Gloria Massamba N'siala

Valentina Grandi

Mirko Iotti

Giuseppe Montanari

Daniela Prevedelli

Roberto Simonini
email: roberto.simonini@unimore.it

Responses of a Northern Adriatic Ampelisca-Corbula community to seasonality and short-term hydrological changes in the Po river

Keywords: macrozoobenthos, temporal scale, river regime, seasonality, Northern Adriatic Sea

During the 20 th century, increased nutrient inflow has increased the frequency and intensity of eutrophication events in the North-western Adriatic Sea. More recently, evidence of a reduction in the Po river flow and a recovery of the benthic environment were reported. We examined the role played by Po river regime and seasonality in determining the temporal pattern of macrozoobenthos variation: samples were collected from a site located 10-12 km off Cesenatico during two years from October 2004 to September 2006, when the mean Po discharge remained exceptionally low. 88 taxa were found, and the community was dominated by few species: Ampelisca diadema, Lumbrineris latreillii, Corbula gibba Aricidea claudiae, Levinsenia gracilis and Nucula nucleus account for about 80% of total abundances. The density of these species exhibited a marked seasonal variability. Moreover, the total abundance and the density of the sensitive species, A. diadema, were negatively affected by several disturbance events (hypoxic conditions, frequent storms and an anomalous winter flood) that occurred between November 2005 and March 2006. The reduction of C. gibba and the increase of A. diadema densities observed in the investigated period, and verified recently by other authors, could be related to the reduction in river inputs as a consequence of climatic changes.

Introduction

The Northern Adriatic Sea is a semi-enclosed and very shallow marine region. Along the North-western side, particularly near the Emilia-Romagna coastline, the terrigenous inputs of the Po and other rivers and the counter-clockwise circulation of the Adriatic Sea favour the occurrence of significant seasonal haline stratification and a high trophic state [START_REF] Russo | Decadal Climatic Anomalies in the Northern Adriatic Sea Inferred from a New Oceanographic Data Set[END_REF][START_REF] Russo | Meteorological and oceanographic condition in the northern Adriatic Sea during the period June 1999-July 2002: influence on the mucilage phenomenon[END_REF][START_REF] Montanari | The currents of Emilia-Romagna coastal strip during the period 1995-2002[END_REF]. Eutrophication of the northern Adriatic Sea, marked by acute events such as red tides, extensive mucus aggregates, anoxic bottom conditions and mass mortalities of benthic animals has been known for centuries (Crema et al., 1991 and reference therein). Yet, after World War II, the Northern Adriatic Sea has been subjected to an increased nutrient inflow from the Po and other rivers, which would have enhanced primary production near the surface and oxygen demand near the bottom. As a consequence, the periodicity and extent of these eutrophication events increased, becoming almost annual between 1970s and 1990s [START_REF] Justic | Long term eutrophication of the Northern Adriatic Sea[END_REF][START_REF] Crema | Long term eutrophication effects on macrofaunal communities in the Northern Adriatic Sea[END_REF][START_REF] Vollenweider | Eutrophication, structure and dynamics of a marine coastal system: result of ten-year monitoring along the Emilia-Romagna coast (Northwestern Adriatic Sea)[END_REF][START_REF] Bucci | A year monitoring of marine eutrophication on the coast of Ancona[END_REF][START_REF] Marchetti | Quantification of the phosphorous and nitrogen loads in the minor rivers of the Emilia-Romagna coast (Italy)[END_REF][START_REF] Justic | Changes in nutrient structure of river-dominated coastal waters: stechiometric nutrient balance and its consequences[END_REF]. These events badly affected not only tourism and fisheries, but also caused a dramatic change in the macrozoobenthic communities, which are widely-used ecological indicators of the "health" of marine and estuarine ecosystems (e.g. [START_REF] Borja | Assessing the environmental quality status in estuarine and coastal systems: Comparing methodologies and indices[END_REF]. Comparisons between data from collections carried out off the coast of Ravenna in [1934][1935][1936] [START_REF] Vatova | La fauna bentonica dell'Alto e Medio Adriatico[END_REF] and in 1989, revealed that the "historical" fine-grain sand (Sables fins bien calibres, SFBC, [START_REF] Pérès | Nouveau manuel de bionomie benthique de la Mer Méditérranée[END_REF]) and coastal-terrigenous mud (Vases terrigènes côtières, VTC, [START_REF] Pérès | Nouveau manuel de bionomie benthique de la Mer Méditérranée[END_REF]) biocenoses had been replaced by assemblages characterized by an abundance of dominant, opportunistic species, such as the bivalve Corbula gibba, that are indicators of sediment instability, organic enrichment and anoxic conditions, [START_REF] Crema | Long term eutrophication effects on macrofaunal communities in the Northern Adriatic Sea[END_REF][START_REF] Cavallini | Evoluzione dei popolamenti macrozoobentonici dell'Adriatico Settentrionale. I. Analisi dei dati storici di Aristocle Vatova (1934-1936[END_REF]. During the 1990s, it was demonstrated that this change in the structure and composition of the macrozoobenthos was general feature over the whole western coastal area of the Northern Adriatic Sea [START_REF] Simonini | Organic enrichment and structure of the macrozoobenthic community in the northern Adriatic Sea in an area facing Adige and Po mouths[END_REF].

