Original scientific article	UDK 597:591.91(262.32)
Received: 2012-10-29
VERTICAL DISTRIBUTION OF SOFT BOTTOM MACROZOOBENTHOS IN THE GULF OF TRIESTE (NORTHERN ADRIATIC SEA)
Lisa FARESI, Nicola BETTOSO & Ida Floriana ALEFFI Osservatorio Alto Adriatico, ARPA FVG, I-34127 Trieste, Via A. La Marmora 13, Italy E-mail: lisa.faresi@arpa.fvg.it
ABSTRACT
The macrozoobenthos is commonly considered to be the main agent of bioturbation in shallow water, and responsible for the enhancement of the interchange of both dissolved and gaseous species between the pore waters and the upper water column. As reported by several authors, this basic phenomenon is usually investigated by describing the vertical distribution of macrobenthic organisms. For this purpose, a site located in the central part of the Gulf of Trieste (depth = 21 m) was selected and, during 1999/2000,9 core samples were collected in triplicate to a maximum depth of 20 cm. The vertical distribution of the macrozoobenthic communities was described by applying the bioturbation activity index (BA). The vertical distribution of macrofauna showed a clear decrease in both number of taxa and abundance when moving downcore. The BA suggested that surficial, detritus-feeding polychaetes have a pivotal role in the bioturbation of muddy sediments, whereas suspension-feeding molluscs are also important if the only top layer is considered. Taking into account the overall results, it can be stated that the BA index application based on the core sampling technique, could be a useful tool in describing bioturbation in the northern Adriatic soft bottom sediments.
Key words: macrozoobenthos, vertical zonation, bioturbation index, feeding guilds, Gulf of Trieste, Adriatic Sea
DISTRIBUZIONE VERTICALE DEL MACROZOOBENTHOS DI FONDO MOBILE NEL
GOLFO DI TRIESTE (ALTO ADRIATICO)
SINTESI
Il macrozoobenthos viene comunemente considerato il maggiore agente dei fenomeni di bioturbazione nelle acque costiere, il quale facilita gli scambi della sostanza disciolta e gassosa tra le acque interstiziali e la sovra-stante colonna d'acqua. Per tale motivo, questi fenomeni di scambio possono essere indagati in via preliminare attraverso la descrizione della distribuzione degli organismi bentonici lungo il profilo sedimentario. A tale scopo è stato selezionato un sito di studio nella parte centrale del Golfo di Trieste. Durante il periodo I999-2ÜÜÜ sono stati condotti 9 campionamenti, considerando un adeguato numero di repliche (3 repliche per campionamento), alla massima profondità di 2Ü cm nel sedimento. In questi campioni è stata descritta la distribuzione verticale del macrozoobenthos e inoltre è stato applicato un indice di bioturbazione (BA). La distribuzione verticale della macrofauna indica un chiaro decremento degli organismi dagli strati superficiali verso quelli più profondi, sia in termini di taxa presenti che di abbondanza degli individui. L'applicazione dell'indice di bioturbazione ha fatto rilevare che i policheti detritivori di superficie possono assumere un ruolo principale nella bioturbazione dei sedimenti fangosi dell'area centrale del Golfo di Trieste, cui fanno seguito i molluschi sospensivori nei soli strati superficiali. Pertanto l'applicazione di tale indice, anche accoppiato alla tecnica di campionamenti con il caro-taggio, potrebbe risultare un utile strumento per descrivere la bioturbazione nei fondi mobili dell'Alto Adriatico.
