temporal succession of phytoplankton in the northern adriatic

Netherlands oTournal of Sea Research I0 (3) : 377-396 (1976)

T E M P O R A L S U C C E S S I O N OF P H Y T O P L A N K T O N IN T H E N O R T H E R N A D R I A T I C

by

NOELIA REVELANTE* (Center for Marine Research, Institute "Rudjer Bo§koviF', Rovinj, Yugoslavia)

and

MALVERN G I L M A R T I N (Australian Institute of Marine Science, TownsviUe, Queensland, Australia)

CONTENTS

I. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . 377 II. Materials and Methods . . . . . . . . . . . . . . . . . . . . . . 378

III. Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . 380 1. Micro- and nannoplankton population densities . . . . . . . . . . 380 2. Microplankton community structure and species succession . . . . . 382 3. Nannoplankton community structure . . . . . . . . . . . . . . 386

IV. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . 387 V. Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . 393

VI. References . . . . . . . . . . . . . . . . . . . . . . . . . . . 394

I. I N T R O D U C T I O N

The first informat ion on the microplankton of the Nor thern Adriat ic was presented by HAUCK (1872) and CASTRACANE (1873), who reported on the li t toral d ia tom flora of the region. These studies were followed by those of STEUER (1902) in the Gu l f of Trieste, and S CHROEDER ( 1911 ) and SCHILLER (1914) Off the west coast of Istria. SCHILLER (1914), analyzing samples collected by the R V Najade (1911-1913), was the first to note an east to west increase in cell density. This gradient was confirmed by ISSEL (1921, 1922, 1925), using a volumetr ic technique. FORTI • ISSEL (1923, 1924) and VATOVA (1928) described the seasonal cycle of the phytop lankton off Rovinj, noted rare forms and described several new species. KAMPTNER (1939, 1940) investigated coccolitho- phorides from the southeast coast of Istria, while ZANON (1940, 1941) produced a d ia tom species list for the vicinity of Rovinj. All of these studies were basically taxonomic in na ture and provided only general

* Present address: Australian Institute of Marine Science, P.O. Box 1104, Townsville, Queensland 4810, Australia.

378 N. REVELANTE & M. GILMARTIN

information on the quantitative aspects of the microplankton. The first semi-quantitative studies of the microplankton were con-

ducted by MOLLER (1938) at a nearshore station off Rovinj, followed by a few cruises (BATTAGLIA, MozzI & VARAONOLO, 1959, 1960, 1961) in the Northern Adriatic with limited attention given to the phytoplankton. ~TIRN (1968) subsequently identified the importance of Nitzschia seriata in the microplankton of the Northern Adriatic, and presented data (~TIRN, 1969) on cell densities and dominant genera during seasonal maxima.

The first truly quantitative data on microplankton cell densities, based on the Uterm6hl technique, were gathered during four seasonal cruises conducted in the Northern Adriatic (VoLTOLINA, 1969, 1970, 1971 a, 1971 b), and a study of the seasonal cycle in the Gulf of Venice (VoLTOLINA, 1971c). Concurrently, LOVASEN (1969) and REVELANTE & KVEDER ( 19 71 ) produced preliminary accounts of the seasonal cycles, microplankton cell densities, and dominant genera off Rovinj.

In virtually all of these studies, constraints imposed by sampling techniques (e.g. net samples only), cruise schedules, or research objectives, restricted the reported data to floristic lists of the more common microplankton species. Not only is almost no information available on the nannoplankton of the Adriatic, which can represent 80 to 99% of the biomass in other regions (MALONE, 197ta, 1971b), but only limited information is available on microplankton species succession, often indicative of major seasonal changes in the environment.

Therefore an 18 month sample set collected along a transect ex- tending across the Northern Adriatic was analyzed to describe seasonal cycles and microplankton species succession, and to consider the relative response of the nanno- and microplankton components of the phytoplankton community to major changes in environmental conditions.

Acknowledgements.--This study was supported by the U.S. National Science Foundation (GA 31947X), and the U.S.-Yugoslav Joint Board for Scientific and Technical Cooperation (P2F069). The authors gratefully acknowledge the assistance of Ms Anica Cerin with phytoplankton enumeration, and the wholehearted support of the crew of the RV "Vila Velebita".

II. MATERIALS AND METHODS

The study transect extended from Rovinj, on the Istrian Peninsula in Yugoslavia, to the region off the Po delta of Italy. The 6 stations (Fig. 1) were selected to reflect gradients in basic oceanographic properties.

P H Y T O P L A N K T O N A D R I A T I C 379

Sampling was conducted on an approximately bi-weekly schedule from June 1972 to December 1973. Details on the general oceanographic programme and analytical techniques are presented elsewhere (GILMARTIN ¢t al., 1972, 1973).