However, the high biomass, species richness and diversity observed at most sites suggest the occurrence of less extreme conditions, compared with other highly-eutrophicated coastal areas, such as the Baltic Sea, the Gulf of Mexico and the Black Sea [START_REF] Moodley | Benthic activity in sediments of the northwestern Adriatic Sea: sediment oxygen consuption, macro-and meiofauna dynamics[END_REF][START_REF] Simonini | Organic enrichment and structure of the macrozoobenthic community in the northern Adriatic Sea in an area facing Adige and Po mouths[END_REF]. More recently, it appeared that the communities were evolving towards a higher degree of structural complexity: the establishment of dense populations of the fossorial amphipod Ampelisca diadema, which is considered as a bioindicator of environmental recovery, and an increase in diversity and evenness indices were reported [START_REF] Occhipinti-Ambrogi | Macrobenthos community structural changes off Cesenatico coast (Emilia Romagna, Northern Adriatic), a six-year monitoring programme[END_REF]. The considerable temporal variability in composition and density of the community of sandy and muddy bottoms of the Northern Adriatic was attributed to the additive effect of 1) inter-annual variation of the hydrographical regime of the Po river and coastal waters [START_REF] Occhipinti-Ambrogi | Macrobenthos community structural changes off Cesenatico coast (Emilia Romagna, Northern Adriatic), a six-year monitoring programme[END_REF]Simonini et al., 2005b) and 2) high seasonal variation in temperature and photoperiod [START_REF] Simonini | Organic enrichment and structure of the macrozoobenthic community in the northern Adriatic Sea in an area facing Adige and Po mouths[END_REF](Simonini et al., , 2005a[START_REF] Simonini | Recolonization and recovery dynamics of the macrozoobenthos after sand extraction in relict sand bottoms of the Northern Adriatic Sea[END_REF]. These factors can modulate biological events, playing an important role in determining the community structure and composition. In general, in temperate coastal areas, the abundance and/or diversity of benthic communities declines over the autumn and winter period, and recovers in summer, when the reproduction, recruitment and settlement of most species occurs [START_REF] Dauvin | Biologie, dynamique, et production de populations de crustacés amphipodes de la Manche occidentale. 1. Ampelisca tenuicornis (Liljeborg)[END_REF][START_REF] Rosenberg | Hypoxia, salinity and temperature as structuring factors for marine benthic communities in a eutrophic area[END_REF][START_REF] Salen-Picard | Responses of a Mediterranean soft bottom community to short-term (1993-1996) hydrological changes in the Rhone river[END_REF]Simonini et al., 2005a,b). The seasonal variations in sedimentation rates and freshwater and terrigenous organic inputs caused by floods and droughts can also affect the population dynamics of the different species, with major consequences at the community level [START_REF] Largier | Estuarine fronts: how important are they?[END_REF][START_REF] Grémare | Long-term comparison of macrobenthos within the soft bottoms of the Bay of Banyuls-sur-mer (northwestern Mediterranean Sea)[END_REF][START_REF] Salen-Picard | Fluctuations of macrobenthic populations: a link between climate-driven river run-off and sole fishery yields in the Gulf of Lions[END_REF][START_REF] Salen-Picard | Responses of a Mediterranean soft bottom community to short-term (1993-1996) hydrological changes in the Rhone river[END_REF][START_REF] Occhipinti-Ambrogi | Macrobenthos community structural changes off Cesenatico coast (Emilia Romagna, Northern Adriatic), a six-year monitoring programme[END_REF][START_REF] Labrune | Long-term comparison of soft bottom macrobenthos in the Bay of Banyuls-sur-Mer (north-western Mediterranean Sea): A reappraisal[END_REF].