Parole chiave: macrozoobenthos, zonazione verticale, indice di bioturbazione, categorie trofiche, Golfo di Trieste,
Mare Adriatico
INTRODUCTION
One of the main factors affecting the species composition within a macrobenthic community is the nature and quality of the substratum (Gray, 1974). It is well established that when fine sediments prevail, the presence of an interstitial fauna is greatly inhibited due to the high packing of the substratum, poor water circulation and, as a consequence, low oxygen content. Conversely, medium and fine sands are characterised by the presence of an abundant fauna, which shows several adaptive strategies (Gray, 1981). The fauna plays a pivotal role in physical phenomena such as bioturbation through building tubes, constructing burrows, feeding pits, transports sediments, thus enhancing the exchange of dissolved (nutrients, trace element etc.) and gas phases between the sea bed and water column. In addition, some abiotic factors (e.g., sedimentation rates, quality and quantity of organic matter, OM) can influence the trophic structure, abundance and biomass of the macrobenthic community and the pattern of colonisation through the sediment layers (Pearson & Rosenberg, 1978; Gray & Mirza, 1979; Warwick, 1986; Marsh & Tenore, 1990; Dauer & Alden, 1995; Flach & Heip, 1996; Dauer, 1997; Dauwe et al, 1998). In particular, the importance of OM becomes evident when it increases and generates more biomass and density of the benthic organisms; under these conditions, dystrophic events - such as hypoxia or anoxia and the strong and prolonged pycnocline of the water column - cause massive reductions and/or elimination of the benthic fauna (Simonini et al., 2004) and important changes in both the physical and chemical characteristics of the top sediment layers (Heip et al., 1995).
To date, knowledge about bioturbation is mainly related to the consequences and the influence on abiotic factors such as physical-chemical parameters, whereas the role of vertical distribution of macrozoobenthic organisms in the sediment has scarcely been investigated. This work provides a first analysis of the vertical zonati-on of the macrozoobenthos in the muddy bottom sediments of the Gulf of Trieste, followed by the application of the bioturbation activity index (BA).
MATERIAL AND METHODS Study area
The Gulf of Trieste is located at the northwestern part of the Adriatic Sea, covers an area of about 600 km2 and reaches a maximum depth of about 25 m in its central part. As reported in Ogorelec et al. (1991), 10 % of the total area has a depth < 10 m. The water circulation system, which is affected by the action of both winds (ENE) and tides (average and spring ranges of 0.5 and 1 m, respectively), is anticlockwise and acts on deep-water layers, which flow permanently at 2-3 cm s-1. Wind-driven superficial currents differen-
tiate the uppermost water mass, down to a depth of about 5 m, flowing anticlockwise with easterly winds and clockwise with westerly winds (Stravisi, 1983). The average salinity ranges between 33 to 38 at the surface and 36 to 38.5 at the bottom (Stravisi, 1983; Cardin & Celio, 1997). The Isonzo River represents the primary freshwater input, with an average annual flow rate estimated at the river mouth (period 1998-2005) of 91.2 m3 s-1 (1.1-665.9 m3 s-1; Comici & Bussani, 2007). The riverine discharge shows significant seasonal variations with two typical flood events: a relatively long spring maximum (March-May) and a shorter, but more intense, autumn maximum (September-November), when the rate of flow can exceed 2,500 m3 s-1 (RAFVG, 1986). The annual water temperatures range from 8 to 24 °C and from 8 to 20 °C at the surface and bottom, respectively. Tidal amplitude is about 1.5 m, which is the highest of the Mediterranean Sea.
Fig. 1: Gulf of Trieste and sampling station AA1. Sl. 1: Tržaški zaliv in vzorčevalna postaja AA1.
The sedimentation rate is mainly controlled by river input rather than by marine currents (Brambati & Cata-ni, 1988). Measurements based on 210Pb determinations estimated it to be 1.84-2.1 mm a-1 in the mid-Gulf (station AA1, Ogorelec et al, 1991; Covelli et al, 2001), and up to 2.5 mm a-1 adjacent to the river mouth (Ogorelec et al., 1991). The soft bottom composition is not homogeneous and varies from sands with patches of beach rocks to mud, predominantly detrital (Brambati et al, 1983). The widespread benthic biocoenoses of the Gulf belong to the DC (Détritique Cotier), DE (Détritique Envasé) and VTC (Vases Terrigenes Cotieres) biocoenoses (Orel & Mennea, 1969; Solis-Weiss et al, 2004).
