original scientific paper UDC 582.261:574(262.3-17) DIATOM ASSEMBLAGES IN COASTAL SHALLOW WATERS AT THE WATER ­SEDIMENT INTERFACE (GULF OF TRIESTE, NORTH ADRIATIC SEA) Elisabetta SDRIGOTTi, Vittorio BARBARIOL & Chiara WELKER Marine Biology Laboratory, IT-34010 Santa Croce, Trieste, Via Auguste Piccard 54 ABSTRACT The resuspended microphytobenthos, mostly consisting of benthic diatoms, plays an important role in the pro­duction of O2 as a food source for pelagic and benthic grazer at the water-sediment interface of the coastal envi­ronment. In the coastal shallow waters of the Gulf of Triestethe possible modifications in diatom community living at the water-sediment interface have been studied in respect of disturbed conditions, such as the presence of sus­pended mussel cultures. Two stations were chosen, one sited below the mussel rafts (st. M), the other (reference site, st. B) in an area free of cultures. The microscopic observations have shown that both stations, where mainly epipelic species belonging to the medium size class have been recorded, are characterised by the species Cylindrotheca closterium, Bacillaria paxiilifera and Gyrosigma acuminatum. The statistical analyses have not shown significant differences as far as biodiversity of communities is concerned. However, the diatom assemblage living under the mussel cultures has been less abundant in comparison with the reference site, presumably due to the presence of benthic grazers drawn to the greater availability of organic matter owing to biodeposition. Key words: Microphytobenthos, mussels, cell density, species composition, biodiversity INTRODUCTION Microphytobenthos, mainly consisting of diatoms, plays an important role as the primary producer in the carbon cycle of nearshore marine sediment systems (Klepper, 1989; De jong et a!., 1994). As producer of the new organic matter that can enter into the benthic and pelagic trophic web, microphytobenthos constitutes a substantial food source for sediment feeders (macro­meiobenthos) (Asmus, 1982; Admiraal et a!., 1983). Be­sides, it can be the main oxygen producer in coastal environments (Varela & Penas, 1985; Johnstone ef al., 1990), controlling also the oxygen balance at the water-sediment interface and allowing the aerobic degradation of autochthonous and sedimented organic matter in sediments (Davis & Mclntire, 1983; Barranguet, 1997). In the past, much research has been carried out into spatial and seasonal distribution patterns of microphy­tobenthos (Sundback, 1984, 1986; Miller et al., 1987; Snoeijs eta!., 1990; Sundback & Snoeijs, 1991; De Song & Colijn, 1994). Moreover, the sediment stabilisation by benthic diatoms and the microstructure of diatom films have been widely studied (Grant ef al., 1986; Paterson, 1989). Although the term microphytobenthos suggests that these algae are confined to the sediments, their presence in the water column as a part of the phyto­plankton has been well-documented (Humell, 1985; De jonge & Van Beusekom, 1992). Recently, some studies have been undertaken to evidence the role of this resus­pended microphytobenthos as an additional food source, in the water column, for pelagic filter- feeding community (De Jonge & Van Beusekom, 1992; De jong ef al., 1994; Stachowitsch & Fucbs, 1995; De Jong & De jonge, 1995). It is known that benthic microalgae can be stirred up into the overlying water by hydrodynamic energy (tidal currents, waves and winds), becoming temporarily part of phytoplankton (De jonge & Van Den Bergs, 1987; De jonge, 1985; Delgado ef al., 1991). The resuspended algae are mainly species able to migrate actively up and down the sediment, or non-motile species reaching the sediment surface in relation to bioturbation features (Paterson, 1986, 1989; Happey-Wood & Jones, 1988). Bsabetu SDRICOTTl p.t .1.: DIATOM ASSEMBLAGES IN COASTAL SHALLOW WATERS AT THE WATER - .... 191-202 Fig. 1: Position of the study area (-k) and location of the reference (st. B) and the mussel culture site (st. M). SI. 1: Slika območja raziskav ter lokacije (-k) referenčne postaje (post. B) in nasadov užitnih klapavic (post M). Several recent studies have investigated the envi­ronmental impacts of bivalve cultures regarding their role as suspension feeders that reduce the amount of phytoplankton (Frechette et al., 1989; Asmus & Asmus, 1991). The massive biodeposition following the estab­lishment of a mussel community also leads to a con­tinuous organic enrichment of sediments, which in­creases the oxygen demand and may cause a temporary anoxia (Svane & Setyobudiandi, 1996; Barranguet, 1997). It has been demonstrated by many authors that organic