These issues have become particularly relevant in recent years when, within a large scale climatic change scenario, an increase in sea water temperature and a drastic reduction in the Po river loads were observed [START_REF] Russo | Decadal Climatic Anomalies in the Northern Adriatic Sea Inferred from a New Oceanographic Data Set[END_REF][START_REF] Russo | Meteorological and oceanographic condition in the northern Adriatic Sea during the period June 1999-July 2002: influence on the mucilage phenomenon[END_REF][START_REF] Montanari | Eutrofizzazione delle acque costiere dell'Emilia-Romagna Rapporto annuale 2004. Agenzia regionale prevenzione e ambiente dell'Emilia-Romagna[END_REF]Montanari et al., , 2006[START_REF] Montanari | Eutrofizzazione delle acque costiere dell'Emilia-Romagna Rapporto annuale 2006. Agenzia regionale prevenzione e ambiente dell'Emilia-Romagna[END_REF].

In this paper we investigated the variation in structure and composition of macrozoobenthic communities sampled from October 2004 to September 2006 in the coastal area of Emilia-Romagna, Italy (Fig. 1). The data were integrated with information on 1) the Po and Northern Adriatic hydrographical regime, and 2) some chemical and physical variables of the water column and sediment properties. The aim was to identify the role played by environmental seasonal changes and the Po river regime in determining the short and medium-term pattern of macrozoobenthos variation, taking into account the community characteristics that had previously been described for the same area.

Materials and methods

Study area and hydrological regime of the Po river during the study period

The sampling activities were performed from October 2004 to September 2006 in the CEBEB (Cesenatico-Bellaria Bianco) area, which is a square 1x1 km sector used as a reference site in a previous investigation on the effects of dumping of dredged harbour sediments on benthic assemblages (Simonini et al., 2005b, Fig. 1). The CEBEB is located 10-12 km off the Cesenatico harbour at a depth of 11-13 m. The sea bottom is composed of infralittoral muds, with a high percentage of silt. The biocenoses living in these bottoms are characterized by high densities of surface-deposit feeder species, such as the ampeliscid amphipod Ampelisca diadema, the corbulid bivalve Corbula gibba, the lumbrinerid polychaete Lumbrineris latreillii and some small-sized spionid and paraonid polychaetes (Simonini et al., 2005b). It represented an ideal site to investigate the natural pattern of temporal variation of benthic communities because of the distance from the coast and from direct source of human disturbance (e.g. aquaculture, drilling platforms, dumping/dredging areas). Moreover, it is representative of the benthic ecosystems of the whole area between the Po river mouth and the promontory of Gabicce at a depth of 8-30 m (Simonini et al, 2005b). This area is strongly influenced by seasonal variation in the main current regime and river runoff [START_REF] Simonini | Organic enrichment and structure of the macrozoobenthic community in the northern Adriatic Sea in an area facing Adige and Po mouths[END_REF](Simonini et al., , 2005b;;[START_REF] Occhipinti-Ambrogi | Macrobenthos community structural changes off Cesenatico coast (Emilia Romagna, Northern Adriatic), a six-year monitoring programme[END_REF][START_REF] Russo | Meteorological and oceanographic condition in the northern Adriatic Sea during the period June 1999-July 2002: influence on the mucilage phenomenon[END_REF]. The Po regime is highly seasonal, with its maximum flow in spring and autumn and minimum in summer and 2006, 2400 m 3 s -1 ) (Fig. 1). The peaks in Po river flow often induce a severe reduction in the superficial water salinity, which can fall to less than 30 psu, and can cause water stratification phenomena. The effect of fresh-water inputs decreases with water depth and, at bottom level, the salinity ranged between 33-38 psu [START_REF] Montanari | The currents of Emilia-Romagna coastal strip during the period 1995-2002[END_REF].