Three main natural factors influence the composition, evolution and persistence of marine life in the Gulf of Trieste: strong winds (mainly Bora), stratification of the water column, leading to occasional hypoxia and/or anoxia events, as well as occasional mucilage production (Solis-Weiss et al, 2001).
This study was carried out at one station (AA1) located in the middle of the Gulf of Trieste (45° 39' 48" N, 13° 35' 42" E) at a depth of about 21 m (Fig. 1). The main solid phase and chemistry characteristics of AA1 are reported in Emili et al. (2011). The sediment texture consists of clayey silt (< 63 |jm, from 87 to 98 %; Hines et al, 2000), whereas C , and C account for about 5.1 and 1.17 %,
tot	org	'
respectively. Previous studies reported the occurrence of hypoxia and anoxia events, such as mucilage aggregate deposition at the bottom (Aleffi et al., 1992).
Sampling
Benthic samples were collected in 1999 (February, June and August) and 2000 (January, June, July, August, October and December) using a KC Haps bottom corer (KC-Denmark, Silkeborg, Denmark) with a polycarbonate sample tube (i.d. = 13.3 cm; sample area = 127 cm2). In order to assess the vertical distribution, three replicate samples were randomly collected. After sediment collection, each core was sectioned in slices (0-1, 1-3, 3-5, 5-10 and 10-20 cm). The sediment was sieved through a 0.5 mm mesh and subsequently stored in 4 % formaldehyde following standard methods (Holme & Mclntyre, 1984). Faunal samples were sorted and identified at the lowest possible taxonomical level.
Analyses
In order to analyse the structure of the communities, several univariate techniques are commonly employed. Among these the abundance, the number of taxa and the diversity index were previously used (Shannon-Wiener diversity index, H', on log2 basis; Shannon & Weaver, 1949). The feeding guild analysis was based on Fauchald & Jumars' (1979), Bachelet's (1981) and Macdonald's et al. (2010) definitions.
Bioturbation activity (BA) was estimated by means of the scoring system outlined in Swift (1993) and Grehan et al. (1994). Briefly, the scores were assigned to all taxa on the basis of individual feeding mode (0-4), mobility (0-3) and burrowing capability (0-4); the maximum value of 11 represents the species characterised by the greatest potential capacity to cause sediment bioturbation.
Tab. 1: Minimum, maximum and mean value of abundance, number of taxa and H' of a core. Tab. 1: Minimalna, maksimalna in povprečna vrednost števila osebkov, števila taksonov in H' jedra vrtine.
	Abundance	No. Taxa	H'
Min	52	15	3.26
Max	270	50	4.65
Mean	163	32	3.98
RESULTS
A total of 1,471 organisms belonging to 90 taxa were identified in the 27 sampled cores. Table 1 displays minimum, maximum and mean values of abundance, number of taxa and H' in the cores. The most abundant taxa were polychaetes (57.8 %) followed by molluscs (24.4 %) and crustaceans (13.2 %), thus accounting together for the 95.4 % of the specimens. Other taxa such as
j= -H A	
	
L^__.	■ ECHINODERMATA
	BCRUSTACEA
lyiHiiiiiiiiiiiiiiiiiiini-1	BOTHERS □ POLYCHAETA
um—i	□ MOLLUSCA
n. individuals
n. taxa
Fig. 2: (A) Average value of abundance, (B) number of main taxa and (C) H' for each layer of the core. Sl. 2: (A) Povprečno število osebkov, (B) število najpomembnejših taksonov in (C) H' za vsak sloj jedra vrtine.
echinoderms, sipunculids, ascidians, anthozoans and nemertines were poorly represented. The polychaetes were also the dominant taxon in term of number of species (44 sp.), followed by molluscs (28 sp.) and crustaceans (11 sp.); together these constituted 91 % of the species.
The vertical distribution of macrofauna showed a clear decrease in both taxa and abundance moving downcore. At the 10-20 cm layer, only polychaetes were found, whereas echinoderms disappeared below 5 cm. H' dropped at the 10-20 cm layer (Fig. 2).
