OPCE-STR.vp Acta Bot. Croat. 68 (1), 29–44, 2009 CODEN: ABCRA 25 ISSN 0365–0588 Phytoplankton composition and biomass of the northern Adriatic lagoon of Stella Maris, Croatia NEDA FANUKO1*, MARKO VAL^I]2 1 Faculty of Arts and Sciences, 51000 Rijeka, Omladinska 14, Croatia 2 Faculty of Maritime Studies, 51000 Rijeka, Studentska 2, Croatia This study provides information on the seasonality of phytoplankton abundance, biomass expressed as cell volume and cell carbon, as well as species composition, in the small, shallow, brackish northern-Adriatic lagoon of Stella Maris near Umag (Croatia). The lagoon is permanently connected with the adjacent sea. Wide seasonal temperature and salinity excursions regulate phytoplankton assemblages. Unlike other Adriatic lagoons, the lagoon of Stella Maris showed moderate phytoplankton abundance, cell volume and carbon content and a high number of species. The specific diatom volumes from the Stella Maris lagoon were higher than those found in other Adriatic lagoons, whereas the specific volumes of dinoflagellates were in the same range. Diatoms represented 55% of all the species found, but there was a considerable contribution of nanoplankton and dino- flagellates in the annual outbursts. Keywords: phytoplankton, taxonomy, cell volume, cell carbon, coastal lagoon, Adriatic Sea Introduction The lagoons of the northern Adriatic Sea are characterized by shallowness, strong influ- ence from the adjacent land and considerable fluctuations in hydrographic conditions. The lagoons of the northwest Adriatic coast have been studied with much attention for over two centuries (NARDO 1847, NINNI 1906, BABI] 1911, KIESSELBACH,1936, BRUNETTI et al.1983, OREL et al. 2001, COVELLI et al. 2005), particularly with respect to lagoon phytoplankton (VATOVA 1940, 1961; MARCHESONI 1954; TOLOMIO 1982; TOLOMIO and BULLO 2001; FACCA et al. 2002, 2003; SOCAL et al. 2006). However, on the eastern coast the few lagoons have been investigated only sporadically (ZANON 1941; MALEJ et al.1979; FANUKO 1979, 1984; DE MENECH 2005; FANUKO et al. 2008). This paper provides information on phytoplankton assemblages, their species composi- tion, abundance, cell volume and carbon in the small, shallow brackish lagoon of Stella Maris near Umag (Croatia). ACTA BOT. CROAT. 68 (1), 2009 29 * Corresponding author, e-mail: fanukon@ffri.hr U:\ACTA BOTANICA\Acta-Botan 1-09\Fanuko.vp 22. travanj 2009 11:20:19 Color profile: Disabled Composite 150 lpi at 45 degrees Materials and methods Study area The study area is a small, natural macro-tidal northern Adriatic lagoon (45°27’06.35’’N, 13°30’59.80’’E), only 15,000 square meters large and 2 m deep in most parts, permanently connected with the adjacent sea by a narrow channel, 6m wide and 40 m long (Fig. 1). The climate of the region is sub-Mediterranean with an average annual air temperature of 16.4 °C and a rainfall up to 1,000 mm per year, distributed mostly over autumn and winter. In the lagoon there are several submarine springs, active mostly during late autumn and winter. The level and water exchange inside the lagoon is influenced generally by the tidal range of up to 2.04 m, while the prevailing weak winds from west and southwest probably represent an additional forcing factor. The euphotic zone comprises the whole water column. The water temperature varies in a wide range, from 4.2 °C in January to 30.2 °C in July and the salinity, ranging from 29 to 37, is directly influenced by daily events: rainfall and subsurface spring activities, with the highest values, above 