input to the sediments enhances a net flux of inorganic nutrients to the water column with a high ammonium efflux (Klump & Martens, 1981; Rizzo, 1990; Hatcher eta!., 1994). In order to identify any changes in diatom assem­blages at the water sediment interface due to the pres­ence of a suspended mussel farm, two stations have been investigated for this paper. Cel! density, species composition, biodiversity and temporal patterns of dia­tom community in the mussel culture have been com­ pared to a nearby reference station, not influenced by mussels. MATERIAL AN D METHODS The study sites (st. M and B) were located in the coastal area of the Gulf, in front of the Marine Biology Laboratory (Aurisina); one sampling station was situated under suspended mussel (Mytilus galloprovincialis) cul­tures (st. M), the other (control station B) in an area of similar depth and sediment type (sandy-pelitic sediment) unaffected by cultures (Fig. 1). The water depth in the sampling sites was 12 m at st. M and 13 m at st. B. Temperature and salinity were measured at the bot­tom layer by means of a Multiparameter Idronaut Ocean Seven mod. 401 probe simultaneously with biological sampling. Sediment samples were collected at two stations at monthly intervals from May to October 1998. At each station, 3 sediment cores (8 cm diameter, 20 cm length) were taken by a diver and the overlying water was sampled using plastic syringes of 60 ml. capacity. The overlying waters of the three cores were mixed together and fixed with a 4 % hexamethylenetetramin-buffered formaldehyde solution. After manual stirring, subsam­ples of 10 mL were placed in a counting chamber, and then the viable ceils at the time of fixing (with plasmatic content), were counted under a Leizt inverted light mi­croscope and expressed as cells t^ , using the Utermohi (1958) method. Three replicates were counted and the absolute numbers of the viable cells were converted into relative abundance values (RA, %). Cells were grouped into five size classes (a < 20, b 21 -70, c 71 -120, d 121 -200 and e > 200 pm) expressed as frequency percentage (%). The different microalgae growth forms were also identified using the literature and experimental observations (Weiker & Nichetto, 1996) and expressed as frequency value (%). The fol­lowing life forms were particularly considered: epipelic (benthic species living on - or moving through - muddy sediments), epipsammic (adnate benthic species grow­ing on sand grains) and planktonic (non-benthic species floating in the water column). Scanning electron microscopy (SEM) was performed on a LEiKA CAMBRIDGE STEREOSCAN- 430i at the Department of Biology, Trieste University (Zingone ef a!., 1990). Community diversity and dominance were measured by the species richness (Margafef index), the Pielou's evenness index (J), the Shannon-Weaver index (H) and the Simpson index (L). The two quantitative indices (Shannon-Weaver index and Simpson index) were cal­culated from relative abundances, using the STAD1V program (Ganis, 1991). Elisabetia SDRiGOlT I et a!.: DIATOM ASSEMBLAGES IN COASTAL SHALLO W WATERS A T THE WATER - ..., 191-2(32 Fig. 2: Hydrological parameters recorded at the bottom layer during the sampling months at the control (a) and at the mussel cultures (b) site. Filled squares; temperature (°C); filled rhombus: salinity (PSU). SI. 2: Hidrološki parametri', izmerjeni v pridnenem sloju v mesecih vzorčenja na referenčni postaji (a) in na lokaciji nasadov užitnih klapavic (b). Črni kvadrati: temperatura (°C); črni rombi: slanost (PSU). RESULTS The investigated stations were characterised, at the bottom layer, by comparable salinity values between the considered months. The temperature values showed a similar increasing trend during the samplings at the two stations, with a strong increase of about 6°C from May to June (Fig. 2). At the mussel culture station (st. M), values of tem­perature slightly higher than at the control station (st. B) were recorded at all times. The temporal pattern of the cellular abundance was quite similar between the months at the stations. The average cell density values at the reference site (st. B) were generally higher than at the mussel culture site (st. M). An average absolute maximum was recorded at the stations in June, respectively with 509333 cells \A at station M and with 1495500 cells I"1 at station 8. Con­versely, a minimum was recorded in October with mean values of 22333 cells I"1 at station M and 37500 cells I"1 at station B respectively (Fig. 3). Fig. 3: Cell density at the control (B) and at the mussel culture (M) site during months of sampling. Standard deviations