The storms driven by the NE and SE winds (Bora and Scirocco), that frequently occur in the Northern Adriatic Sea from December to late March, could cause a significant stress to the benthic ecosystem due to sediment re-suspension and sedimentation phenomenon [START_REF] Matteucci | Fluxes of suspended materials in the north Adriatic sea (Po prodelta area)[END_REF][START_REF] Simonini | Recolonization and recovery dynamics of the macrozoobenthos after sand extraction in relict sand bottoms of the Northern Adriatic Sea[END_REF]. The storm intensity, estimated using the wave-height recorded at the AGIP (Azienda Generale Italiana Petroli) "GaribaldiC/Cervia B" oil rig (2 km SE to CEBEB), was at a maximum between late autumn and early spring. In fact, between November 2004 and April 2005, three severe storms (waves more than 3 m high) were registered. During the December 2005-March 2006 period, five storm events were recorded (Fig. 1). The data on the Po river flow and wave height were obtained from the annual report on the eutrophication of the Emilia-Romagna coastal waters [START_REF] Montanari | Eutrofizzazione delle acque costiere dell'Emilia-Romagna Rapporto annuale 2004. Agenzia regionale prevenzione e ambiente dell'Emilia-Romagna[END_REF](Montanari et al., , 2006[START_REF] Montanari | Eutrofizzazione delle acque costiere dell'Emilia-Romagna Rapporto annuale 2006. Agenzia regionale prevenzione e ambiente dell'Emilia-Romagna[END_REF] http://www.arpa.emr.it/Daphne/).

Sampling plan and data acquisition

Taking into account the high heterogeneity of the main biotic and abiotic variables, not only at inter-but also at intra-seasonal level [START_REF] Montanari | The currents of Emilia-Romagna coastal strip during the period 1995-2002[END_REF], in order to obtain an homogeneous (but not regular) distribution of sampling dates within seasons and ensure the representativeness of samples, we adopted a two-stage sampling strategy.

For each month (of each season) the date of sampling was randomly chosen and the time between two consecutive samplings ranged from 2 to 6 weeks. Because sampling times are not spaced at equal intervals, the inadequate estimation of the intra-season temporal variation should not be a problem [START_REF] Morrisey | Temporal variation in soft-sediment benthos[END_REF]Underwood, 1996).

Twenty-four sampling surveys were carried out using the oceanographic M/S ''Daphne II'' of ARPA (Regional Agency for Prevention and the Environment of Emilia-Romagna). The sampling for macrozoobenthos and sediment was carried out using a Box-Corer with a surface of 200 cm 2 . Seven replicates were collected at each month: six were used for the macrozoobenthos, one for granulometric analysis and Total Organic Carbon (%TOC). Each replicate was subdivided in two layers: superficial (0-5 cm depth) and deep (> 5 cm depth): only the superficial layer was considered because previous investigation had highlighted that the 90-95% of organism, in terms of abundance and biomass, occurred in the upper 5 cm of sediment [START_REF] Simonini | Organic enrichment and structure of the macrozoobenthic community in the northern Adriatic Sea in an area facing Adige and Po mouths[END_REF](Simonini et al., , 2005b)). The samples for macrozoobenthos analysis were processed through sieves with mesh size of 0.5 mm and the retained material was preserved in 4% CaCO 3 buffered formalin in seawater. The near-bottom chemical and physical data (temperature, salinity, pH, conductibility, dissolved oxygen, Chl a) were measured using a Hydronaut, model Ocean Seven316 CTD. Water transparency was obtained using the Secchi disk method [START_REF] Montanari | The currents of Emilia-Romagna coastal strip during the period 1995-2002[END_REF]. The granulometric and %TOC analyses were performed in accordance with the ICRAM (Istituto Centrale per la Ricerca scientifica e tecnologica Applicata al Mare) guidelines (http://www.icram.org/Acrobat/Libro/Metodologie_analitiche/metodologie_analitiche.h tm). The residual material obtained from the sample sieving was sorted in the laboratory, by separating the organisms according to phylum. The macrofauna was preserved in 70% ethylic alcohol. As a rule, the organisms were identified to species level: in a few cases the identification was limited to genus or family. The data from each sampling month were obtained by the sum of their respective replicates.

Data analysis

The degree of differentiation of benthic communities on year and seasonal scales was 2 nd ]). The three samplings performed in each season were considered as replicates (Date). This approach was chosen over an orthogonal Year x Season design because preliminary analyses performed according with the orthogonal design highlighted significant Season and/or Year effects, but no significant interaction between the two factors for any of the investigated variables, suggesting that the season effect was the same in the two years (data not shown). Moreover, the nested design allowed the direct comparison of the magnitude of variation on the two temporal scales [START_REF] Morrisey | Temporal variation in soft-sediment benthos[END_REF]. The critical values of the F tests performed according with the nested design were: F crit 1, 6, <0.05 =5.99 for the factor Year; F crit 6, 16, <0.05 =2.74 for the factor Season.