33, observed in summer. The la- goon is located in the middle of a tourist resort, where bungalows are inhabited only during spring and summer. In 1979 the lagoon and the channel were deepened, a pier and lateral quays were erected, transforming the lagoon into a small marina, equipped with water and electricity supply, accessible to vehicles, with one hundred moorings for smaller boats an- choring between April and October, reaching the maximum number in August. During the cold part of the year the lagoon is entirely abandoned and the only human activity inside is sporadic fishing. 30 ACTA BOT. CROAT. 68 (1), 2009 FANUKO N., VAL^I] M. Fig. 1. Location of Stella Maris lagoon with sampling site U:\ACTA BOTANICA\Acta-Botan 1-09\Fanuko.vp 16. travanj 2009 8:49:33 Color profile: Disabled Composite 150 lpi at 45 degrees The sandy to muddy sediments are populated by eelgrass Cymodocea nodosa which dominates the macroalgae Chaetomorpha sp. and Cystoseira sp. in winter and spring and Padina pavonica in summer. In autumn and winter, when anthropogenic influence is sparse, the lagoon becomes a habitat for 3 species of water birds: Tachybaptus ruficollis, Aythya fuligula and Larus genei. Occasionally from May to September a mucilage phe- nomenon extending from surface to bottom is observed, in the same days but to a greater extent than in the outside sea. Phytoplankton From September 2004 to September 2005, with the exception of October, January and February, the sampling was carried out once or twice a month at the 1 m deep station. The phytoplankton samples for microscopic analysis were preserved with buffered formalde- hyde (1.5% final concentration) and the subsamples (50 mL) were settled overnight in sedi- mentation chambers. The entire bottom chamber plate area was counted at 250´ magnifi- cation for cells larger than 10 mm, whereas for smaller cells (< 10 mm) one transect of the chamber bottom was scanned at 500´ magnification. The species were identified and clas- sified according to STREBLE and KRAUTER (1984) for cyanobacteria, THRONDSEN (1997) for naked flagellates, HEIMDAL (1997) for coccolithophorids, PERAGALLO and PERAGALLO (1908), HUSTEDT (1930), HENDEY (1964) and HASLE and SYVERTSEN (1997) for diatoms, STEI- DINGER and TANGEN (1997) for dinoflagellates. Cells of approximately 2 mm in size that were hard to identify were reported as minute nanoplankton. During each count, linear measurements of cell size, made by ocular micrometer, were made for 3 to 5 specimens of perennial species and every specimen of rare species. These values were converted to specific average biovolume using the geometric formula of either a sphere, a parallelepiped, a cylinder, a cone or truncated cone, an ellipsoid or two compos- ite geometric bodies. The average cell volume was converted to cell carbon using the con- version factor of 0.13 pg C mm –3 for armoured dinoflagellates and 0.11 pg C mm –3 for other phytoplankton groups (ANDERSSON and RUDEHÄLL 1993). Results Species composition and phytoplankton successions The phytoplankton assemblage of the Stella Maris lagoon was composed of 151 taxa (Tab. 1). Diatoms were the dominant group (55% of all the species found), followed by dinoflagellates (28%) and prymnesiophytes (7%). The shallow lagoon assemblage was characterized by 21 genera of pennate diatoms that appeared throughout the year in low but steady number and were obviously well adapted to the fluctuating abiotic variables. The microscopic observations revealed that the winter specimens of these pennate diatoms had larger chloroplasts, which were more abundant and more intense in colour than those ob- served in the cells of the same species that appeared in summer. The outbursts of abundance, cell volume or phytoplankton carbon were caused by other groups. In March 2005, when the sea temperature was 12 °C and the salinity 34.4, the coccolithophorid Acanthoica aculeata reached its maximum of 1.52´10 5 cells L –1 , while in May, when water temperature and salinity rose over 20 °C and 33 respectively, dinofla- ACTA BOT. CROAT. 