obtained from three replicates are indicated by error bars. SI. 3: Gostota celic na referenčni postaji (B) in na lokaciji nasadov užitnih klapavic (M). Označene so standardne deviacije srednjih vrednosti, izračunane iz treh ponovitev. The size classes that appeared with constant and significative frequency were mainly of "a", "b" and "c" classes. The smallest (< 20 (jm) and 21-70 pm size groups increased from May to October, whereas the medium size diatoms (71-120 pm) indicated an evident decrease. The other size classes ("d" and "e") occurred with irregular and very low frequency during all exam­ined period (Fig. 4). The diatom assemblages were mostly characterised by the prevalence of epipelic species with "raphe sys­tem"; these motile diatoms varied in mean frequency from 49.7% to 96.7% at station M and from 70.7% to 98.0% at station 8 (Fig. 5). The epipsammic growth forms, consisting of diatoms attached firmly to the sub­strate, appeared with a mean relative frequency always higher at the mussel cultures site in respect to the refer­ence site, reaching the highest frequency (49.8%) in July. Living planktonic species were constantly present at both locations with a variation ranging from 0.5% to 3.8% at station M and from 0,2% to 14.2% at station B. Nevertheless, the highest mean frequency (14.2%) of CHsabetia SORICOTTI e( il : DfATO M ASS EM BLACKS IN COASTAL SHALLOW WATERS ATTH E WATER - .... 191-202 Fig. 4: Relative abundance of size groups at the control (B) and at the mussel cultures (M) site. All values are means of three replicates. SI. 4; Relativna abundanca velikostnih razredov na referenčni postaji (B) in na lokaciji nasadov užitnih klapavic (M). Vse vrednosti so srednje vrednosti treh ponovitev. planktonic species was recorded at the control station (st. B) in May (Fig. 5). The two sampling sites were characterised by the presence of the following epipelic species: Cylin­drotbeca closterium (EHR.) REIMANN & LEWIN, Bacil­laria paxillifera (O. F. MULL.) HENDEY and Cyrosigma acuminatum (KUNTZ.) RABENHORST (Figs. 6 a-f). At the control station, C. closterium showed the highest relative abundance in June with the mean value of 82.05%, while B. paxillifera and G. acuminatum reached the highest values in July (mean RA 16.37% and 18.16%). The relative abundance of these three species was followed by a progressive decrease during the other months (Tab. 1, Fig. 7). At the mussel culture station, comparable relative abundances were noted. In particular C. closterium reached its maximum in May (mean RA 84.15%), while B. paxillifera and G. acuminatum reached it in August (mean RA 19.25%) and September (mean RA 19.88%) respectively. In spite of the constant presence of B. paxillifera and G. acuminatum at both stations during the sampling period (May-October), C. closterium was never found on either site (Tab. 1, Fig. 7). These species may be considered characteristic of the diatom-assem­blages living at the water-sediment interface, in order to determine the features of the community, even if they Fig. 5: Change in life forms frequency (%) at the control (B) and at the mussel cultures (M) site. All values are means of three replicates. SI. 5: Spremembe v frekvenci življenjskih oblik (%) na referenčni postaji (B) in na lokaciji nasadov užitnih klapavic (M). Vse vrednosti so srednje vrednosti treh ponovitev. are not necessarily the most abundant species. These communities may be even defined by merely one of the characteristic species. The epipsammic forms detached with large amounts were mainly characterised by the genera Amphora and Navicula. Particularly at station M, Amphora was the prevalent genus in July (mean RA 48.15%) with the highest percentage of A. ostrearia De BREBiSSON (mean RA 46.98%; Tab. 1, Fig. 5). The diatom communities at the water-sediment inter­face showed similar values in species richness (Margalef index) at both sites. The absolute maximum was re­corded in September (3.4) at station B and in July (3.3) at station M, whereas the absolute minimum was noticed in August (1.5) at station B and in May (1.3) at station M. The evenness measures ranged from 0.2 to 0.8 at the reference site, while at the mussel site they ranged from 0.3 to 0.8. Pielou's index values showed a similar tem­poral pattern to H-values, indicating that abundance was more evenly distributed among the species from May to October. The species abundance was unevenly distri­buted in June (0-2) and in May (0.3) at B and M sites re­spectively, as indicated by the very low Pielou's index values. A better abundance distribution among the spe­cies was evident in September at station B and in August at station M with the corresponding values near to one. Fig. 6: Micrographs by inverted light and scanning electron microscope of following characteristic species: Cylin­drotheca ciosferium (a-b), Bacitlaria paxiilifera (c-d) and Cyrosigma acuminatum (e-f). Scale bar (a, c and e) = 33 pm; (b, d and f) = 10 pm. SI. 6: Posnetki invertnega svetlobnega mikroskopa in elektronskega mikroskopa značilnih vrst: Cylmdrotheca clostcrium (a-b), Bacillaria paxiilifera (c-d) in Cyrosigma acuminatum (e-f). Merilo (a, c in e) = 33 pm; (b, d in f) ~ 10 pm. ElisabaKa 5DR1GOTTI el o!.: DIATOM ASSEMBLAGES IN COASTA L SHALLO W WATERS AT THE WATER - 191-202 11» B 80 ­« • 1 8 Sncilferiti IwrtSilfirv. 