Feeding guilds were mostly represented by suspension and surface deposit feeders, both in terms of abundance and taxa, followed by carnivores, sub-surface deposit feeders and grazers. Suspension feeders were the dominant guild at the top. Surface deposit feeders showed similar abundance in 0-1 and 1-3 cm layers, but dropped in 3-5 cm. Carnivores and sub-surface deposit feeders showed a constant decrease down core, whereas grazers were scarcely represented and disappeared below 10 cm (Fig. 3).
As reported in Table 2, twelve species represented about 50 % of the total abundance. Among these, some suspension feeders such as the bivalves Corbula gibba and Venerupis aurea and the amphipods Ampelisca spp. were mostly abundant at the topmost layers (0-3 cm). C. gibba and Ampelisca spp. completely disappeared after 3 cm while V. aurea was found until 5 cm (Fig. 4). The
Tab. 2: Total abundance (A), frequency (F), percentage (%) and cumulative percentage (% cum) of the twelve most abundant species. Feeding guilds (F. guilds): SDF = surface deposit feeders, SF = suspension feeders, G = grazers, sSDF = subsurface deposit feeders, C = carnivores. Taxa: Cru = crustaceans, Mol = molluscs, Pol = polychaetes.
Tab. 2: Skupno število osebkov (A), pogostost (F), odstotki (%) in kumulativni odstotki (% cum) dvanajstih vrst z največjim številom osebkov. Prehranski cehi (F. guilds): SDF = površinski detritivori, SF = suspenziofagi, G = strgalci, sSDF = podpovršinski detritivori, C = karnivori. Taksoni: Cru = raki, Mol = mehkužci, Pol = mno-goščetinci.
F. guilds	Taxa	Species	A	F	%	% cum
SDF	Pol	Prionospio cirrifera	162	7	11.0	11.0
SF	Cru	Ampelisca spp.	135	9	9.2	20.2
SF (SDF)	Mol	Mysella bidentata	135	7	9.2	29.4
SF	Mol	Corbula gibba	57	9	3.9	33.2
SDF	others	Aspidosiphon muelleri	51	7	3.5	36.7
G	Mol	Hyala vitrea	44	7	3.0	39.7
SF	Mol	Venerupis aurea	36	7	2.4	42.2
sSDF	Pol	Maldane glebifex	32	6	2.2	44.3
C	Pol	Lumbrineris gracilis	27	4	1.8	46.2
SDF	Pol	Laonice cirrata	24	3	1.6	47.8
C	Pol	Lumbrineris latreilli	23	5	1.6	49.4
C	Pol	Eunice vittata	22	5	1.5	50.9
0	10	20	30	40	50	60
n. individuals
Fig. 3: Average abundance of feeding guilds for each layer of the core.
Sl. 3: Povprečno število osebkov prehranskega ceha za vsak sloj jedra vrtine.
bivalve Mysella bidentata and several surface deposit feeders species such as the polychaetes Laonice cirrata, Prionospio cirrifera and the sipunculid Aspidosiphon muelleri muelleri gradually decreased with depth, until 10 cm. M. bidentata and P. cirrifera peaked the abundance in 1-3 cm. Taking into consideration the carnivores, the polychaetes Lumbrineris gracilis and Eunice vittata gradually decreased from the top till 5 and 20 cm respectively, whereas the Lumbrineris latreilli distribution did not show any significant trend along the se-
10-20
niAmpelisca spp. hC. gibba HIV. aurea
10
n. individuals
15
20
□	A. muelleri
□	L. cirrata ■ P. cirrifera EM. bidentata
10	15
n. individuals
20
25
10-20
0L. latreilli HL. gracilis HE. vittata
2	4	6
n. individuals
10
4	6
n. individuals
Fig. 4: Average abundance of the dominant species for each layer of the core. Sl. 4: Povprečno število osebkov prevladujoče vrste za vsak sloj jedra vrtine.
dimentary sequence. The polychaete Maldane glebifex mostly represented sub-surface deposit feeders within the upper 10 cm without any clear decreasing gradient. Conversely, the grazer gastropod Hyala vitrea showed an increase in abundance from the top until 10 cm, below where it disappeared.