68 (1), 2009 31 PHYTOPLANKTON IN STELLA MARIS LAGOON U:\ACTA BOTANICA\Acta-Botan 1-09\Fanuko.vp 22. travanj 2009 11:20:20 Color profile: Disabled Composite 150 lpi at 45 degrees 32 ACTA BOT. CROAT. 68 (1), 2009 FANUKO N., VAL^I] M. Tab. 1. List of the phytoplankton species found in the Stella Maris lagoon, their average cell volume and carbon content T a x o n cell volume (mm³) cell carbon content (pgC) C Y A N O B A C T E R I A Aphanizomenon gracile Lemmermann 25 3 Dactylococcopsis acicularis Lemmermann 157 17 Oscillatoria sp. 19 2 Phormidium faveolarum Montagne ex Gomont 6 1 Synechococcus aeruginosus Nägeli 462 51 C R Y P T O P H Y C E A E Hillea fusiformis Schiller 14 2 C H R Y S O P H Y C E A E Dictyocha fibula Ehrenberg 2094 230 Meringosphaera tenerrima Lohmann 268 29 Mesocena polymorpha Lemmermann 3534 389 Uroglena volvox Ehrenberg 28 3 P R Y M N E S I O P H Y C E A E Acanthoica aculeata Kamptner 133 15 Calyptrosphaera oblonga Lohmann 1047 115 Calciosolenia murrayi Gran 209 23 Emiliania huxleyi (Lohmann) Hay et Mohler 268 29 Michaelsarsia adriatica (Schiller) Manton, Bremer et Oates 335 37 Ophiaster formosum Gran 34 4 Ophiaster hydroideus (Lohmann) Lohmann 26 3 Prymnesium parvum Carter 56 6 Rhabdosphaera stylifera Lohmann 524 58 Syracosphaera pulchra Lohmann 717 79 B A C I L L A R I O P H Y C E A E C e n t r a l e s Biddulphia biddulphiana (Smith) Boyer 395640 43520 Biddulphia titiana Grunow 339120 37303 Cerataulina pelagica (Cleve) Hendey 44179 4860 Chaetoceros affinis Lauder 15708 1728 Chaetoceros brevis Schütt 3402 374 Chaetoceros compressus Lauder 2650 291 Chaetoceros curvisetus Cleve 2011 221 Chaetoceros decipiens Cleve 282743 31102 Chaetoceros peruvianus Brightwell 261979 28818 Chaetoceros simplex Ostenfeld 34 4 Chaetoceros tetrastichon Cleve 942 104 Chaetoceros tortissimus Gran 877 96 Chaetoceros wighami Brightwell 1356 149 Coscinodiscus excentricus Ehrenberg 9770 1075 U:\ACTA BOTANICA\Acta-Botan 1-09\Fanuko.vp 16. travanj 2009 8:49:33 Color profile: Disabled Composite 150 lpi at 45 degrees ACTA BOT. CROAT. 68 (1), 2009 33 PHYTOPLANKTON IN STELLA MARIS LAGOON T a x o n cell volume (mm³) cell carbon content (pgC) Coscinodiscus perforatus Ehrenberg 1286 141 Guinardia flaccida Castracane (Peragallo) 1781283 195941 Hemiaulus hauckii Grunow 80592 8865 Leptocylindrus danicus Cleve 7853 864 Leptocylindrus minimus Gran 125 14 Melosira nummuloides Agardh 785 86 Melosira sulcata (Ehrenberg) Kützing 1155 127 Odontella mobiliensis (Bailey) Grunow 196250 21587 Proboscia alata (Brightwell) Sundström 6283 691 Pseudosolenia calcar avis (Schultze) Sundström 1178097 129591 Rhizosolenia styliformis Brightwell 105029 11553 Skeletonema sp. 2356 259 Thalassiosira decipiens (Grunow) Jørgensen 17671 1944 P e n n a l e s Achnantes brevipes Agardh 117810 12959 Achnantes longipes Agardh 376991 41469 Amphiprora sulcata O’Meara 4385 482 Amphora crassa Gregory 2880 317 Amphora hyalina Kützing 3240 356 Amphora marina (W Smith) Van Heurck 78540 8639 Amphora ostrearia Brébisson 165360 18190 Amphora ovalis Kützing 180 20 Amphora sulcata (Brébisson) Cleve 5000 550 Amphora sp. 2880 317 Auricula adriatica Peragallo 19250 2117 Auricula insecta (Grunow) Cleve 24000 2640 Campylodiscus adriaticus Grunow 28260 3109 Cocconeis scutellum Ehrenberg 943 104 Cylindrotheca closterium (Ehrenberg) Reimann et Lewin 524 58 Diploneis bombus Ehrenberg 6250 687 Diploneis crabro Ehrenberg 14400 1584 Entomoneis paludosa (W. Smith) Reimer 11025 1213 Fragilaria crotonensis Kitton 707 78 Grammatophora marina (Lyngbye) Kützing 8000 880 Grammatophora oceanica Ehrenberg 16000 1760 Licmophora communis (Heiberg) Grunow 1600 176 Licmophora flabellata (Carmichael) Agardh 12087 1330 Licmophora lyngbyei (Kützing) Grunow 27500 3025 Licmophora paradoxa (Lyngbye) Agardh 6480 713 Licmophora quadriplacata Mereschkowsky 126 14 Licmophora remulus Grunow 30000 3300 Licmophora sp. 6480 713 Tab. 1. – continued U:\ACTA BOTANICA\Acta-Botan 1-09\Fanuko.vp 16. travanj 2009 8:49:33 Color profile: Disabled Composite 150 lpi at 45 degrees 34 ACTA BOT. CROAT. 68 (1), 2009 FANUKO N., VAL^I] M. T a x o n cell volume (mm³) cell carbon content (pgC) Lioloma pacificum (Cupp) Hasle 5655 622 Mastogloia asperula Grunow 6000 660 Mastogloia citrus Cleve 2400 264 Navicula cancellata Donkin 3000 330 Navicula lyra Ehrenberg 3900 429 Navicula spp. 6000 660 Nitzschia incerta Grunow 6000 660 Nitzschia longissima (Brébisson) Ralfs 3351 369 Pleurosigma angulatum (Quekett) W. Smith 255563 28112 Pleurosigma balticum Smith 180000 19800 Pleurosigma elongatum W. Smith 144000 15840 Pleurosigma formosum W. Smith 194000 21340 Podocystis adriatica Kützing 73476 8082 Pseudo-nitzschia sp. 1 147 16 Pseudo-nitzschia sp. 2 1800 198 Striatella unipunctata (Lyngbye) Agardh 252500 27775 Surirella fluminensis Grunow 15000 1650 Synedra crystallina (Agardh) Kützing 19110 2102 Synedra fasciculata (Agardh) Kützing 4500 495 Synedra hennedyana Gregory 22973 2527 Synedra tabulata (Agardh) Kützing 5655 622 Synedra toxoneides Castracane 1050 115 Synedra sp. 5655 622 Thalassionema nitzschioides (Grunow) Mereschkowsky 120 13 Thalassionema frauenfeldi (Grunow) Hallegraeff 3750 412 Toxarium undulatum Bailey 22973 2527 Tropidoneis lepidoptera (Gregory) Cleve 60000 6600 E U G L E N O P H Y C E A E Euglena viridis (O.F. Müller) Ehrenberg 3142 346 Eutreptia lanowii Steuer 1571 173 D I N O P H Y C E A E Alexandrium minutum Halim 3462 450 Ceratium furca (Ehrenberg) Claparéde et Lachmann 36559 4753 Ceratium fusus (Ehrenberg) Dujardin 9739 1266 Ceratium macroceros (Ehrenberg) Vanhöfen 39270 5105 Ceratium massiliense (Gourret) E.G. Jørgensen 188495 24504 Ceratium tripos (Müller) Nitzsche 150795 19603 Dinophysis caudata Seville-Kent 104720 13614 Dinophysis fortii Pavillard 111910 14548 Dinophysis hastata Stein 85910 11168 Dinophysis schroederi Pavillard 91630 11912 Goniodoma polyedricum (Pouchet) Jorgensen 38288 4977 Tab. 1. – continued U:\ACTA BOTANICA\Acta-Botan 1-09\Fanuko.vp 16. travanj 2009 8:49:33 Color profile: Disabled Composite 150 lpi at 45 degrees ACTA BOT. CROAT. 68 (1), 2009 35 PHYTOPLANKTON IN STELLA MARIS LAGOON T a x o n cell volume (mm³) cell carbon content (pgC) Goniaulax polygramma Stein 22725 2954 Gymnodinium simplex (Lohmann) Kofoid et Swezy 589 65 Gymnodinium sp. 589 65 Gyrodinium fusiforme Kofoid et Swezy 21206 2333 Gyrodinium sp. 21206 2333 Oxytoxum longiceps Schiller 1571 204 Oxytoxum tesselatum (Stein) Schütt 1140 148 Oxytoxum variabile Schiller 697 91 Phalacroma rotundatum (Claparede et Lachmann) Kofoid et Michener 6936 902 Prorocentrum arcuatum Issel 29438 3827 Prorocentrum balticum (Lohmann) Loeblich 173 22 Prorocentrum