1 f Q CyUwlroihccv chslenun-. f B Gyrojiffnn a&trnimthtm J 411 • J I 20 ••••••< ! 0 . H.issai. M 1 B,!11.3,1-=^ A S O Fig. 7: Relative abundance of characteristic species at the control (B) and at the mussel cultures (M) site. All frequency values are means of three replicates. SI. 7: Relativna abundanca značilnih vrst na referenčni postaji (B) in na lokaciji nasadov užitnih klapavic (M). Vse vrednosti so srednje vrednosti treh ponovitev. The Shannon-Weaver's diversity (IT) index varied be­tween 0.9 and 3.0 at site B, whereas at site M the same index varied between 0.8 and 2.7. The H-values showed an increasing trend from May to October at both sites. At the two stations the highest diatoms community biodi­versity (H) was reached at different times. In particular at station B, the Shannon-Weaver index was equal to 3.0 in September, while at the other station the highest value was recorded in August (H = 2.7). Inversely, the lowest H-index values were recorded in june (H = 0.9) and in May (H = 0.8) at sites B and M respectively. The Simpson index values showed an evident op­posite temporal trend in view of the trend observed for the Shannon-Weaver index. The lowest Simpson index values were reached in September at st. B (L = 0.07) and in August at st. M (L = 0.09), the months in which the highest H-values were revealed. As these quantitative indices are inversely correlated, this corresponds with the absence of any dominant species. In addition, the highest L-index values (0.67 at st. B in june and 0.71 at st. M in May) together with the lowest H-index values were registered in the same months, indicating a clear dominance of some species (Tab. 2). DISCUSSIO N The between-site differences in totaf cell density in­dicated that the diatoms community at the water-sedi­ment interface, affected by the mussel cultivations, was always less abundant than the one observed at the refer­ence site. The diatom assemblage under the suspended mussel cultures seemed to be limited in growth, in spite of the enhanced sedimentation of organic material and furthermore active nutrients regeneration at the water-sediment interface (Klump & Martens, 1981; Rizzo, 1990; Hatcher el a!, 1994; Svane & Setyobudiandi, 1996). O n the other hand, the sediments are usually considered an inexhaustible source of inorganic nutri­ents for benthic algae so that nutrient limitation should not arise (Sundback, 1986; Sundback & Snoeijs, 1991). The lower diatom abundance, at the mussel site, could be explained by considering the presence of the other limiting factors. As already seen by several authors (Sundback, 1986; Paterson, 1986; De jonge & Colijn, 1994), light intensities may be considered a limiting factor for microphytobenthos, which can survive shad­ing for long periods and recover rapidly when light conditions improve. The mussel site, although placed at a lower depth than the other one (st. B) and also condi­tioned by higher bottom temperature, might be unfa­vourable to photo synthetic activity, presumably for a diminished fight irradiance at the bottom layer. This limitation might be due to the shading effect of the floating mussel systems and the continuous release and biodeposition of organic matter (faeces and pseudofae­ces) from the water column to the bottom layer. As far as the incident light intensity reaching the water sediment interface is concerned, Barranguet (1997) highlighted, in a recent study on microphytobenthic primary produc­tion in a mussel culture, that the incident light percent­ages lower at a mussel station than at a reference station are a limiting factor for microphytobenthos production. Besides, enriched sediments by biodeposition are usu­ally inhabited by a quantitatively rich associated fauna consisting of both infauna in the sediment and attached epifauna on the shells (Svane & Setyobudiandi, 1996). In addition, direct underwater observations showed that the soft-bottom under the investigated