?'° .8 I~,,.*

Fig. 1. The Northern Adriatic and position of stations: 6 (45002'45" N, 13022'30" E) A15 (45002"25 " N, 13016'25" E), 7 (45001'30" N, 13°09'15" E), 8 (45000'30 " N 12°59'15" E), 9 (44059'30" N, 12o49'00" E) and 10 (44o46'30" N, 12041'30 " E)

Samples for taxonomic analysis and phytoplankton cell density determinations were collected with Van Dorn bottles from the surface and 30 m depth (near bottom), and preserved with Lugol's solution made basic with sodium acetate. Cell densities were estimated by the Uterm6hl settling method (UTERMOHL, 1958). Fifty or 100 ml of the sample was settled in Opton R sedimentation chambers for approximately 72 hours. The entire bottom area was usually counted for microplankton (larger than 20 #m), and 100 random ocular grids for nannoplankton (smaller than 20 #m). Microplankton was examined at 100 to 160 ×, and nannoplankton at 400 ×. Phytoplankton densities were expressed as cells per litre. Similarity indexes were calculated according to SORENSEN (1948).

Approximately 95% of the number of microplankton species en-

380 N. R E V E L A N T E & M. G I L M A R T I N

countered during the 18 months are reported herewith, with the rarer species (ca 50/o) reported elsewhere (REvELANTE, 1975). Due to difficulties in identifying the smaller nannoplankton even to the generic level with the Uterm6hl technique, these forms are grouped as: diatoms, dinoflagellates, microflagellates, and "others". All naked nannoplanktonic organisms with visible flagella (except dinoflagellates) were classified as microflagellates, while those lacking visible flagella or whose taxonomic identification was doubtful were designated "others".

Estimates of chlorophyll a standing crops were based on two 0.5 to 1.0 1 sub-samples. The first was filtered directly through a Whatman GF/C glass filter. The second was fractionated by passing the sea water through a 20 #m net disc (microplankton), and then through the glass filter (nannoplankton). The microplankton chlorophyll a concentrations were calculated as the difference between the unt)action- ated and fractionated values. Chlorophyll a concentrations were measured on a Turner Model 111 fluorometer, standardized against a Beckman DU spectrophotometer (STRICKLAND &PARSONS, 1968).

III. RESULTS

1. MICRO- AND NANNOPLANKTON POPULATION DENSITIES

During the period of study, nannoplankton dominated the phytoplankton community, both in terms of cell densities and biomass as reflected by the chlorophyll a standing crop (Table I). The average nannoplankton contribution ranged from 74 to 88%, with a slight decrease westward along the transect.

The average microplankton and nannoplankton cell densities increased four to six fold westward along the transect, reaching maximum

T A B L E I

Regional variations (with range in parentheses) in the micro- and nannoplankton cell density, and the relative contribution to total community biomass (measured as

chlorophyll a).

Station Cell density Nannoplankton/total Number ( × 10 5 1-1) (%) of

- samples Micro- Nanno- Cell Chloro-

plankton plankton density phyll a

6 1~6(0.04-20.3) 9.6i0107~-- 23.8 ) 88 76 29 - 7 2.6(0.06-20.1) 12.4(0.07- 44.4) 83 68 28 8 3.3(0.01-34.5) 10.7(0.07- 32.1) 82 69 28 9 4.3(0.06-32.7) 24.6(0.08-107) 82 62 28

10 9.5(0.01-46.6) 41.4(0.6 -284) 74 64 12

PHYTOPLANKTON ADRIATIC 381

concentrations of 47 and 284 × 105 ceils per litre, respectively, at station 10.

The westward increase in the range of cell densities, in both microplankton and nannoplankton, resulted primarily from a bimodality of cell frequency distributions at the western stations. At station 10, a marked bimodal distribution of densities showed nannoplankton maxima in the 32 to 100 and 320 to 1000 × 104 classes, with micro-

MICROPLANKTON 4060 t 2 0 1 - 3 1 1 0 : 1 1 ~ 3 - 1 0 I 10-32 I//.f_~32-100 \1100-3201320"100011000-32001-- t '~, STATION3200+10

STATION 9 40 ~2o

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NANNOPLANKTON ~ Fig. 2. Surface cell density frequency distribution of micro- and narmoplankton at

stations A15, 7, 9, and 10.

plankton peaking in the 10 to 32 and 100 to 320 × 108 classes (Fig. 2). Stations 6 and 9 can be considered representative of the eastern and

western portions of the transect. The seasonal cycle of surface and bottom cell densities at these stations is presented in Fig. 3.

