original scientific paper UD C 574.5(262.3-17) 546.26:504.4(262.3-17) CONTRIBUTIO N O F PHYTOPLANKTO N T O PARTICULAT E ORGANI C CARBO N IN TH E GUL F O F TRIESTE (NORTHER N ADRIATI C SEA) Christinamaria SALVI Department of Geological, Environmental and Marine Sciences, IT-34127 Trieste, Via Weiss, 2, PAD Q Serena FONDA UMANi & Sara C'OK Marine Biology Laboratory, iT-34010 Santa Croce, Trieste, 54 Via Auguste Piccard ABSTRACT In the Gulf of Trieste (Northern Adriatic Sea) an investigation was carried out to determine the contribution of particulate phytoplankton carbon (PRC) to particulate organic carbon (POC) and their seasonal evolution. Water samples were collected monthly from March to October 1990 in three coastal stations at four depths. The suspended matter, temperature, salinity, chlorophyll a, phytoplankton composition and abundance were estimated and phytoplankton carbon content was calculated. in the Gulf of Trieste, variations in the amounts and quality of total suspended matter (TSM) and its C/N ratio reflectedriverdischarge changes and phytoplankton community succession. In 1990 high fresh water inputs were observed in spring and autumn, which corresponded to high TSM concentrations mostly due to inorganic fraction. In spring and autumn, even if phytoplankton blooms occurred, PPC was only a small part of the POC, because of the small dimensions of diatoms and the presence of high terrestrial contributions. The prevalence of nanoplankton and the very low PPC values observed in March and in April can also confirm this hypothesis. In summer, high PPC values, corresponded to large-sized diatom species although present at low concentrations. Key words: Suspended matter, particulate organic carbon, particulate phytoplankton carbon, C/N ratio INTRODUCTION The bulk of suspended particles in marine waters (seston) consists of mineral and organic fractions (Posedel & Faganeli, 1991). In the coastal zone, the mineral part of seston, is carried out into the sea by means of land drainage, coastal erosion and atmos­pheric inputs, or it is stirred up from the bottom and, to a lesser extent, formed in situ (Fabiano et at., 1986). The particulate organic matter, found in the open oceans, consists mainly of iiving organisms, like phyto- and zooplankton, bacteria and fungi (Faganeli, 1983; Honjo, 1980; Sreepada et at., 1996). In coastal waters, such as the Gulf of Trieste, particulate organic matter is mostly derived from the terrigenous supply of rivers and resus­pension of bottom sediments (Fonda Umani et ai, 1985; Airoldi ef at., 1995), except during phytoplankton blooms when living organic matter prevales. The phytoplankton community in temperate areas is influenced by nutrient availability (Malone, 1980) and by environmental parameters: microphytoplankton blooms occur in spring and autumn, after high river inputs and in optimal chemical and physical conditions. In this situation, eukaryotic phytoplankton can constitute up to 80% of the POC (Flobson eta!., 1973; Laws etai, 1988). On the other hand, nano- and picoplankton are abun­dant especially in summer months, but their contribution to POC is generally low due to their small size. The aim of the present investigation is to estimate the contribution of particulate phytoplankton carbon (PPC) to particulate organic carbon (POC) and its seasonal evolution in a coastal area of a semi-enclosed Gulf. C.SA L VI, S. FOND A UMANI, S. COK : CONTRIBUTIO N OF PHYTOPLANKTO N T O PARTICULATE ORGANI C CARBO N ..., 19-30 53° 13°30* Fig. 1: Location map showing the sampling stations in the Gulf of Trieste. SI. 1: Položaj vzorčevalnih postaj v Tržaškem zalivu. DESCRIPTION OF THE STUDIED AREA The Gulf of Trieste (Fig. 1) is a shallow semi-en­c.losed basin (max. depth of 25 nn) in the north-eastern part of the Adriatic Sea. It receives conspicuous fresh water inflows driven by the Isonzo river, with a mean annual flow of 100 mV 1 (Olivotti et ai, 1986), by the Timavo river from the northern part of the Gulf and to, a lesser extent, by the Rosandra and Rižana rivers from eastern and southern parts. Urban sewages, industrial and agricultural effluents coming from the neighbouring area are discharged into the Gulf, generally through un­derwater pipelines. The circulation system is typical of the northern Adriatic Sea characterized by two major currents: a deep one, anticlockwise, with an average vectorial ve­locity of about 2 cm sec'1 and a surface one, generally flowing clockwise (average velocity 5 cm sec"1) which plays a fundamental role in the sediment distribution in shallow waters, particularly in association with strong winds and high river inputs {Stravisi, 1988). The water column is characterized by a strong verti­cal temperature gradient (up to 8°C difference between surface and bottom water) during spring and summer and by vertical mixing, mostly produced by the Bora wind, during autumn and winter. Local winds in the Gulf are sea and land breezes, almost along the direc­tion WNW-ESE respectively. South winds, from "Scirocco" (SE) to "Libeccio" (SW), are significant in the whole Adriatic basin (Stravisi, 1991 ). MATERIALS AN D METHODS Sampling was carried out at monthly intervals from March to October 1990 at three stations (1, 2, 3) in the Gulf of Trieste. The stations were located along a NE­SW transect (Fig. 1). The water depth at these stations was 20, 17 and 19 m, respectively. Water sampling was performed using a Niskin bottle (5 I) at four different depths (surface, 5, 10 and 15m). Total Suspended Matter (TSM) samples were pre-fii­tered through a 200 pm mesh net. One litre was filtered onto precombusted (3 hours at 450'"C) and pre-weighed 47 mm Whatman GF/F glass-fibre filters (Fabiano & Povero, 1992). After filtration, the salt was removed by washing the filter with distilled water. TSM was estimated by gravimetry and was separated into inorganic (ISM-lnorganic Suspended Matter) and 20 C.SA L VI, S. FOND A UMANJ, S. COK : CONTRIBUTIO N O F PHYTOPIANKTO N T O PARTICULATE ORGANI C CARBO N .... 19-3(1 Fig. 2: Particulate Organic Carbon (POC) at the stations 1, 2 and 3. Si 2: Partikulatni organski ogljik (POC) na postajah 1, 2 in 3. organic (POM-Particulate Organic Matter) components, after removing carbonates with I N HCI (Hedges & Stern, 1984) and measuring the organic residual by reweighting the filters (Smetacek ef a/., 1978). Finally, the ISM lost during acidification was calculated accord­ing to the difference between TSM and POM. Analyses of Organic Carbon (POC) and Nitrogen (PN) in particu­late matter were performed, on HC! treated filters, using a Perkin-Elmer 2400 CH N Elemental Analyzer for dry combustion (950°C). Phytoplankton samples (250 ml) were fixed with buffered formalin at 4% final concentration, 50-100 ml had been settled, counted according to Utermohl (1958) using an inverted microscope (320x), and determined generally at the species level for microphytoplankton and at group level for nanoplankton. Particulate phyto­plankton carbon (PPC) was obtained by calculating the cell volume (CV) for every species, where each one was Fig. 3: Particulate Nitrogen (PN) at the stations 1, 2 and 3. Si 3: Partikulatni dušik (PN) na postajah 1, 2 in 3. expressed as a simple or composed solid geometric body. For the majority of the species, the cell volume is equal to plasma volume (CV=PV). Regarding the dia­toms only 90% of the vacuolar volume (VV), which contains generally non nutritious cell sap, was sub­tracted from cell volume (Mullin ef a/., 1966; Smayda, 1978; Edler, 1979). The carbon content of each species was calculated according to Strathmann (1967). The to­tal PPC was obtained by summing individual values of species encountered in each sample. Sea water samples for Chlorophyll a (Chi a) determi­nation (21) were filtered onto a 47 mm 0.45 pm Mil­lipore HA filter. Chlorophyll a analyses were carried out using a Perkin-Elmer Lambda 2 spectrophotometer, ac­cording to Magazzu (1978). Temperature and salinity were measured throughout the water column using a CT D probe (Hydronaut mod. 401). C . SALVI, S. FOND A UMAN1, S. COK : CONTRIBUTIO N O F PHYTOPIANKTO N T O PARTICULATE ORGANI C CARBO N .., 19-30 Station 1 Station 2 Station 3 Months Depth. Temp. Sal. Temp. Sal. Temp. Sal. (°C) (°C) (°C) March 0m 14.22 35.28 13.29 35.90 12.79 37.78 22.03.90 5m 11.26 38.31 10.72 38.07 11.20 38.27 10m 10.91 38.28 10.29 38.07 10.91 38.29 15m 10.66 38.25 10.32 38.09 10.87 38.27 April Om 12.01 27.21 11.97 36.84 12.27 38.01 11.04.90 5m 11.55 37.96 11.36 37.90 11.92 38.05 10m 11.44 37.98 11.23 37.99 11.83 38.10 15m 11.41 38.01 11.20 38.04 11.79 38.12 May 0m 19.02 35.79 19.22 35.58 18.51 37.52 30.05.90 5m 17.81 36.37 18.65 36.44 17.79 37.52 10m 16.48 37.44 17.96 36.93 17.50 37.73 15m 14.07 37.97 13.79 38.01 14.97 38.12 June 0m 19.29 33.79 19.39 31.03 20.55 32.27 14.06.90 5m 19.10 36.40 18.90 36.79 18.90 37.05 10m 18.14 37.41 18.31 37.35 18.69 37.52 15m 15.82 37.79 15.89 37.74 18.60 37.60 July 0m 21.38 37.13 24.32 37.01 23.04 37.46 05.07.90 5m 20.83 37.43 22.53 36.89 22.52 37.46 10rn 19.98 37.82 20.16 37.49 20.65 37.86 15m 18.50 37.77 18.41 37.74 18.46 37.89 August 0m 23.55 37.95 24.40 37.68 24.84 37,93 02.08.90 5m 23,35 37.95 23.92 37.64 23.30 38.01 10m 23.23 37.97 23.67 37.73 23.09 38.00 15m 23.00 37.92 22.78 37.87 21.49 38.23 September 0m 23.53 36.20 24.63 36.78 24.89 37.78 30.09.90 5m 23.25 37.75 23.25 3 7.71 24.07 37.77 10m 22.76 37.80 22.94 37.77 23.29 37.81 15m 22.11 38.05 22.30 37.90 21.91 38.07 October 0m 19.85 36.94 18.94 35.48 18.96 34.56 19.10.90 5m 19.48 37.18 19.73 37.02 19.90 37.14 10m 19.77 37.51 19.83 37.36 19.97 37.46 15m 19.74 37.52 19.71 37.47 20.14 37.59 Tab. 1: Temperature and Salinity at the stations 1, 2 and 3. Tab. 1: Temperatura in slanost morja na postajah 1, 2 in 3. RESULTS ture was 11.97°C {St. 2, April) while maximum was 19.22°C (St. 2, May). O n the contrary, minimum bottom Hydrological features value was 10.32°C (St. 2, March) while maximum was 14.97DC (St. 3, May). The difference between surface In spring 1990 three water masses were recognized and bottom layers was on average 2.69[>C in the three in the Gulf of Trieste, each characterized by different stations. Salinity was generally low at the surface be-hydrological and nutrient regimes. Fresh water