mussel cul­tures was covered by living mussels and shells that had fallen down from the rafts. This modified substrate was an optimal support of adhesion for the settling of epibenthic communities. This community, known as O ­R-M (Ophiothrix, Reniera and Microcosmus genus) community (Stachowitsch & Fuchs, 1995), was largely made of mobile and sessile filter or suspension feeders that had aggregated in the form of the so-called multi-species clumps (F. Aleffi, pers. comm.). Therefore, the diatom community living at the water-sediment interface below the mussel site might be subject to a major seiec­ Elisatwas 50RIGOTTS el a!,: DIATOM ASSEMBLAGES IN COASTAL SHALLOW WATERS AT THE WATER - .... 191-20? Diatom taxon Control station (st. ß) Mussel culture station (st. M) M j J A S O M j I A S O Amphora coffeaeformis (Agardh) Kürzing 0,2 0,2 - 0,3 0,9 1,3 1,4 0,9 0,9 2,3 0,6 3,7 Amphora ostrearia de Brébisson - - 0,2 - 2,9 0,4 - - 47,0 11,7 1,2 0,7 Amphora ovalis (Kützing) Kützing 0,1 - 0,1 0,3 2,6 2,2 0,2 0,4 0,3 - 1,2 0,7 Amphiprora etat a Külztng - - - - - - - 0,2 - - - Amphiprora paludosa W. Smith - - 0,0 - - - - - 0,1 - - - Bacillariapaxillifera (O. F. Müller) Hendey 1,6 5,2 16,4 10,2 12,0 6,7 0,7 8,6 3,8 19,2 2,8 4,5 Caropylodiscus sp. - 0,0 1,2 0,3 - 2,7 - 0,5 - - - 2,2 Cocconeis placentuia Ehrenberg - - - - - - - - 0,3 - - - Cocconeis sp. - - - - 0,3 0,4 - - - - - 1,5 Cydotella sp. - - 0,1 - - - - - - - - - Cylindrotbeca ciosterium (Ahrenberg) Reimann et lewin 59,0 82,1 37,9 43,3 0,9 - 84,2 53,0 0,5 7,0 2,4 - Cymbella. cfr. cistula (Hemprich) Grunow - - 0,3 - - - - - - - - - Cymbella sp. - - - 0,8 1,4 0,9 - 0,1 0,2 - 0,3 - Diploneis bombus Ehrenberg - 0,0 - 0,3 - 0,9 - - - - 0,3 - Diphneis crabro Ehrenberg - - - - - - - 0,2 - - - - Diphneis ovalis (Hälse) Cleve 0,3 0,0 0,9 1,9 2,0 - 0,5 0,6 0,3 - 0,6 6,0 Diphneis smilhii (de Brébisson) Cleve 0,2 0,0 0,2 - 1,4 0,9 0,4 - 0,3 1,9 0,6 - Diphneis sp. - 0,0 • 0,3 - - - - - - Eunotia sp. - - - - - 9,8 - - - - - - Grammatophora sp. - 0,0 - - - - - - 0,3 - - - Gyrosigma acuminatum (Kützing) Rabenhorst 14,8 3,3 18,2 6,2 6,9 4,0 1,6 13,9 0,8 8,5 19,9 3,7 Gyrosigmafasciola (Ehrenberg) Griffith et Henfrey 0,1 - 0,8 - 0,6 - - 0,2 4,6 - - - Gyrosigma macrum W. Smith - 0,0 0,4 - 0,6 1,3 - 0,4 4,5 1,4 2,4 1,5 Gyrosigma obliquum (Grunow) Boyer - - - - - - - - 0,4 - - - Licmophora gracilis Grunow 0,0 - - - - - - - - - Licmophora sp. - - - - - 0,4 - - - - - 0,7 Melosira moniliformis (O. F. Müller) Agardh - 0,1 0,3 - 14,6 7,1 1,8 3,5 1,1 - 3,1 3,0 Navícula cancellata Donkin - - - - 0,3 - - - - - - - Navícula cfr. directa (W. Smith) Ralfs 1,3 1,0 2,3 4,3 8,9 5,3 0,7 2,5 2,7 4,7 3,1 - Navícula s!mutans Donkin 0,1 0,1 0,8 - 2,6 4,0 - 0,4 0,3 - - Navícula sp. 1 2,6 1,5 0,8 7,3 2,3 4,0 3,2 1,6 0,3 5,2 4,6 29,1 Navícula sp. 2 2,0 0,3 1,5 16,4 5,4 21,3 1,5 1,1 5,0 12,2 35,5 15,7 Navícula sp. 4 - - - - - - - - - 0,9 - - Navícula sp. (tubo) 0,0 - - - - - - - - Nitzschía angulada (A. Schmidt) W. Smith - 0,1 0,9 - 2,6 - - - 1.1 - 1,2 - N!Uschia gracUis Hantzsch 0,3 0,8 - - 0,4 - 0,5 0,4 0,5 - Nitzschía irresoluta Hustedt 0,3 0,5 0,2 - 1,4 0,9 - - 2,0 1,4 1,5 2,2 Nitzschka lanceola Grunow - 0,1 0,3 - - 0,4 0,1 - - - - Nitzschía longissima (de Brébisson) Ralfs - 0,1 0,2 - 0,6 0,9 - 0,3 0,1 - - - Nitzschía hrenzíana Grunow 0,3 0,9 0,8 0,3 1,4 0,4 0,2 0,2 2,7 3,8 1,2 - Nitzschía obtusa (A, Schmidt) W. Smith - 0,1 0,6 - 0,6 - - - - - - - Nitzschía cfr. patea (Kützing) W. Smith - - - - 1,1 - - - - - - - Nitzschía pandurt jormií Gregory 0,2 - 0/1 - - - - 0,1 0,2 - - 2,2 Nitzschía recta Hantzsch 0,3 0,8 1,0 0,8 0,3 2,7 0,5 1,8 2,4 1.9 1,8 2,2 Nitzschía sigma (Kützing) W. Smith 0,4 1,1 8,6 1,3 5,1 4,0 - 2,4 2,5 4,7 1,8 4,5 Nitzschíasigrna var. íntercedens Grunow - - - - - - 0,1 1,6 - 0,3 - Nitzschíasigrna var. sígrriatella Grunow • - - - - - - 0,5 1,4 - - - Niteschía sigmoidea (Ehrenberg) W. Smith 0,1 0,1 0,5 - 6,6 - - 1,1 '! ,4 1.4 1,2 - Nitzschía tryblioneila Hantzsch 1,1 0,8 0,9 3,0 3,7 7,6 0,5 1,5 3,1 4,2 5,5 8,2 Nitzschía vermicularis (Kützing) Grunow - 0,7 0,5 - 4,6 - - 1,4 1,4 0,9 1,2 - Pinnularia cfr. leptosoma Grunow 0,2 - - - 0,3 - - 0,3 - - - - Pinnularia viridis(Nitzsch) Ehrenberg - 0,0 - - - - - 0,3 - 0,5 - - Pinnularia sp. - - 0,3 - - - - - - - Pteurosigma aestuaríi (de Brébisson) W. Smith - - - - 0,9 - 0,3 - - - 0,7 Pteurosigma angulatum (Quekett) W. Smith - - - - 0,3 - - - - - Pteurosigma formosum W. Smith - - 0,4 - - - - 0,1 0,5 0,5 - - Pteurosigma minutum Grunow - - - - - - - - 0,3 - - - EUsaten a SORICOTT ! eta l: LHAÎOM ASSEMBLAGES !N COASTAL SHALLO W WATERS AT THE WATER - ..., T9I-20 2 Proboscia ahta (BrighiweSI) Sundström - 0,0 - - - - - - - - - - Rhopabdia gibba (Ehrenberg) 0 . Müller 0,1 0,0 - - 0,3 1,3 - 0,2 - 0,6 - Surirella sp. - 0,7 - - - - 0,3 0,1 - - - Syncdra sp. - - - - - 3,1 - - - - - - Thatassiosira eccentrica (Ebrenberg) Cleve 0,2 0,0 - - 0,3 0,9 0,7 - 0,4 0,9 0,3 2,2 Thalassiosira sp. 14,2 0,1 0,3 2,4 1,! 