The microplankton surface populations show three marked seasonal maxima in spring, mid-summer, and autumn, with the maxima being most pronounced at western stations. In contrast, microplankton bottom communities showed only spring and summer maxima, which usually followed surface maxima. No autumn bottom maximum was observed. Surface cell densities usually exceeded bottom population densities.

At western stations, the nannoplankton reached a pronounced surface maximum in late spring, between the early spring and early summer microplankton maxima. No comparable maximum was ob-

3 8 2 N. R E V E L A N T E & M. G I L M A R T I N

served at depth. Nannoplankton surface populations at eastern stadons did not differ markedly from bottom populations at the same stations, and in general the amplitude of seasonal changes in bottom populations was the same along the transect. The seasonal changes in both micro- and nannoplankton cell densities were most pronounced in surface populations.

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2. M I C R O P L A N K T O N C O M M U N I T Y S T R U C T U R E A N D S P E C I E S

S U C C E S S I O N

The seasonal succession of microplankton species at station 6 (Table II) and station 9 (Table III) are presented as representative of the eastern end of the transect and of the region close to the Po delta at the western end of the transect. For convenience and clarity, only those

P H Y T O P L A N K T O N A D R I A T I C 3 8 3

species representing at least 5 % of those noted in the community are presented and discussed.

Throughout the period of study, the microplankton community was dominated by the diatom component, except during June and July when dinoflagellates were prominent. Silicoflagellates and coccolitho- phores appeared in the late autumn and early winter, and represented less than 2 % of the total population.

In about October, a 4 to 5 month "winter" period commenced, during which a relatively rapid succession of dominants occurred, each tending to dominate the communities for only 3 to 4 weeks, being relatively unimportant throughout the rest of the year. Usually from 6 to 9 species formed a clearly discernable successional series, with the number of species in the series decreasing westward along the transect.

In contrast, the "late winter" period, lasting from about January- February to April, was dominated by only 2 to 3 species, with co- dominants sporadically appearing and re-appearing. The "spring" period, lasting from about May through June-July, was marked by the almost simultaneous appearance and persistence of from 4 to 7 species ofdinoflagellates, and the "summer" period, starting in June-July, was characterized by the dominance of a single diatom species, Nitzschia seriata. This species, often co-occurring with several dinoflagellate species, usually persisted through early October when the "winter" succession of species again commenced.

During the "winter" period, 4 diatom species (Nitzschia longissima, Rhizosolenia stolterfothii, Leptocylindrus danicus, and Lauderia borealis) dominated at all stations along the transect, tending to appear in that order throughout the period. Rhizosolenia stolterfothii was relatively more important along the eastern portion of the transect, and Leptocylindrus danicus and Lauderia borealis on the western portion. In the latter region, Chaetoceros curvisetus appeared as an important co-dominant late in the period. Only one other species, Thalassionema nitzschioides, was common to all stations, appearing as a minor co-dominant in November- December.

During the entire "late winter" period, Chaetoceros sp. dominated the communities along the eastern portion of the transect, and was gradually replaced by Skeletonema costatum westward. The latter species became exclusively dominant at stations 9 and 10, off the Po delta, during the period. Chaetoceros curvisetus existed as an important co- dominant at the beginning of the period in the eastern and middle portions of the transect.

The most characteristic "spring" species was Prorocentrum micans, which increased in relative importance westward along the transect. At the eastern stations, the dinoflgellate Geratiumfurca, and such dia-

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toms as Chaetoceros compressus, Rhizosolenia alata f gracillima, and Hemiaulus hauckii were common co-dominants, while to the west only the dinoflagellate Peridinium steinii was an important co-dominant.

Nitzschia seriata was the dominant species at most stations throughout the 1972 and 1973 "summer" periods. However, co-dominants dif- fered between the two years. In 1972, Chaetoceros diversus and Chaetoce- ros curvisetus tended to co-dominate on the eastern portions of the transect, and Skeletonema costatum on the western portion. In 1973, Leptocylindrus danicus was the single most important co-dominant along the entire transect.

3. N A N N O P L A N K T O N C O M M U N I T Y S T R U C T U R E

The relative proportions of the major components of the nannoplankton community did not differ markedly along the transect, and the distribution for station 9 (Fig. 4) is representative.

Microflagellates were the single most important component of the community, ranging from a low of 38% at the start of the "winter" period in 1972 to complete dominance during the "summer" periods of both 1972 and 1973.

100-

75-

50-

25-

O- 5oJ

25

0

Fig. 4. Seasonal

1972 1973 J I J I A I S I O I N I DI J I F I M I A I M I J I J I A I S I O I NJ~J MICROFLAGELLATES

DIATOMS

DINOFLAGELLATES

0

changes in the surface nannoplankton community structure at station 9,


Diatoms, mainly small naviculoids, made significant contributions to the community in September-October 1972, January-February 1973, and December 1973, when they represented 20 to 35% of the community.