compressum (Bailey) Abé ex Dodge 22808 2965 Prorocentrum dactylus (Stein) Dodge 18850 2450 Prorocentrum gracile Schütt 2566 334 Prorocentrum lima (Ehrenberg) Dodge 14158 1841 Prorocentrum micans Ehrenberg 13090 1702 Prorocentrum minimum (Pavillard) Schiller 2545 331 Prorocentrum scutellum Schröder 20944 2723 Prorocentrum triestinum Schiller 785 102 Protoperidinium crassipes (Kofoid) Balech 174411 22673 Protoperidinium depressum (Bailey) Balech 184103 23933 Protoperidinium diabolus (Cleve) Balech 150795 19603 Protoperidinium divergens (Ehrenberg) Balech 110733 14395 Protoperidinium globulus (Stein) Balech 2617 340 Protoperidinium kofoidi Fauré-Fremiet 233674 30378 Protoperidinium leonis (Pavillard) Balech 11641 1513 Protoperidinium pallidum (Ostenfeld) Balech 102108 13274 Protoperidinium solidicorne (Mangin) Diwald 43422 5645 Protoperidinium steinii (Jörgensen) Balech 63617 8270 Protoperidinium tuba (Schiller) Balech 3393 441 Protoperidinium sp. 63617 8270 Scripsiella trochoidea (Stein) Loeblich 6283 817 dinoflagellate cyst 1 8831 971 dinoflagellate cyst 2 14130 1554 C H L O R O P H Y C E A E Carteria marina Diesing 188 21 Chlamydomonas sp. 385 42 Dunaliella sp. 198 22 Tetraselmis sp. 385 42 minute nanoplankton 6 1 incertae sedis 785 86 Tab. 1. – continued U:\ACTA BOTANICA\Acta-Botan 1-09\Fanuko.vp 16. travanj 2009 8:49:33 Color profile: Disabled Composite 150 lpi at 45 degrees 36 ACTA BOT. CROAT. 68 (1), 2009 FANUKO N., VAL^I] M. 0 2 4 6 8 10 12 14 16 2004 2005 sampling days total phytoplankton minute nanoplankton 0 8 se p 2 5 n o v 1 0 d e c 1 5 m a r 1 9 a p r 1 5 m a y 2 7 m a y 2 ju n 1 5 ju n 1 7 ju l 3 0 ju l 2 a u g 1 3 a u g 1 3 se pA b u n d a n c e (1 0 c e ll s L ) 5 – 1 Fig. 2. Annual variation of phytoplankton abundance 2004 2005 sampling days 160 250 0 10 20 30 40 50 60 70 0 8 se p 2 5 n o v 1 0 d e c 1 5 m a r 1 9 a p r 1 5 m a y 2 7 m a y 2 ju n 1 5 ju n 1 7 ju l 3 0 ju l 2 a u g 1 3 a u g 1 3 se p diatoms 0 0.010 0.020 0.030 0.040 0.050 silicoflagellates 150 0 1 2 3 4 5 6 7 coccolithophoridae 0 4 8 12 16 18 armoured dinoflagellates 0 200 400 600 800 1000 1200 1400 minute nanoplankton 0 8 se p 2 5 n o v 1 0 d e c 1 5 m a r 1 9 a p r 1 5 m a y 2 7 m a y 2 ju n 1 5 ju n 1 7 ju l 3 0 ju l 2 a u g 1 3 a u g 1 3 se pa b u n d a n c e (1 0 0 0 c e ll s L ) – 1 0 8 se p 2 5 n o v 1 0 d e c 1 5 m a r 1 9 a p r 1 5 m a y 2 7 m a y 2 ju n 1 5 ju n 1 7 ju l 3 0 ju l 2 a u g 1 3 a u g 1 3 se p 0 8 se p 2 5 n o v 1 0 d e c 1 5 m a r 1 9 a p r 1 5 m a y 2 7 m a y 2 ju n 1 5 ju n 1 7 ju l 3 0 ju l 2 a u g 1 3 a u g 1 3 se p 0 8 se p 2 5 n o v 1 0 d e c 1 5 m a r 1 9 a p r 1 5 m a y 2 7 m a y 2 ju n 1 5 ju n 1 7 ju l 3 0 ju l 2 a u g 1 3 a u g 1 3 se p 2004 2005 sampling days Fig. 3. Seasonality of some phytoplankton groups U:\ACTA BOTANICA\Acta-Botan 1-09\Fanuko.vp 22. travanj 2009 11:20:21 Color profile: Disabled Composite 150 lpi at 45 degrees gellates (Goniaulax, Gymnodinium, Prorocentrum, Protoperidinium, Scripsiella) became more abundant though their number never exceeded 5´10 4 cells L –1 . In July and August the two centric diatoms were blooming: Skeletonema sp. and Chaetoceros simplex with 1.57´10 5 cells L –1 and 2.33´10 5 cells L –1 respectively. Skeletonema sp. appeared when the temperature and salinity conditions were among the highest registered (30.2 °C and 36.2). In terms of abundance the minute nanoplankton cells were the most conspicuous group throughout the year (Fig. 2), contributing up to 91% of the average annual phytoplankton abundance. Seasonality in diatoms showed bimodal annual pattern