Multivariate analyses were performed with 2 nd -root transformed abundance data.

Outputs from non-metric multidimensional scaling (n-MDS) ordination models of the Bray-Curtis similarity matrix were obtained. The differentiation of communities on inter-annual and season scale was tested using a two-way nested ANOSIM (analysis of similarity) randomisation-permutation test. One-way ANOSIM was also performed to compare the communities from different seasons a posteriori. The % similarity or dissimilarity within/between groups (years or season within years), and the species making the greatest contribution to dissimilarity among years or seasons, were investigated using the similarity percentage breakdown procedure (SIMPER). The relationships between multivariate community structure and environmental variables were examined using the BIOENV procedure. The BIOENV compares the (rank) similarity matrices from biotic and abiotic variables and chooses that subset of environmental variables which provides a good match between them, which is expressed in term of a matching coefficient (the Spearman coefficient, s ) [START_REF] Clarke | A method of linking multivariate community structure to environmental variables[END_REF]. The multivariate analyses were carried out using PRIMER V6 software [START_REF] Clarke | Change in marine communities: an approach to statistical analysis and interpretation[END_REF]. The multivariate analyses were carried out using PRIMER V6 software [START_REF] Clarke | Change in marine communities: an approach to statistical analysis and interpretation[END_REF].The univariate analyses were performed by means of R (http://www.rproject.org/) and Microsoft Excel software.

Results

Environmental variables

Macrozoobenthos

Throughout the course of the investigation 10988 individuals, belonging to 88 taxa, distributed among 9 phyla were found (in order of abundance: arthropods, annelids, molluscs, nemertines, echinoderms, sipunculids, phoronids, cnidarians, echiurians, ascidians). The polychaete annelids were the most important group in term of taxa (47 taxa, 53% of the total), followed by the molluscs (25 species, mainly bivalves) and the arthropods (7 taxa). Yet, arthropods were the most important group in term of abundance with 5465 individuals; most of them (97%) being the amphipod Ampelisca diadema. The remaining 6 phyla were represented by 9 taxa and contributed marginally to the overall abundances.

The multivariate analyses performed on 2 nd -root transformed data highlighted that the macrozoobenthic communities varied among season, but most of the variability was observed between years (ANOSIM; Years: R=0.62, p<0.05; Season (Y) R=0.51, p<0.01). The Bray-Curtis dissimilarity between the two years was 48%, while the dissimilarity between seasons ranged from 31 to 38% in the first year and from 38 to 55% during the second year (SIMPER, Table 1). The MDS ordination plot relating to each sampling date showed 1) two main groups corresponding to the first and the second year, respectively, 2) several sub-groups corresponding to the year-season period of sampling: in particular the point referring to the 2 nd year winter, characterized by several disturbance events, segregated apart from the others (Fig. 3).

The BIOENV analysis, that relates macrobenthos structural changes to the variability of environmental parameters, highlighted that the correlation between the environmental and biological similarity matrix was rather low. The transparency (related to storm events and Po river flow), temperature (correlated with dissolved oxygen) and mean Po river flow were the combination of variables that gave the highest rank correlation ( =0.421, 2 nd root transformed abundances).

The SIMPER analysis highlighted that the community was characterized by the presence of a few dominant species typical of off-shore muddy bottoms. Only 12 species contributed at least 3% to the total abundances observed at each season, and they accounted for most of the dissimilarity between season and years. For most of the species, the minimum density was observed during the autumn and winter of the second year, which was the period characterized by the disturbance events, and this feature explains the low similarity between the 2 nd year winter and the other sampling times (Table 1-2, Fig. 34). Few species, such as the polychaete Nephtys hystricis, showed similar density throughout the investigation. Other species, such as the amphipod Pseudolirius kroyerii and the bivalve Nucula nucleus, were subjected to occasional demographic blooms (Table 2). Yet, most of the inter-annual and season variability of the community could be associated with the oscillation in the abundance of some of the most abundant and characteristic taxa such as Ampelisca diadema, Lumbrineris latreillii, Corbula gibba Aricidea claudiae, Levinsenia gracilis and N. nucleus. The first three species dominated the community and contributed to 65% of total abundances. Overall, the six species account for about 80% of total abundances (Table 2).