The dinoflagellates exhibited two marked maxima during the period of study. The largest, in May to July, occurred during the "spring" period in 1973. The smaller bracketed the end of the "summer" period and the start of the "winter" period in 1972.

The last nannoplankton community component, "others", was relatively insignificant during "spring" and "summer" periods, and reached maxima during the "winter" periods of 1972 and 1973.

IV. DISCUSSION

The oceanographic regime of the Northern Adriatic is dominated by 3 basic processes: (1) a seasonal cycle of water column stability, characterized by complete mixing in the early winter of each year, followed by the development of a highly stratified two layer system which lasts from late spring to the following period of winter mixing; (2) an east to west horizontal gradient in nutrient concentrations and salinity, created by the discharge of the Po river (D•GoBBIS, 1974; ~KRIVANId, 1971); and (3) an annual Po river discharge cycle characterized by spring and autumn maxima (ANON., 1972, 1973). This basic regime is modified by a general current pattern characterized by a northwest inflow from the central Adriatic along the eastern side, and a southeast outflow along the western side of the basin (ZORE-ARMANDA, 1956). The biological communities, especially the phytoplankton, respond to these conditions; and the resulting temporal and spatial variations in community density and structure can be used as indicators of these conditions.

Changes in the physical properties of the water column, such as density (Fig. 5) and stability (Table IV) are reflected in the microplankton community structure; therefore changes in community structure can be used as indicators of changes in the physical environment. Increased similarity between the surface and bottom communities (Table IV), and an increase in the proportion of the diatom fraction (Table V), marked the change from stratified to mixed conditions. In all likelihood, increased vertical turbulance not only intro- duced diatoms from bottom communities into the water column, but also functioned to maintain the denser passive fraction of the community (e.g. diatoms) in suspension. In particular, such tychopelagic diatom species as Melosira sulcata, Diploneis bombus, Coscinodiscus excen- tricus~ C. radiatus, C. nodulifer, Pleurosigma normanii, and P. angulatum, and

0.

5 -

A 1 0 -

i 15-

Q 2 0 -

25-

3C

0

5-

"~ 10-

i 15-

a 2 0 -

25 -

3 0 -

3 5 - -


1972 J I A I 'S I 0 N ,l ° 1973

F I M A M I J I J I A f 5 l O N I D

. . . . . . \~'~k,./ • ' "

Fig. 5. Vertical distribution of density at stations 6 and 9.

the silicoflagellate Dictyocha fibula, are good indicators of the onset of this period.

The tendency for diatoms to represent a larger fraction of the microplankton bottom communities during the stratified period (Table V) may reflect their higher sinking rates relative to the dinoflagellates. In particular the sinking of chain-forming diatom species, dominating during the spring and summer blooms, probably resulted in the post- bloom "bottom" maxima observed at western stations (Fig. 3).

' F A B L E I V

Similarity indices between surface (0 m) and bottom (30 m) microplankton communities, and water column stability (0 to 30 m).

Date Similarity index Stability ( :lat ) (1973) at stations: at stations :

6 9 10 6 0 10

3 Ju ly 54 38 27 4.8 3.3 5.8 19 Ju ly 40 47 34 5.4 4.6 5.5 2 Aug. 52 56 5.5 4.8

21 Aug. 48 51 22 5.2 5.b~ 3.2 11 Oct. 57 53 0.9 1.7 31 Oct. 67 90 75 0.9 0.9 0.3 16 Nov. 82 86 80 0.05 0.1 0.6 6 Dec. 80 83 82 0.2 0. ! 0.6


T A B L E V

The number of diatom, dinoflagellate and silicoflagellate species in surface (0 m) and bottom (30 m) communities, and w~ter column stability (0 to 30 m) at station 9.

Date Number of species Stability (1973) (Aat)

Surface Bottom

Total Diat. Dinof, Silic. Total Diat. Dinof. Silie.

3 July 24 10 14 0 39 27 11 1 3.3 19July 17 9 8 0 40 27 12 1 4.6 2 Aug. 35 17 18 0 46 30 14 1 4.8

21 Aug. 37 19 18 0 27 24 2 1 5.8 11 Oct. 31 23 5 2 29 22 6 1 1.7 31 Oct. 33 24 9 1 30 28 1 1 0.9 16 Nov. 26 20 4 2 33 24 7 2 0.1 6 Dec. 13 9 3 1 15 14 0 1 0.1

While vertical mixing primarily influenced changes in phytoplankton community structure at a given station, the Po river discharge was responsible for a marked east to west gradient in the micro- and nannoplankton densities, chlorophyll a (as a measure for standing crop), and different species successional series.