inputs cause of the riverine inflows and it was higher at the were predominant in April, June and October and the bottom, owing to the presence of a high salinity nucleus intensity of each episode was weaker than in the previ-with winter characteristics. Thermohaline stratification ous and following years (Cardin & Celio, 1997). of the water column lasted also in June while during the During the spring months (March, April and May) the warmest months the water column was more homoge­water column was stratified with higher temperature at neous. The highest surface temperature was observed in the surface and lower at the bottom related to residual September (24.89°C, St. 3), whereas the lowest value winter water nuclei (Tab. 1). Minimum surface tempera-was reported in June (19.29°C St. 1). The highest bot­ ANNALES 13/'98 C . SALVI, S. FOND A U M ANI . S. ČOK : CONTRIBUTIO N O F PHYTOPLANKTO N T O PARTICULAT E ORGANI C CARBO N ..., 13-30 torn water temperature was observed in August (23.00°C, St. 1) whereas the lowest one was found in June {15.82°C, St. 1). Salinity values were generally ho­mogeneous in July and August while in the other months the lowest values were found at the surface, especially in April (27.21) and in June (33.79). in October, a pro­gressive mixing of the water column was evident {Tab. 1). 22.0.1.90 n.ai.W K1.GS.9Q ROIi.lW Oj^T.au GI.OS.W Jfl.OiKOl) tsuo.yo Fig. 4: C/N ratio at the investigated stations. • •• Dktoms .. 1 " ... jr .loflageUaies w tenoplanfäa» s n 1 S ' -•• ,. -linoflageihues S Narraptenkton Fig. 5: Relationship between Particulate Phytoplankton Carbon (PPC - pgC/l) on the left and the abundance of phytoplankton cells (cells/1) on the right at the station 1 (surface and 5 meters). Slotiro i . CM rot«? Î2.0i,54> n.W.90 jü.Oi. 10 and low PPC values were observed, indicating the prevalence of the detritic frac­tion over the living one (Calvo et a!., 1991; Fabiano & Povero, 1992). This hypothesis was also supported by the high POC/Chi a ratio, which underlines the preva­lence of the detrital material, as previously stated by Fabiano et a!,, (1993) and by Galois eta!., (1996). Simi­lar situations were also observed by Faganeli & Malej (1981) and Faganeli (1983} in the Gulf of Trieste. O n the other hand, the contribution of PPC to POC was higher in autumn than in spring, due to the pres­ence of large phytoplankton species. Simultaneously C/N ratio was high in all stations because of the con­tinuous terrigenous supply of riverine origin. In summer, especially in August, there were the low­est TSM observed throughout the whole period, and the general prevalence of the organic fraction (expecialiy in St. 1 and 2). The low C/N ratios observed in summer (June-September) supported the idea of a strong domi­nance of the living fraction over the detritic one as well as the high PPC values. In particular, in September the highest values of PPC, corresponding to low C/N ratio, were found at the bottom layer, confirming that primary production was higher below the pycnocline. High pri­mary production in the deepest layers, due to both higher nutrient availability and reduced photolimitation, is usually observed in the North Adriatic Sea (Cabrini et al., 1989) at the end of summer. The seasonal distribution of the phytoplankton community confirmed previous observations on the same area (Milani et al., 1989; Cabrini et al., 1989; Fonda Umani, 1992; Fonda Umani et