The BA score assigned to each taxon is shown in Table 3. BA estimated for feeding guilds showed a mean of 8 for SSDF, 5 for SDF and 2 for remaining guilds; with regard to main taxa the average was 4 for polychaetes and crustaceans, 3 for echinoderms and others and 2 for molluscs.
The trend of BA downcore showed a significant average decrease for every feeding guild. Moreover, it seems to be clear that SDF represents the most important class, which contributes to the index value in the whole of the layers investigated. Finally, BA calculated on main taxa indicates that polychaets are the main responsible for the bioturbation phenomena, whereas the contribution of molluscs seems to be important only in the upper layer (Fig. 5).
DISCUSSION
The vertical zonation of benthic organisms within the sediment has long attracted the attention of marine biologists. Since macrobenthic and meiobenthic bioturbation is an important phenomenon in surficial marine sediments (Cullen, 1973), even geologists and geoche-mists have emphasised the necessity for information on patterns of vertical distribution in the marine benthos (Huys et al, 1986).
Several authors have described in the past the different macrozoobenthic communities in the Gulf of Trieste (e.g., Orel & Mennea, 1969; Fedra et al, 1976; Fedra, 1978; Orel et al., 1987), often in connection with anoxia conditions (e.g., Stachowitsch, 1984; Aleffi et al., 1992; Orel et al, 1993; Stachowitsch & Fuchs, 1995). Recently, the status and distribution of macrobenthic communities was discussed through the application of GIS techniques (Solis-Weiss et ai, 2001) and a faunistic, biocoenotic and ecological survey on soft bottom macrozoobenthos was conducted in the southern part of the Gulf (Mavric et al, 2010). Apart from some studies concerning the
vertical distribution of meiobenthos in the southern area of the Gulf (Vrišer, 1983-1984), there are very few data regarding the vertical zonation of soft bottom macrozoobenthos.
The development of the macrobenthic community at station AA1 was followed from 1990 to 2001. This area was affected by anoxia in September 1990, when oxygen (O2) concentrations at the bottom dropped to 0.37 cm3 l-1. In 1991 a mucilage event occurred during the summer and the lowest value of O2 was recorded in October (1.28 cm3 l-1), whereas in 1992 the minimum value was measured in August (3.46 cm3 l-1). During the period 1999-2001, no O2 concentration at the bottom below 2.00 cm3 l-1 was recorded, except in October 2001, when 1.69 cm3 l-1 was found. In addition, mucilage aggregates occurred in June 2000 (Bettoso et al, 2003). A clear difference was revealed between the benthic community of the investigated periods 1990-1993 and 1999-2001 at the same site of the present study. Average H' and J values were 1.89, 0.56 and 2.9, 0.79 in the for-
Fig. 5: (A) Average BA for feeding guilds and (B) main taxa in each layer of the core. Sl. 5: (A) Povprečen indeks BA za vsak prehranski ceh in (B) najpomembnejši taksoni za vsak sloj jedra vrtine.
mer and latter period, respectively. A remarkable result during the period 1990-1993 (characterised by frequent hypoxia-anoxia events) was the huge abundance of the suspension feeder bivalve Corbula gibba, which is a well-known indicator of environmental instability condition (Aleffi & Bettoso, 2000). From 1999-2001 and in the present study, the environmental conditions were more stable, without any noticeable chemical-physical stress. Accordingly, a notable decrease of the C. gibba dominance was detected. Similar trends were revealed for the polychaetes Maldane glebifex and Eunice vitta-ta, which were found to be dominant together with C. gibba in the early 1990s (Aleffi et al., 1996). In contrast, several species, never recorded or very scarce, were abundant in 1999-2001. Among these, the gastropod Aporrhais pespelecani, the sipunculid Aspidosiphon muelleri muelleri and the polychaetes Sternaspis scu-tata and Sthenolepis yhleni are the most important. Moreover, the constant presence of Atrina pectinata was also remarkable because this large bivalve seems to be very sensitive to low oxygen levels (Bettoso et al., 2003). During 1999-2001 the presence of Ampelisca spp. was also remarkable. Moodley et al. (1998) observed that C. gibba abundance may be negatively affected by the presence of large population of Ampelisca. The authors consider the two species as suspension feeders that compete for food supply. Moreover, they suggested that dense aggregations of Ampelisca, which build tubes in the more superficial layers of sediment, could both occupy space and influence the structure and density of the community down core (Dauvin, 1988; Dauvin & Bellan-Santini, 1990; Massamba N'Siala et al, 2008).