The two-fold surface nannoplankton density gradient westward along the transect, compared with a six-fold microplankton density gradient, suggests that Po river nutrient input is the major factor in- fluencing the increased relative importance of the microplankton. The relative increase in microplankton westward is also reflected in primary production rates (REVELANTE & GILMARTIN, 1976).

The marked bimodal distribution of phytoplankton population densities at western stations (Fig. 2) probably results from the more intense seasonal changes in the environmental conditions of the region, in particular marked fluctuations in the rate of nutrient input which result in population "explosions" of microplanktonic diatoms. In addition, these high growth rates may uncouple the phytoplankton from grazing populations and allow the development of very dense blooms. In contrast, eastern stations may represent a quasi-steadystate, with grazing rates more closely coupled to, and ultimately restruc- turing the size of, the phytoplankton crop.

During both the stratified and mixed periods, the most pronounced seasonal and regional changes in abundance of both size classes occurred in the surface layer. During the stratified period these changes reflect variations in the rate of Po river nutrient input, and distance from the source of input, with the nutrients tending to remain in the less dense surface waters (GILMARTIN et al., 1972, 1973). During the

390 N . R E V E L A N T E & M . G I L M A R T I N

mixed period, with nutrient concentrations relatively constant throughout the water column, low winter incident radiation, and increased turbidity due to sediment lc~ading, tend to restrict the euphotic zone to the upper portion of the water column.

Species succession in the Northern Adriatic is assumed to start during the late autumn, probably triggered by intense vertical mixing. The 4 periods of"winter" , "late winter", "spring" and "summer" may be regarded as 4 stages in the annual succession, similar to the succession observed by MARGALEF (1958) in western Mediterranean coastal waters. Vertical mixing appears to induce a succession of diatom species which lasts until the end of the spring. High nutrient concentrations, resulting from a combination of vertical mixing and increased Po river discharge, probably select for species with high division rates and K8 values conducive to the utilization of higher nutrient concentrations. The effect of temperature changes on the development of species succession is difficult to evaluate since low temperatures were always associated with high nutrient concentrations (Figs 6 and 7). In general, several dominants are characteristic of this period: Rhizosolenia stolterfothii, Leptocylindrus danicus, Nitzschia longissima, Thalassionerna nitzschioides, Chaetoceros curvisetus, and Melosira sulcata.

The "late winter" period may be considered as the second stage in

1972 1973

i I I _ _ _ _ [ _ _ _ _ _ L _ _ 4 . . . . . . . . . 2 _ _ . _ £ . . . . . ~ . . . . . . ~ . • 1 _ _ _ ~ . . . . . . . L 2BI J A S 0 ~ N l O J F M A M J s A S 0 N D

24t 0 METRE i ~ - - I

0 . . . . . . . . . . . . . . . . STATION g

282

24,

~ZO"

16.

w m 12. IL

v- 8 .

STATION 6

Fig. 6. Temperature at the surface (0 m) and bottom (30 m) at stations 6 and 9.

PHYTOPLANKTON ADRIATIC 391

the succession, developing in association with stabil ization of the water co lumn and relatively high rates of Po river discharge. Charac- teristic species of this stage were: Skeletonema costatum, Chaetoceros sp., Chaetoceros affnis, and Bacteriastrum delicatulum. Reduced vertical mixing and the delimitation of Po river nutrients to the western side of the Adriatic confined the dominance of Skeletonema costatum to stations in that region.

1972 19'/3 i J i A , S , O i N ~ D ] J e F i M i A i M i J i J i A l $ , O i N e D I

>20 9-2 STAT]ON 9 6- 0 METRE

5,

,z 3 . f

I

0 0.20~

~ 0"0] ~0""1

f~o.] 0,%t,]

0

z

~o~

STATION 6 0 METRE

~0 i i i i i i I ' i i i , , i r r J i

Fig. 7. Surface phosphate, nitrate and silicate concentrations at stations 6 and ~.


The "spring" period is the third stage in the succession. During this period increased stability and the start of nutrient depletion in the surface layer are associated with the appearance of dinoflagellate species. Characteristic species were: Prorocentrum micans, Ceratium furca, and Peridinium steinii. The series ended with the "summer" period and the appearance of such diatom species as Nitzschia seriata, Chaeto- ceros diversus and Leptocylindrus danicus, whose growth might have been stimulated by the autumn Po river discharge maximum.