1,3 1,6 0,3 0,5 3,8 3,7 1,5 Triceratium sp. - - - - 0,3 - - - - - - . Tropidoneis lepidoptera (Gregory) Cleve - - 0,3 - - - - - - - 0,3 3,0 Tropidoneis longa Cleve - - 0,3 - - - - - - - - - Tropidoneis cfr. longa Cleve - - - - 2,0 - - - 2,2 - - - Tropidoneis sp. 0,2 0,0 - 0,3 0,9 1,3 - 0,3 2,0 0,5 0,6 - Tab. 1; List of diatom taxa with mean relative abundance value (%). Tab. 1: Seznam diatomejskih vrst in njihove relativne vrednosti abundance (%), Site 8; Date Mean cell densities Number of Species richness Pielou's index 0) Shannon-Weaver Simpson index (L) (cells L-1} species evenness index (H) M 331000 24 1,8 0,4 1,4 0,39 ) 1496000 36 2,5 0,2 0,9 0,67 J 334000 36 2,8 0,6 2,1 0,21 A 62000 18 1,5 0,7 1,9 0,24 S 58340 38 3,4 0,8 3,0 0,07 O 37500 32 2,9 0,8 2,9 0,08 Site M: Date Mean cell densities Number of Species richness Pielou's index (J) Shannon-Weaver Simpson index (L) (cells L'1) species evenness index (H) M 94670 16 1,3 0,3 0,8 0,71 J 509300 36 2,7 0,5 1,9 0,31 J 185200 41 3,3 0,6 2,4 0,24 A 35500 24 2,2 0,8 2,7 0,09 S 54500 29 2,6 0,7 2,4 0,18 O 22330 22 2,1 0,8 2,5 0,13 Tab. 2: Summary tables of statistic indices calculated from relative abundance, using the STADIV program. Tab. 2: Pregled statističnih indeksov, izračunanih iz relativne abundance z uporabo STADIV programa. tive pressure by benthic invertebrates (Asmus, 1982; was not recorded from September on; the overlying Admiraal eta!., 1983, De Jong ef al., 1994). water appeared homogeneous and remained constantly The highest total abundances showed by the diatom unstratified during the ensuing months, in addition, few assemblage at the water-sediment interface in June were days before October sampling, an aggravation of the probably due to the sudden increase in temperature in meteorological conditions (characterised by low baro­the study area. As already demonstrated, changes in metric pressure, increasing precipitation and wind in-temperature seem to be correlated with the increase of tensity) was recorded (M. Celio, pers. comm.). Therefore rnicrophytobenthic density, until blooming begins at the alt these physical variations could act as a disturbance water-sediment interface (De Jong & Admiraal, 1984; phenomenon in diatom community monitored, as al-Deliavalle et al., 1993; De Jong & De jonge, 1995; ready seen by Deliavalle ef al. (1993) for a fouling Weiker & Nichetto, 1996). community of the Gulf of Trieste. The relative minimum in diatoms abundance accord-Besides, due to some previous studies (Hudon & ing to our results in October might be related to physical Bourget, 1983; Deliavalle et al., 1993) we also know variations, such as light attenuation and vertical insta-that physical disturbance, in diatom assemblage, leads bility of the overlying water due to the active mixing of usually to a fail in highly motile and chain forming spe­the winds, cooling and mechanical stirring, in fact, cies in diatom community. This observation would be in water column stratification that was established in May agreement with our results concerning growth of life ElisabeHa SDRSGOTTI eíüí.: DIATOM ASSEMBLAGES IN COASTAL SHALLO W WATERS AT THE WATER . ..., 191-302 forms. In fact, in fall, the epi pel ic species reached their minimum frequency favouring the epipsammic growth forms development, presumably better adapted to exis­tence under unfavourable conditions (such as low light intensity or very fast current) and continual disturbance (Miller etal., 1987; Paterson, 1989). However, the presence of epipelic species at the water-sediment interface is probably linked to the verti­cal active migrations towards the sediment surface that take place whenever the sediment is disturbed, finding a way out into the overlying water. Besides, a highly stimulated photosynthefic activity at the sediment sur­face may in itself function as a selective pressure on the microalgal community by selectively transporting away easily suspendible species (Sundback & Snoeijs, 1991). Conversely, when the conditions at the water-sediment interface are not yet favourable, owing to chemical and physical variations, these motile diatoms are able to take cover in the sediment. On the other hand, De jonge (1985) highlights how the capacity of epipelic diatoms to migrate could be interpreted as a survival strategy to prevent populations from being resuspended. These