and they were most abundant in summer and autumn; the silicoflagellates appeared in modest abundances in autumn, the coccolithophorids appeared from March to August, while the armoured dinoflagellates were most abundant in May (Fig. 3). Abundance, cell volume and carbon stock Maximum abundances of the small nanoplankters (1.3´10 6 cells L –1 and 1.1´10 6 cells L –1 ) were registered in June and August with a sharp decrease in July, which coincided with the increase of oligotrich ciliate density (results not shown) and could be explained as a ACTA BOT. CROAT. 68 (1), 2009 37 PHYTOPLANKTON IN STELLA MARIS LAGOON c a rb o n st o c k ( g C L ) m – 1 0 50 100 150 200 250 300 350 0 8 se p 2 5 n o v 1 0 d e c 1 5 m a r 1 9 a p r 1 5 m a y 2 7 m a y 2 ju n 1 5 ju n 1 7 ju l 3 0 ju l 2 a u g 1 3 a u g 1 3 se p 2004 2005 sampling days Fig. 5. Seasonal changes in phytoplankton carbon stock c e ll v o lu m e (m m L ) 3 – 1 0 1 2 3 0 8 se p 2 5 n o v 1 0 d e c 1 5 m a r 1 9 a p r 1 5 m a y 2 7 m a y 2 ju n 1 5 ju n 1 7 ju l 3 0 ju l 2 a u g 1 3 a u g 1 3 se p 2004 2005 sampling days Fig. 4. Seasonal changes in phytoplankton cell volume U:\ACTA BOTANICA\Acta-Botan 1-09\Fanuko.vp 16. travanj 2009 8:49:34 Color profile: Disabled Composite 150 lpi at 45 degrees consequence of the high microzooplankton grazing pressure. All the other taxonomic groups reached the maximum abundances in late spring and summer, with the exception of prymnesiophytes which had their peak in late winter. Seasonal dynamics of phytoplankton volume and carbon showed a quite different pat- tern. In May, an explosive growth of large-sized dinoflagellates occurred and despite their low number (up to 5´10 4 cells L –1 ), they provoked a marked increase in phytoplankton vol- ume, rising up to 2.7 mm 3 L –1 (Fig. 4) and consequently in carbon stock, reaching its maxi- mum of 347 mg C L –1 (Fig. 5). In other months the carbon content never exceeded 100 mg L –1 . Thus, in terms of biovolume and carbon stock, the dinoflagellates were the most prom- inent group, contributing up to 90% of total carbon stock in May, between 20% and 80% in other months (Fig. 6), and with an average annual contribution of 73% (Fig. 7). Discussion In winter season the global solar irradiance of the area is 15 KJ cm–2, while in summer it reaches a fivefold value, of 80 KJ cm–2 (FANUKO 1986). In the phytoplankton assemblages of the adjacent open sea an inverse fivefold increase in cell chlorophyll content was ob- 38 ACTA BOT. CROAT. 68 (1), 2009 FANUKO N., VAL^I] M. 0 10 20 30 40 50 60 70 80 90 100 % o f c a rb o n st o c k others minute nanoplankton dinoflagellates pennatae centricae 0 8 se p 2 5 n o v 1 0 d e c 1 5 m a r 1 9 a p r 1 5 m a y 2 7 m a y 2 ju n 1 5 ju n 1 7 ju l 3 0 ju l 2 a u g 1 3 a u g 1 3 se p 2004 2005 sampling days Fig. 6. Annual phytoplankton composition given as % of carbon stock centricae 7% pennatae 17% dinoflagellates 74% others 2% Fig. 7. Average annual contribution of different taxonomic groups to the carbon stock U:\ACTA BOTANICA\Acta-Botan 1-09\Fanuko.vp 16. travanj 2009 8:49:34 Color profile: Disabled Composite 150 lpi at 45 degrees served in winter months. While in July the average monthly concentration of chlorophyll a per cell was 1.1 pg per cell–1, in December it rose to 5.5 pg per cell–1 (FANUKO 1986). More numerous and larger chloroplasts in the winter pennate diatoms of the lagoon of Stella Maris could be the adaptation of this group to the reduced light conditions in winter. The same phenomenon was never observed in other taxonomic groups. The