The seasonal succession of microplankton species indicated regional differences in the dominant species. The western stations, characterized by higher nutrient concentrations (Fig. 7), support a vigorous growth of such diatom species as Skeletonema costatum and .Vitzschia seriata, which usually exclusively dominated the blooms. The importance of Nitzschia seriata in the Northern Adriatic phytoplankton was previously reported by MULLER (1938), ~TIRN (1968), and VOLTOLINA (1970, 1971a, 1971b, 1971c), with the highest concentrations noted at western stations. The occasional unusual increase in the abundance of these species in the surface layer of the eastern Adriatic (VOLTOLINA, 1971b; REVELANTE • GILMARTIN, 1976) is associated with a spring- summer cyclonic circulation pattern which transports water condi- tioned by Po river discharge to the eastern side of the Northern Adria- tic. The dominance of these species at the western end of the transect may be related to their higher K., values. In this connection, it has been observed that Skeletonema costatum is the dominant species in the eutrophic Venetian lagoon (VoLTOLINA, 1973a, 1973b, 1973c), dominated the bloom after the Nile flood, comparcd with its low occurrence before the food (HALIM, 1960), and dominates under bloom conditions in the western Mediterranean (JACQUES, 1969), in the Black Sea (FINENKO ~% KRUPATKINA-AKININA, 1974), Narragansett Bay (MARTIN, 1966), and in the California Current (REVELANTE, 1975).

Higher nutrient concentrations or "bloom" conditions at eastern stations appear to favour the dominance of diatom species less nu- merous at the western side, such as Rhizosolenia stolterfothii, Chaetoceros curvisetus, Chaetoceros diversus, and Chaetoceros sp. In addition, under conditions of low nutrient concentrations associated with the stratified period, eastern stations were also characterized by a relatively high proportion of such warm-water diatom species as Chaetoceros compressus, Rhizosolenia alata f. gracillima, Hemiaulus hauckii, and Chaetoceros anasto- mosans, characteristic of the middle Adriatic (PUCHER-PETKOVIC, 1966; SOLAZZI & ANDREOLI, 1970), as well as a high percentage of such dinoflagellate species as Ceratiumfurca and Ceratiumfusus.

Nannoplankton succession can only be evaluated as "group suc-


cession". The high frequency of diatoms and "others" in the nannoplankton fraction during the period of vertical mixing, compared with the exclusive dominance of flagellated forms under stratified conditions, indicate that nannoplanktonic diatoms (like microplanktonic diatoms) respond more quickly to increased nutrient concentrations. The increased abundance of "others" during the mixed period suggests that vertical turbulence introduces a high percentage of cysts, resting spores and other resting stages into the euphotic zone.

The foregoing discussion illustrates the importance that changes in the vertical structure of the water column have on basic characteristics of the phytoplankton community, and emphasizes the large effect that a Po river imposed oceanographic gradient has on regional distribu- tional patterns and species succession in the Northern Adriatic.

V. S U M M A R Y

The seasonal succession of marine phytoplankton was studied along a transect across the Northern Adriatic from the Istrian peninsula, Yugoslavia, to the Po delta, Italy. The phytoplankton communities were dominated by the nannoplankton (74 to 88%), with seasonal blooms resulting primarily from increases in the microplankton component.

Phytoplankton communities exhibited a 4 to 6 X gradient in cell densities along the transect, associated with oceanographic conditions created by Po river discharge. Ranges in cell densities increased westward, and a marked bimodality of cell density distribution occurred at those stations close to the Po delta.

The annual cycle was composed of "winter", "late winter", "spring" and "summer" periods, starting with a clearly defined microplankton diatom species succession in the "winter" and ending with dinoflagellate dominated communities during the "summer". The nannoplankton group succession was similar along the transect, being dominated by microflagellates. Maxima of the diatom and dinoflagellate components coincided with maxima in the same components in the microplankton.

Regional differences in microplankton successional series occurred with Skeletonema costatum and Nitzschia seriata being dominant at western stations, and warmer water species (characteristic of the Central Adriatic) being relatively more common at eastern stations.

Changes in community structure and similarity indexes were indicative of major changes in the oceanographic environment, in particular the transition from stratified to vertically mixed periods.

394 N. REVELANTE & M. GILMARTIN

VI. R E F E R E N C E S

ANON., 1972. Boll. idrol, mens. (issued monthly, June-December). Ministero dei Lavori Pubblici, Servizio idrografico (Italia) : 1-8. , 1973. Boll. idrol, mens. (issued monthly, January-December). Ministero dei Lavori Pubblici, Servizio idrografico (Italia) : 1-8.

BATTAGLIA, B., C. Mozzi & A. M. VARAGNOLO, 1959. Osservazioni sulla distribuzione e composizione del plancton nell' Adriatico.--Atti Ist. veneto Sci. 177:111-124.

- - , 1960. Prime osservazioni sul materiale planctonico raccolto durante la crociera talassografica adriatica del 1955.--Rapp. P.-v. Rfiun. Comnm int. Explor. scient. Mer M~diterr. 15 (2) : 309-315.