considerations, which were eventually confirmed by other authors (Paterson, 1986; De jonge & Van Den Bergs, 1987; Happey-Wood & Jones, 1988; Delgado ef a/., 1991; De jong & De Jonge, 1995), would explain the decrement in epipelic species frequency, proceeding from the summer stability to the fall unfavourable situ­ation. Adversely, the epipsarnmic species, firmly bound to the substrate with persistent mucilaginous attachment structures, would be found at the water-sediment inter­face only after stirred up by hydrodynamic forces. This would explain their higher abundance at the end of summer and early autumn, periods characterised by an increasing instability at the bottom layer (M. Celio, pers. comm.). Besides, the epipsarnmic percentage values, greater at the mussel culture station than at the reference station, might be a result of the changed substrate con­ditions which, for the presence of the mussel shells and their harvesting remains, could act as an additional ad­hesion support. Similar results were already obtained in the recent work by Barranguet (1997), in which he highlighted how under the mussel culture area the mi­crophytobenthos is composed mainly of diatoms, with a high abundance of epiphytic forms. The planktonic species, which can be found at the water-sediment interface, are usually the consequence of the sinking phenomena involving either the vegeta­tive cells at the end of the blooming period or the resting cells (non-growing diatom cells) (Smayda, 1971; Smeta­cek, 1985). During our investigation, these diatoms ap­peared to be slightly more abundant at the reference site than at the mussel culture site, suggesting that phyto­plankton biomass is probably influenced by the pre­sence of these filter feeders (Frenchelte et a!., 1989; Asmus & Asmus, 1991). Large amounts of planktonic diatoms were particularly evident in May, at the reference site, following the spring diatom bloom at the subsurface layer (S. Cok, pers. comm.). The temporal increasing pattern of the smallest and medium-small diatoms could be related to the corre­sponding increase in temperature at the bottom layer. The specific growth rate, [i, is a convenient parameter for characterising the growth potential of a species as a function of environmental variables and it provides a means of comparing one species with another. Among the environmental factors, temperature is the main vari­able that can affect the growth rate of diatoms. Each species show a maximum specific growth rate, probably genetically determined, at its temperature optimum. Changes in temperature affect the specific microalgal growth rate, inducing slight shift within both the plank­tonic and benthic communities. In addition, larger cells grow at a slower rate than small-celled species of both pennate and centric diatoms. Therefore, small size dia­toms respond more quickly to a sudden rise in tempera­ture with respect to the larger ones. A benthic diatom community can modify its composition, especially within size classes, when changes in temperature occur (Eppley, 1977). In natural environment, small size dia­toms can have precedence over the larger ones and can characterise the whole community (Malone et ai., 1993). As regards species composition, the diatom com­munities described in this study did not show substantial differences, as confirmed by similar richness values. These diatom assemblages are typical of muddy sedi­ment in the Gulf of Trieste, and comparable to those found by Welker & Nichetto (1996) in deeper offshore waters in the same area. Since variation in diatom species composition is a good indicator of the environmental condition (Snoeijs et a!., 1990), the large amounts of epipsarnmic species A. ostrearia (only at the mussel culture site) suggest that this species seems to prefer enriched sediments with higher organic content. On the other hand, A. ostrearia was often found, in large quantities, in oyster-bed cul­ture (Maestrini & Robert, 1987), perhaps favoured by photoheterotrophic growth at low light intensities (Admiraai eta/., 1984; Sundback, 1986). Although the density of benthic diatoms is appar­ently not correlated with nutrient availability at the bot­tom layer, it seems to be influenced to a certain extent by the changes in temperature (De jong & Admiraai, 1984; Sundback, 1986; Sundback & Snoeijs, 1991; Deilavaile et a/., 1993; De Jong & De }onge, 1995; Welker & Nichetto, 1996). However, the species C. closterium seems positively affected by nutrient avail­ability. In fact, it reached the maximum abundance value in early summer, period characterised by highest ammonium and phosphate concentration