Despite a low number of species detected with this sampling method, the percentage abundances of the most abundant taxa in the cores were comparable to those recorded by grab sampling (Aleffi et al, 1996; Bettoso et al., 2003). Unfortunately, no recent data for this sampling station were available, although no stress condition due to oxygen depletion was recorded in the last decade.
The infaunal macrozoobenthos distribution indicates that the colonisation of the sediment occurs at the maximum depth of 20 cm, according to Moodley et al. (1998) for the northwestern Adriatic Sea; however, as already observed by Simonini et al. (2004) in the area close to the Adige and Po River mouths, it essentially involves the first 5 cm, whereas a sharp decline of macrofauna occurred below 10 cm.
The vertical distribution of feeding guilds followed a well-defined pattern in which the suspension and surface deposit feeders are mostly found in the surface layers, whereas sub-surface deposit feeders and carnivores-omnivores followed below. Among suspension feeders Ampelisca spp., C. gibba and Venerupis aurea were not encountered in deeper layers as also observed by Moodley et al. (1998). In contrast, the bivalve Mysella bidentata was not confined to a specific layer. These
Tab. 3: Bioturbation activity (BA) assigned to taxa as outlined in Swift (1993) and Grehan et al. (1994): M = mobility, F = feeding mode, B = burrowing capability. For explanations (F. guilds, Taxa except Ech = echino-derms) see Table 2.
Tab. 3: Bioturbacija (BA), kot so jo posamičnim taksonom pripisali Swift (1993) ter Grehan et al. (1994): M = mobilnost, F = način prehranjevanja, B = zmožnost kopanja rovov. Za razlago (Prehranski cehi, Taksoni razen Ech = iglokožci) glej Tabelo 2.
M	F	B	BA	F. guilds	Taxa			Species
2	l	l	4	u	othe		rs	Nemertea ¡ndet.
l	O	l	2		Mo			Nassarius reticulatus
l	O	l	2		Mo			Cylichnina umbilicata
O	O	O	O		Mo			Akera bullata
l	O	O	l		Mo			Scaphander lignarius
l	4	l	e		Mo			Antalis inaequicostata
S	O	2	s		Po			Glycera sp.
O	O	O	O		Po			Ophiodromus flexuosus
O	l	2	S		Po			Sigambra tentaculata
O	O	2	2		Po			Svll ¡dae ¡ndet.
2	O	O	2		Po			Nereis lamellosa
2	O	O	2		Po			Nereis rava
2	O	O	2		Po			Nereis sp.
2	O	2	4		Po			Micronephtys sp.
O	O	O	O		Po			Sthenolepis' yhleni
O	O	O	O		Po			Nothria conchylega
O	O	O	O		Po			Eunice vittata '
2	O	2	4		Po			Lumbrineris gracilis
2	O	2	4		Po			Lumbrineris latreilli
2	O	2	4		Po			Lumbrineris tetraura
l	2	O	S		Cru			Gammaridae ¡ndet.