- - - - , 1961. La distribuzione del plancton nell'Adriatico in rapporto con la concen- trazione dei sali nutritivi.--Rapp. P.-v. R~un. Commn int. Explor. scient. Met Mckliterr. 16 (2) : 93-95.

CASTRACANE, F., 1873. Le Diatomee litorale dell'Istria e della Dahnazia.--Att. Accad. pontif. Accad. Nuov. Lincei 26: 335-345, 399-411.

DEGOBBIS, D., 1974. The nitrogen cycle and budget in the Northern Adriatic. Univ. Zagreb, Yugoslavia: 1-154 (thesis).

FINENKO, Z. Z. l~z D. K. KRUPATKINA-AKININA, 1974. Effect of inorganic phosphorus on the growth rate ofdiatoms.--Mar. Biol. 26 (3) : 193-201.

FORTI, A. ~; R. ISSEL, 1923. Di alcuni elementi rari osservati nel microplancton del mare Adriatico di Rovigno.-- Nuova Notarisia 34:58-61. , 1924. Di alcuni elementi nuovi osservati nel microplancton del mare Adriatico di Rovigno.--Nuova Notarisia 35: 23-27.

GILMARTIN, M., S. KVEDER, D. DEGOBBIS, N. REVELANTE • N. SMODLAKA, 1972. Hydrographic data collected during cruises in the Northern Adriatic, July- December 1972.--Thalassia jugosl. 8 ( 1 ) : 149-213. , 1973. Hydrographic data collected during cruises in the Northern Adriatic, January-December 1973.--Thalassiajugosl. (in press).

HALIM, Y., 1960. Observations on the Nile bloom of phytoplankton in the Medi- terranean.~] . Cons. perm. int. Explor. Mer 26 (1): 57-67.

HAUCK, F., 1872. Aufz~hlung einiger in dem sogenannten Seeschleime der Adria vorkommenden Diatomeen.--Ost. bot. Z. 10: 331.

ISSEL, R., 1921. Le variazioni del plancton delle acque di Rovigno e i problemi relativi del ptancton Adriatico.-- Memorie R. Com. talassogr, ital. 88: 1-26. , 1922. Nuove indagini sul plancton nelle acque di Rovigno (1920-21).--Me- morie R. Com. talassogr, ital. 102: 1-36. , 1925. Ricerche sulle variazioni del plancton nelle acquc di Rovigno e di Quarto (1922-23).--Memorie R. Com. talassogr, ital. 115: 1-41.

JACQUES, G., 1969. Aspects quantitatifs du phytoplancton de Banyuls-sur-Mer (Golfe du Lion). III . Diatomdes et dinoflagelles de Juin 1965 aJu in 1968.--Vie Milieu 20 (I) : 91-126.

KAMPTNER, E., 1939. l~ber die Coccolithineen der Stidwestktiste yon Istrien.--Sber. 6st. Akad. Wiss. 148 (3/4): 119-121.

- - - - , 1940. Die Coccolithineen der Stidwestktiste yon Istrien.--Annln naturh. Mus. Wien 51 : 54-149.

LOVA~EN, Z., 1969. Les conditions hydrographiques et biologiques en Adriatic du Nord. IV. Variation saissonni&re du phytoplancton.--Thalassia jugosl. 5 :213- 218.

MALONE, T. C., 1971a. The relative importance of nannoplankton and netplankton as primary producers in the California Current system.--Fishery Bull. Fish Wildl. Serv. U.S. 69 (4) : 799-820.


- - , 1971b. The relative importance ofnannoplankton and netplankton as primary producers in tropical oceanic and neritic phytoplankton communities.--Limnol. Oceanogr. 16 (4) : 633~39.

I~ARGALEF, R., 1958. Temporal succession and spatial heterogeneity in phytoplankton. In: A. A. BUZZATI-TRAVERSO. Perspectives in marine biology. Univ. Calif. Press, Los Angeles: 323-349.

MARTIN, J. H., 1966. Phytoplankton-zooplankton relationships in Narragansett Bay.--Limnol. Oceanogr. 10 (2) : 185-191.

MOLImR, H., 1938. Einige Beobachtungen fiber die Schichtung des Phytoplanktons nahe der Ktiste bei Rovigno d ' Is t r ia . - -Note Ist. italo-germ. Biol. mar. Rovigno 2 (11): 1-20.

PUCHER-PETKOVld, T., 1966. V6getation des diatom6es p61agiques de l 'Adriatique moyenne.--Acta adriat. 13 (1) : 1-97.

REVELANTE, N., 1975. A comparative study of the effects of upwelllng and river eutrophication on phytoplankton community structure and primary production. Univ. Zagreb, Yugoslavia: 1-252 (thesis).