at the bottom layer (S. Predonzani, pers. comm.). This is in agreement Eíisabeila SORÍCOTT t ef nI.: DIATO M ASSEMBLAGES IN COASTA L SHALLO W WATERS A T THE WATER - .... W-2Ü 2 with previous observations, in which highly motile C. closterium cells were able to benefit from the inorganic nutrient enrichment and laboratory experiments that, also showed their capability to increase its growth rate with higher salinity and temperature (jong & Admiraal, 1984; Sundback & Snoeijs, 1991). The between-site variation of diversity was small, but slightly lower diversity at the mussel culture site might have been caused by a combination of several factors, such as the occurrence of intense mechanical distur­bance by grazing, shading effect, high organic content, etc. The slight decrease in diversity that according to some previous studies occurred near the shore could perhaps be interpreted as an indicator of stress, such as from organic pollution (Pielou, 1975). Decrease in di­versity has also been observed in benthic diatom com­munities under eutrophic conditions (Sundback, 1984). Although there are few studies that overlook diatom communities diversity of mud substrates, the Shannon-Weaver's index values obtained during our investiga­tions were comparable to those reported for diatom communities of sandy substrates (Sundback & Snoeijs, 1991). The Simpson index values also confirmed the net dominance of C. closterium, above all in early summer, at both sites. CONCLUSION S The assemblages of diatoms at the water-sediment interface were at both investigated stations composed mainly of epipelic species belonging to the medium size group, represented by the following characteristic spe­ cies: C closterium, B. paxillifera and G. acuminatum. Regarding the possible effects of the presence of the suspended mussel cultures on the two diatom commu­nities, no evident variations were noted during the study regarding the species composition as underlined by Margalef index, and temporal pattern. The only disturb­ing effect due to the presence of mussel rafts, which also causes a between-site dissimilarity, was noted in algal cell density. In spite of a rather similar biodiversity index value (Shannon-Weaver index) within both stations, the lower diatom abundance observed at the mussel site might be due indirectly to their presence. Among all the disturbance factors, such as shading effect, biodeposi­tion and grazing by benthic animals, the latter is the only one that does not affect the species composition of the diatom assemblages. A light attenuation at the bot­tom layer, generally, can favour some shading-adapted species. On the other hand, the continuous deposition of organic matter (faeces and pseudofaeces) may select some species characterised by a photo heterotrophic metabolism. Suspension and filter feeders invertebrates are not able to select the desirable diatom species, as food source, so the grazing effect on the community at the water-sediment interface is relevant only for the cell abundance. Therefore it can be assumed that the diatom com­munity under the suspended mussel cultures is limited in cell density due to a larger grazing activity by the rich associated fauna, drawn to higher availability of organic matter, and by filter-feeders of epibenthic O-R-M com­munity. KOPIČENJ E DIATOME J N A VMESN I PLAST I VODNI H USEDLI N V OBALNI H VODA H TRŽAŠKEG A ZALIV A (SEVERN I JADRAN ) Elisabetta SDRIGOTTi, Vittorio BARBARIOL & Cbiara WELKER Morski biološki laboratorij, IT-34010 Santa Croce, Trst, Via Auguste Piccard 54 POVZETEK Resuspendirani mikrofitobentos, ki sestoji predvsem iz bentoških diatomej, igra pomembno vlogo v proizvajanju O2 kot hrane za peiagične in bentoške prehranjevalce na vmesni plasti vodnih usedlin v morskem obrežnem okolju. Da bi preverili morebitne spremembe v kopičenju diatomej na vmesni plasti vodnih usedlin zaradi tam domujočih suspendiranih kultur užitnih klapavic, je bilo opravljenih več raziskav na dveh izbranih postajah v Tržaškem zalivu. Mikroskopske analize so pokazale, da so za obe postaji, kjer so bile zabeležene predvsem epipelične vrste srednje velikosti, značilne vrste Cylindrotheca closterium, Bad Ilaria paxillifera in Gyros igma acuminatum. Statistične ana­lize niso pokazale kakih večjih razlik glede biotske pestrosti teh združb. Pa vendar so bile diatomeje, živeče pod kulturami užitnih klapavic, manj številne, če jih primerjamo s tistimi na referenčnih postajah, nemara zaradi pojavljanja bentoških prehranjevalcev, ki jih je privabila večja razpoložljivoist organskih snovi zaradi biodepozicije. 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