O	O	O	O		Cru			Athanas nitescens
O	O	O	O		Cru			Eualus cranchii
O	O	O	O		Cru			Processa macrophthalma
l	l	l	S	u	Mo			Hyala vitrea
l	O	O	l		Mo			Calyptraea chinensis
l	O	O	l		Mo			Capulus ungaricus
l	2	O	S	U- Q 'SI	Mo			Euspira pulchella
O	S	l	4		Mo			Nucula nucleus
O	S	l	4		Mo			Nuculana pella
O	2	l	S		Mo			Thyasira flexuosa
l	O	O	l		Mo			Mysella bidentata
O	2	l	S		Mo			Tellina distorta
O	S	l	4		Mo			Abra alba
O	S	l	4		Mo			Abra prismatica
l	O	l	2		Mo			Thracia pubescens
2	S	O	s		Po			Laonice cirrata
2	S	O	s		Po			Polydora flava
2	S	O	s		Po			Prionospio cirrifera
2	S	O	s		Po			Prionospio malmgreni
2	S	2	Z		Po			Magelona sp.
2	S	O	s		Po			Poecilochaetus serpens
S	S	2	ß		Po			Aricidea sp.
S	S	2	ß		Po			Levinsenia gracilis
S	S	2	s		Po			Paradoneis lyra
S	S	2	s		Po			Paraonidae indet.
l	2	l	4		Po			Chaetozone setosa
l	2	l	4		Po			Tharyx killariensis
l	2	l	4		Po			Cirratu l ¡dae ¡ndet.
2	l	2	s		Po			Ophe l ¡¡dae ¡ndet.
O	2	l	S		Po			Ampharete acutifrons
O	2	2	4		Po			Melinna palmata
O	2	O	2		Po			Amphitrite variabilis
O	2	O	2		Po			Terebe l l ¡dae ¡ndet.
2	l	2	s		Po			O l ¡gochaeta ¡ndet.
O	2	O	2		ot	hers		Aspidosiphon muelleri muelleri
2	2	2	e		ot	hers		Sipunculus nudus
S	4	4	ll		Cru			Cal l ¡anass¡dae ¡ndet.
2	2	2	e		Ec			Amphiura chiajei
O	2	O	2		Ec			Amphipholis squamata
O	O	O	O	U- 'Sl	othe		rs	Cerianthus membranaceus
O	O	O	O		Mo			Anomia ephippium
l	2	l	4		Mo			Loripes lacteus
l	2	l	4		Mo			Myrtea spinifera
l	O	O	l		Mo			Kellia suborbicularis
O	O	l	l		Mo			Acanthocardia paucicostata
O	O	l	l		Mo			Dosinia lupinus
O	O	l	l		Mo			Pitar rudis
O	O	l	l		Mo			Venerupis aurea
l	O	O	l		Mo			Corbula gibba
2	S	O	s		Po			Polydora sp.
2	S	O	s		Po			Sp¡on¡dae ¡ndet.
O	O	O	O		Po			Sabe l l ¡dae ¡ndet.
O	2	O	2		Po			Spiochaetopterus costarum
l	2	O	S		Cru			Cumacea ¡ndet.
O	O	O	O		Cru			Ampelisca spp.
S	4	4	ll		Cru			Upogebia tipica
O	O	l	l		Ec			Leptopentacta elongata
2	S	2	Z	U- Q 'SI	Po			Cap¡tell¡dae ¡ndet.
2	S	2	Z		Po			Euclymene sp.
2	4	2	s		Po			Maldane glebifex
2	4	2	s		Po			Maldan¡dae ¡ndet.
2	l	2	s		Po			Sternaspis scutata
2	4	S	9		Po			Pectinaria auricoma
2	4	S	9		Po			Lagis koreni
indet.					Mo			Gastropoda ¡ndet.
					Mo			B¡valv¡a ¡ndet.
					Cru			Harpact¡co¡da ¡ndet.
					Cru			Tana¡dacea ¡ndet.
					Cru			Amph¡poda ¡ndet.
species are known to live in association with amphiu-rids or sipunculids (Hayward & Ryland, 1990), and their numbers are correlated in some areas (Ockelmann & Muus, 1978) but not in others (Rosenberg, 1995). The presence of a large fraction of the Mysella population in deeper sediment layers suggests that this bivalve is not restricted to filter feeding but can also feed on deposit particles (Rosenberg, 1995). Most Hyala vitrea specimens were deeper than 5 cm and this species could be a predator (Moodley et al, 1998, Koulouri et al. 2006) and not a grazer, as classified in Macdonald et al. (2010). Polychaetes such as Prionospio cirrifera, Eunice vittata and Lumbrineris latreilli were recorded along the whole sediment profile and these species are very common in the sandy and pelitic bottoms of the northern Adriatic Sea (Aleffi et al, 2003).