al., 1992; Malej ef al., 1995): a prevalence throughout the whole period of nanoplankton and diatoms and an increase of din­oflagellates in summer. In particular diatoms show a late spring and autumn bloom and they are, with the din­oflagellates, the main component of PPC. in general the temporal pattern of PPC followed both the quantitative and qualitative fluctuation of phyto­plankton, even if it appeared to be more related to cell volume of the phytoplankton species than to cell abun­dance, as previously observed by ViliLid (1985). Conse­quently it was strongly coupled with phytoplankton suc­cession because each species was characterized by dif­ferent PPC contents. During the studied period, a sea­sonal variability of PPC values was observed; tower in early spring (with the lowest percentage on POC) and higher in summer. Andersson & Rudehall (1993) also observed a marked seasonal variability in the components of the POC pool. The percentages of PPC, similar to those ob­served in our case, can vary from 3.4% in March and April to a maximum of 50% in summer (Zeitzschel, 1970; Sarma & Nageswara, 1989) when species with large cell volume prevail. The same situation was also reported by Eppley et al. (1992) (PPC range - 25-50%). in our case, as observed also by Andersson & Rude-hall (1993), POC can be used as an index of phyto­plankton biomass only in summer months, when large C . SALVI, S, FOND A UMANt, S. ČOK : CONTRIBUTIO N OF PHYTOPLANKTO N T O PARTICULATE ORGANI C CARBO N ..., 19-30 phytoplankton species occur, in early spring and toraggio sullo stato cbimtco, fisico e biologico deile ac­autumn POC appears to be more related both to riverine que deil'Alto Adriático, in relazione al fenomeno di inputs and probably to nanopiankton and to small-sized formazione degli ammassi geiatinosi". microalgae. Consequently it can give limited informa-The authors wish to thank Dr. Marina Monti for the tion on phytoplankton biomass. phytoplankton analysis, Dr. Romana Melis for sus­ pended matter and particulate organic carbon and nitro-ACKNOWLEDGEMENTS gen analysis and Dr. Sergio Sichenze for calculating PPC. This work was supported by the Osservatorio The authors also thank Prof. Paul Wassmann and dell'Alto Adriatico within the framework of the Alpe Prof, jadran Faganeli for comments on the manuscript Adria project: "Campagna scientifica di ricerca e moni-and useful suggestions to increase its reading. Station 1 Station 2 Station 3 Months Prof. POC PPC Chi. a POC/Chi. a POC PPC Chi. a POC/Ch!. a POC PPC Chi. a POC/Chl. a March 0m (Mgfl) 487 iwA) 13,78 {\>m 1.80 ratio 270 ÍMS/I) 533 ÍMS/1) 5.45 ÍMn/!) 0.80 ratio 666 ÍMn/l) 410 19.90 0.80 ratio 512 22.03.90 5m 831 12.69 0.40 2075 155 2.54 0.40 387 204 9.26 0.20 1016 10m 199 1.54 0,80 248 137 3.89 0,60 228 177 1.48 0.20 886 15m 176 1.98 0.60 293 207 7.76 1.40 147 2421 11.08 0.40 6045 April 0m 690 9.71 0.71 967 838 2.37 1.00 837 371 3.61 0.00 / 11.04.90 5m 632 6.25 0.45 1417 596 1.92 0.80 742 863 11.90 0.40 2155 10m 15rn 625 386 /5.29 1.00 0.45 624 864 560 482 1.28 2.60 1.20 1.00 466 481 921 891 8,76 9.00 0.60 0.40 1533 2224 May 30.05.9 Orn 5rn 700 1055 142.02 99.47 0.40 0.80 1747 1317 777 861 148.02 116.00 0.60 1.00 1293 859 823 664 19.78 8.84 0,60 0,80 1370 829 10m 784 152.90 0.20 3912 1165 101.74 0.40 2909 396 10.20 0,60 659 15m 889 138.87 0.40 2220 739 73.37 0.40 1846 775 54.76 0,80 968 June 0m 613 150.73 1.80 340 502 71.39 0.40 1253 468 