REVELANTE, N. & M. GILMARTIN, 1976. The effect of Po river discharge on phytoplankton dynamics in the Northern Adriatic Sea. - -Mar . Biol. 34:259-271.

R~.V~.LANTE, N. &. S. KV~DER, 1971. Hydrographic and biotical conditions in North Adriatic. XI. Some relations between phytoplankton abundance, primary productivity and plant pigments in Rovinj a rea . - -Rapp. P.-v. Rdun. Commn int. Explor. scient. Mer Mdditerr. 20 (3) : 331-334.

SCHILLER, J. 1914. Bericht tiber Ergebnisse der Nannoplanktonuntersuchungen anliisslich der Kreuzungen S.M.S. Najade in der Adr ia . - - In t . Revue ges. Hydrobiol. Hydrogr. 6 (Suppl.) : 1-15.

SCHROEDER, B., 1911. Adriatisches Phytoplankton.--Sber. 6st. Akad. Wiss. 120: 1-57.

gKRIVANIC, A., 1971. Hydrographic conditions in the North Adriatic (1968-1970).-- Thalassiajugosl. 7: 567-589.

SOLAZZI, A. &. C. Andreoli, 1971. Produttivittt e ciclo annuale del fitoplancton nel medio Adriatico occidentale.--Quad, lab. tech. pesca, Ancona I (suppl.) : 1-90.

SORENSEN, T., 1948. A method of establishing groups of equal amplitude in plant society based on similarity of species content . --K, danske Vidensk. Selsk. Skr. 5 (4): 1-34.

STEUER, A., 1902. Quantitative Planktonkunde im Golf von Triest .--Zool. Ariz. 25: 372-375.

gTmN, J. , 1968. The importance of N. seriata Clev. in the northern Adriatic phytoplankton.---Rapp. P.-v. R6un. Commn int. Explor. scient. Mer M6diterr. 19: 577-580. , 1969. The north Adriatic pelagial, its oceanological characteristics, structure and distribution of the biomass during the year 1965.--Dissn Acad Sci. Art. Slov. 12 (2) : 1-92.

STRICKLAND, J. D. H. & T. R. PARSONS, 1968. A practical handbook of sea water analysis.--Bull. Fish. Res. Bd Can. 167: 1-311.

UT'~R~6HL, H., 1958. Zur Vervollkommnung der quantitativen Phytoplankton- Methodik.--Mit t . int. Verein theor, angew. Limnol. 9: 1-38.

VATOVA, A., 1928. Compendio della flora e fauna del mare Adriatico presso Rovigno. - -Memor ie R. Com. talassogr, ital. 143: 1-614.

VOLTOLINA, D., 1969. Distribuzione quantitativa e qualitativa del fitoplancton nell 'Adriatico settentrionale. I. Estate 1965.--Areho Oceanogr. Limnol. 16 (2) : 173-187. , 1970. Distribuzione quantitativa e qualitativa del fitoplancton nell 'Adriatico settentrionale. II . Autunno 1965.--Archo Oceanogr. Limnol. 16 (3) : 227-246.


. . . . . , 1971a. Distribuzione quanti tat iva e quali tat iva del fitoplancton nell 'Adriatico settentrionale. I I I . Inverno 1966.--Archo Oceanogr. Limnol. 17 (1) : 71-93.

- - - - , 1971 b. Distribuzione quanti tat iva a quali tat iva del fitoplancton nell 'Adriat ico settentrionale. IV. Primavera 1966.--Archo Oceanogr. Limnol. 17 (2) : 169-177.

- - - - . 1971e. Phytoplankton in the Gulf of Venice, April 1965-June 1966.--Archo Oceanogr. Limnol. 17 (1) : 43-70.

- - , 1973a. Phytoplankton concentrations in the Malamocco Channel of the Lagoon of Ven ice .~Areho Oceanogr. Limnol. 18 ( 1 ) : 1 - 18.

- - - - , 1973b. A phytoplankton bloom in the Lagoon of Venice . - -Archo Oceanogr. Limnol. 18 (1) : 19-37.

- - - - , 1973c. Suspended matter in the lagoon of Ven i ce .~At t i Ist. veneto Sci. 131: 29-34.

ZANON, D. V., 1940. Diatomee di Rovigno. Terzo contributo alia flora diatomologica del l 'Adria t ico.- -Thalass ia 3 (a) : 3-72.

- - - - , 1941. Diatomee dello stagno Palfi (Rovigno) . - -Thalassia 5 14) : 1-30. ZOR~.-ARMANDA, M., 1956. On gradient currents in the Adriatic s e a . ~ A c t a adriat.

8 (6): 1-38.

temporal succession of phytoplankton in the northern adriatic

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