Lee & Schwartz (1980) suggested a scheme of guilds of macrofauna depending on individual modes of feeding, mobility and position in and or the sediment. In this study, the taxa were coded and scored using the scheme of Swift (1993). The sum of the scores for each taxon ranked indicate that the sub-surface deposit feeders Pectinaria auricoma, Pectinaria koreni and M. glebifex have the greatest bioturbatory effects over the largest depth range, whereas the suspension feeders C. gibba, V. aurea and Ampelisca spp., among the most abundant, have a modest bioturbation in the surface layer.
Considering the relative abundance of each species, BA indicated that SDF polychaetes have a pivotal role in
the bioturbatory activity in muddy sediment of the central area of the Gulf of Trieste, followed by SF molluscs if only the top layer is considered.
The results of this work suggest the following final remarks:
(1)	The muddy sediment of the investigated area (central Gulf of Trieste) is normally inhabited down to 20 cm depth, with the exception of a sporadic presence of some very large specimens;
(2)	The strata analysis of cores successfully investigated and explained the vertical zonation of macrofauna;
(3)	The application of BA index to estimate the degree of bioturbation could also be a useful tool when grab-sampling techniques is applied. The availability of a larger dataset and the calculation of BA derived using the grab technique could provide a more detailed depiction of bioturbation for the northern Adriatic soft bottom sediments.
ACKNOWLEDGEMENTS
The authors are very grateful to Dr. Alessandro Acquavita and Francesco Tamberlich for their valuable revision of the manuscript. This study was carried out as a part of the European Community INTERREG II Italy -Slovenia project.
VERTIKALNA DISTRIBUCIJA MAKROBENTOŠKIH ORGANIZMOV MEHKEGA DNA V TRŽAŠKEM ZALIVU (SEVERNO JADRANSKO MORJE)
Lisa FARESI, Nicola BETTOSO & Ida Floriana ALEFFI Osservatorio Alto Adriatico, ARPA FVG, I-34127 Trieste, Via A. La Marmora 13, Italy E-mail: lisa.faresi@arpa.fvg.it
POVZETEK
Makrobentoški organizmi običajno veljajo za najpomembnejše povzročitelje bioturbacije v plitvih vodah, spodbujajo pa tudi premešavanje raztopljenih spojin in plinov med porno vodo in zgornjim slojem vodnega stolpca. Kot je omenilo že več avtorjev, se ta temeljni pojav najustrezneje opiše s preučevanjem vertikalne distribucije makrobentoških organizmov. V ta namen smo izbrali lokacijo v osrednjem delu Tržaškega zaliva (globina 21 m) in v letih 1999/2000 vzeli 9 vzorcev jedra vrtine v treh primerkih z največjo globino 20 cm. Pri opisu vertikalne distribucije makrobentoških združb smo upoštevali bioturbacijski indeks (BA). Iz vertikalne distribucije makrofavne je jasno razvidno, da se z upadom globine manjša tako število taksonov kot število osebkov. Indeks BA nakazuje, da imajo ključno vlogo pri bioturbaciji muljastih sedimentov površinski detritivorni mnogoščetin-ci, medtem ko v zgornjem sloju odigrajo pomembno vlogo suspenzivorni mehkužci. Če vzamemo v obzir vse rezultate, lahko trdimo, da je upoštevanje indeksa BA ob uporabi metode vzorčenja jedra vrtin lahko uporabno orodje pri opisu bioturbacije sedimentov na mehkem dnu severnega Jadrana.
Ključne besede: makrobentoški organizmi, vertikalna conacija, bioturbacijski indeks, prehranski cehi, Tržaški zaliv,
Jadransko morje
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