number of phytoplankton species found in the lagoon of Stella Maris appeared to be high compared with other Mediterranean lagoons (Tab. 2). A higher number of species was found only in the southern part of the Lagoon of Venice (TOLOMIO and BULLO 2001), in a pool approximately 70 times larger than the Stella Maris lagoon and within 506 samples taken daily throughout the year. Among the 198 taxa found in the adjacent open sea of the Gulf of Trieste (FANUKO 1986), 151 (76%) were also registered in the Stella Maris lagoon. In two shallow bays of the Gulf of Trieste a total of 100 species of armoured dinoflagellates were registered (FRANCE and MOZETI^ 2006) while 212 phytoplankton taxa were reported off-shore in the Gulf of Venice (BERNARDI AUBRY et al. 2006). In the more southern area off Rovinj 689 phytoplankton species were found (RELEVANTE 1986) whereas 888 species were registered for the whole Adriatic sea (VILI^I] et al. 2002). In an in situ enrichment experiment, exhibited in the nearby lagoon of Strunjan (FANUKO 1984), the phytoplankton was significantly altered in terms of reduction of spe- cies diversity, cell density and chlorophyll biomass in the experimental lagoon which re- ceived settled municipal sewage during the period of two years. Similarly, the dystrophic ACTA BOT. CROAT. 68 (1), 2009 39 PHYTOPLANKTON IN STELLA MARIS LAGOON Tab. 2. Phytoplankton diversity in several Mediterranean lagoons. 1 this study; 2 FANUKO 1980; 3 SARNO et al. 1993; 4 ANDREOLI et al. 1989; 5 TOLOMIO et al. 1990; 6 TOLOMIO and BULLO 2001; 7 ANDREOLI and TOLOMIO 1988 Taxonomic group Lagoon S te la M a r is 1 S tr u n ja n 2 S tr u n ja n e x p .2 F u sa r o 3 V a ll e P o z z a ti n i4 V a r a n o 5 V e n ic e la g o o n C h io g g ia a r e a 6 V e n ic e la g o o n V a ll e D o g à 7 Cyanobacteria 5 0 0 2 2 0 0 0 Cryptophyceae 1 0 0 3 0 0 0 0 Chrysophyceae 4 1 0 9 0 1 2 0 Prymnesiophyceae 10 9 6 2 2 12 6 0 Bacillariophyceae (Centrales) 27 28 16 38 10 15 35 7 Bacillariophyceae (Pennales) 55 47 43 18 37 50 148 96 Euglenophyceae 2 1 1 3 1 1 3 2 Dinophyceae 43 29 20 31 9 20 42 13 Chlorophyceae 4 0 0 4 1 1 0 1 Total number of species 151 115 86 110 62 100 236 119 U:\ACTA BOTANICA\Acta-Botan 1-09\Fanuko.vp 16. travanj 2009 8:49:34 Color profile: Disabled Composite 150 lpi at 45 degrees status of the shallow Santa Giusta lagoon in Sardinia (SECHI et al. 2001) remained un- changed even after waste water diversion, with blooms of the toxic Cochlodinium polykri- koides and Chattonella marina, as well as the nitrophile macrobenthic alga Ulva rigida, whose massive proliferation in other eutrophic Mediterranean lagoons is attributed to in- dustrial, agriculture and domestic wastes introduced in shallow lagoon waters (ACRI et al. 1995, SOCAL et al. 1999, SFRISO et al. 2003, FACCA et al. 2003). None of these algae has ever been observed in the lagoon of Stella Maris. The coccolithophorids, predominantly oceanic in distribution (HEIMDAL 1993), are completely absent in some Mediterranean lagoons (SARNO et al. 1993), but they are common in the Stella Maris lagoon. During the investiga- tion period, 9 different species of coccolithophorids were found, among which Acanthoica aculeata predominated. Tychopelagic pennate diatoms, found in the Stella Maris lagoon, are also a representa- tive and perennial group in the Mediterranean (ANDREOLI and TOLOMIO 1988, ANDREOLI et al. 1989, SARNO et al. 1993, SOCAL et al. 2006) and some other lagoons worldwide (CONDE et al.1999, MACEDO et al. 2001) due to their capacity to support large and highly frequent