106,14 1.20 390 14.06.90 5m 573 70.55 0.40 1430 408 46.78 0.40 1019 281 56.72 0.60 468 10m 429 48.71 0.20 2143 461 51.04 0.80 576 172 38.40 0.20 859 15m 557 74.07 0.60 926 372 91.63 0.80 464 276 64.03 0.60 460 July 05.07.90 0m 5 m 343 341 374.06 311.58 2.40 0.60 143 568 80 198 87.62 49.31 0.20 0.20 40 Î 990 110 172 162.24 46.88 0,40 0.20 275 857 10m 514 273.31 0.60 856 378 252.60 1.00 377 269 177.18 0.20 1345 15m 412 415.78 0.80 514 484 99.57 3.00 161 527 180.54 1.20 439 August 0m 785 133.24 0.40 1959 318 136.94 0.80 397 291 34.79 0.27 1089 02.08.90 5m 340 186.24 0.80 424 501 235.64 0.20 2501 376 103.18 0.27 1404 10m 1114 166.18 0.20 5560 557 187.23 0.60 927 567 203.33 0.20 2833 15m 882 189.89 0.60 1468 929 122.09 1.20 773 231 31.91 0.20 1154 September 30.09.90 Orn 5m 260 392 218.34 216.86 0.60 0.20 433 1955 663 708 214.59 281.51 0.20 0.20 3314 3535 177 282 99.61 202.95 0.80 0.20 221 1409 10m 270 266.87 0.40 675 442 440.78 0.40 1104 787 88.41 0.20 3928 15m 372 539.43 0.80 465 689 183.81 0.60 1147 467 78.95 0.40 1166 October 0m 270 181.92 0.80 336 418 75.97 0.60 696 858 37.91 0.60 1428 19.10.90 5m 469 238.61 0.60 780 549 249.43 0,20 2742 499 35.68 0.40 1246 10m 552 96.22 0.80 689 528 186.03 0.60 879 719 60,64 0.71 1006 15m 738 224.56 1.00 737 571 244.85 1.00 571 271 52.81 0.20 1355 Tab. 5: Concentrations of Particulate Organic Carbon (POC), Particulate Phytoplankton Carbon (PPC), Chlo­rophyll a (Chi. a) and POC/Chl. a rations. Tab. 5: Koncentracije partikulatnega organskega ogljika (POC), partikulatnega fitoplanktonskega ogljika (PPC), klorofila a (Chi a) in razmerja POC/Chl a. C. SALVI, S. FOND A UMANI, S. ČOK : CONTRIBUTIO N O F PHVTOPLANKTO N T O PARTICULATE ORCANI C CARBO N ..., 19-30 SEZONSK I DELE Ž FITOPLANKTON A V PARTIKULATNE M ORGANSKE M OGLJIK U V VODAH TRŽAŠKEGA ZALIVA Christinamaria SALVI Fakulteta za geologijo in raziskovanje okolja irt morja, 1T-34127 Trst, Via VVeiss, 2, PAD Q Serena FONDA UMANI & Sara ČOK Laboratorij Morske biološke postaje, IT-54 34010 Santa Croce, Trst, 54, Via Auguste Piccard POVZETEK Avtorice pričujočega članka so K Tržaškem zalivu s temeljito raziskavo ugotavljale delež partikulatnega fito­planktonskega ogljika (PPCj v partikulatnem organskem ogljiku (POC) in njuno sezonsko spreminjanje. Vsak mesec od marca do oktobra 1990 so na treh postajah v obrežnih vodah Tržaškega zaliva jemale vzorce morske vode na štirih različnih globinah. Izmerjene so bile količine suspendiranih snovi, temperatura, slanost, klorofil a, sestava fito­planktona in njegova relativna gostota ter vsebnost ogljika. V Tržaškem zalivu so razlike v količinah in kvaliteti skupnih suspendiranih snovi (TSM) in njihovo razmerje C/N odsevale spremembe K rečnih odplakah in sukcesijo fitoplanktonske združbe. Spomladi in jeseni leta 1990 so bile ugotovljene velike količine sladke vode, kar se je ujemalo z visokimi koncentracijami TSM zlasti zaradi anorganske frakcije. Spomladi in jeseni je količina PPC, tudi v primeru cvetenja fitoplanktona, dosegala le manjši del POC, predvsem zaradi majhnih dimenzij diatomej in večjih količin zemeljskih snovi. To hipotezo je mogoče potrditi tudi s prevladujočim nanoplanktonom in z zelo nizkimi vrednostmi PPC, opaženimi v marcu in aprilu. Poleti so se visoke vrednosti PPC ujemale z majhnimi količinami padavin in z nizkimi koncentracijami mikroalg, vendar so pre­vladovale vrste večjih dimenzij. 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