changes in the physical conditions of the environment (BONILLA et al. 2005). Nevertheless, as shown in our study as well, other groups are responsible for the phytoplankton peaks: the small-sized pico- and nanoplankton cells (VAQUER et al. 1996) as primary producers in the microbial loop, centric diatoms, especially chain-forming Chaetoceros spp. and Skeleto- nema sp. (SOCAL et al. 1999, BEC et al. 2005, SOCAL et al. 2006) and finally dinoflagellates (CARRADA et al. 1991) which, due to their possible toxicity or other palatability issues, may be subject to relatively low grazing pressure (BADYLAK and PHLIPS 2004). In the northern Adriatic, Skeletonema has been recently identified as S. marinoi (SARNO et al. 2005). The phytoplankton abundance of the Stella Maris lagoon was comparable to the oligotrophic Mar Chiquita lagoon in Argentina (DE MARCO et al. 2005). Compared to the western Adriatic lagoons (SOCAL et al. 1999, 2006) and the eutrophic Thau lagoon in France (VAQUER et al. 1996, BEC et al. 2005), values in the Stella Maris lagoon were lower by two or three orders of magnitude. Even when the unialgal blooms occurred (for example Skeletonema sp.), their abundances in the Stella Maris lagoon never resulted in brown tides, as was the case in the industrial area of the lagoon of Venice (SOCAL et al. 1999). The seasonal pattern of cell abundance is similar to that of other lagoons of the temperate zone, showing low winter values and summer peaks (FACCA and DE CASABIANCA 2003, FACCA et al. 2004). The specific volume of diatom cells in the Stella Maris lagoon was higher than those found in the artificially fertilized fish ponds of the Po estuary (ANDREOLI et al. 1989), whereas the volumes of dinoflagellate species were in the same range (Tab. 3). Neglecting the possible inaccurate microscopy measurements (MONTAGNES et al. 1996) and great vari- ations in cell size (VILI^I] 1985), cell volume may give better phytoplankton quantification than abundance. Great differences in diatom cell size could be explained by different dia- tom division rates in different environments. Probably the diatoms in the nutrient-rich pond multiplied more rapidly, with more frequent reduction in cell size. As far as the total phytoplankton volumes and the estimated carbon content are con- cerned, both parameters were still lower in the Stella Maris lagoon than in Mediterranean (SARNO et al.1993, ANDREOLI et al. 1989) and Atlantic (BADYLAK and PHLIPS 2004, BONILLA et al. 2005) lagoons. 40 ACTA BOT. CROAT. 68 (1), 2009 FANUKO N., VAL^I] M. U:\ACTA BOTANICA\Acta-Botan 1-09\Fanuko.vp 16. travanj 2009 8:49:34 Color profile: Disabled Composite 150 lpi at 45 degrees Presuming that nutrients are not limiting in such a shallow environment and consider- ing the high tidal dynamics, additionally enhanced by winds, low phytoplankton abun- dances might be the result of low residence time of the water inside the lagoon and its rapid export in the adjacent sea. Acknowledgements Thanks to Giuliano Orel and Romina Zamboni for their assistance in the project and for making laboratory facilities available. The research was supported by the Ministry of Sci- ence, Education and Sport of Croatia, grant no. 0236001. References ACRI, F., ALBERIGHI, L., BASTIANINI, M., BIANCHI, F., BOLDRIN, A., CAVALLONI, B., CIOCE, F., COMASCHI, A., RABITI, S., SOCAL, T., TURCHETTO, M. M., 1995: Variazioni ad alta frequenza dei parametri idrobiologici nella Laguna di Venezia. Atti della Società Italiana di Ecologia 16, 31–34. 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