The analysis of the Spearman rank correlation between the main univariate indices, the density of the most important species and the environmental variables that gave the highest correlation between the environmental and biological similarity matrix (BIOENV), highlighted that: 1) the diversity and the number of taxa were not significantly correlated; 2) there were negative correlations between the Shannon Diversity Index, the total abundance and the density of A. diadema; 3) the density of A. ind.m -2 , and then declined in the following months (Fig. 4).

Also the lumbrinerid polychaete L. latreillii exhibited demographical variation with season. The highest densities were observed during summer (400-500 ind. m -2 ), the lowest in winter (60-180 ind. m -2 ) (Fig. 4).

The pattern of temporal variation of the paraonid polychaete A. claudiae was quite unusual (Fig. 4). At the beginning of the investigation, it was one of the most abundant taxa (more than 1000 ind. m -2 ). Subsequently, its density declined and a minimum of less than 20 ind. m -2 was found in February 2006. Then, the density increased during spring and fall during summer 2005 (ANOVA, log transformation: Year, F=9.5, p<0.05; Season, F=4.3, p<0.01).

The dynamic of the other paraonid polychaete L. gracilis exhibited seasonal trends with maximum densities during spring and minimum densities during autumn in both years (ANOVA, log transformation: Year, F=2.4, p>0.17; Season, F=3.5, p<0.05; Fig. 4).

The ANOVA did not highlight any significant Year and Season effect on the density of the bivalve N. nucleus (log transformation: Year, F=1.9, p>0.21; Season, F=1.6, p>0.21). From October 2004 to March 2006, the density of N. nucleus ranged from 25 to 170 ind. m -2 (with the exception of February 2006, when no specimens were found).

Then a demographic bloom was observed: the density peaked to more than 500 ind. m -2 in June 2004, then fell to 100 ind. m -2 in September 2006 (Fig. 4).

Discussion

Most of the previous investigations on the soft-bottom macrozoobenthos in the Northwestern Adriatic Sea adopted a sampling strategy with sampling intervals of three months or more [START_REF] Moodley | Benthic activity in sediments of the northwestern Adriatic Sea: sediment oxygen consuption, macro-and meiofauna dynamics[END_REF][START_REF] Prevedelli | Relationship of non-specific commensalism in the colonization of the deep layers of sediment[END_REF]Simonini et al., , 2005b)). In some cases, this approach can make it difficult to identify the seasonal patterns of variation in the benthic community [START_REF] Occhipinti-Ambrogi | Macrobenthos community structural changes off Cesenatico coast (Emilia Romagna, Northern Adriatic), a six-year monitoring programme[END_REF]. The sampling strategy adopted in the present study provides some insight into the patterns of temporal variability and the role of biotic interactions and abiotic factors in determining community structure and the population densities of the most important species.

The macrozoobenthic assemblages at the CEBEB site showed clear seasonal trends.

Densities rose sharply at the end of spring/begin of summer, peaked in summer and then strikingly declined, the lowest values being recorded during autumn and winter. Similar seasonal patterns were observed also at other North-western Mediterranean and Northern Adriatic shallow sites, and were relate to seasonal reproductive events, that were driven by the cyclical annual variation of most of the environmental variables [START_REF] Sardà | Seasonal dynamics of macroinfaunal key species inhabiting shallow soft-bottoms in the Bay of Blanes (NW Mediterranean)[END_REF]Simonini et al., 2005a). Multivariate and univariate analyses also highlighted significant inter-annual variability in the benthic community: in particular, the total abundance and the density of one of the most abundant taxa, Ampelisca diadema, were higher during the first year relative to the second year. The high interannual variability of North-western Adriatic sea benthic assemblages was observed also in previous investigations, and was attributed to several different perturbations, such as Po river floods [START_REF] Occhipinti-Ambrogi | Macrobenthos community structural changes off Cesenatico coast (Emilia Romagna, Northern Adriatic), a six-year monitoring programme[END_REF], hypoxic/anoxic conditions [START_REF] Moodley | Benthic activity in sediments of the northwestern Adriatic Sea: sediment oxygen consuption, macro-and meiofauna dynamics[END_REF][START_REF] Simonini | Organic enrichment and structure of the macrozoobenthic community in the northern Adriatic Sea in an area facing Adige and Po mouths[END_REF], thermal anomalies and severe storms (Simonini et al., 2005 a,b). Findings from a recent 6-years study suggested that the macrobenthic community in the Emilia-Romagna coastal area was mainly determined by the onset of very high Po river discharge values, which caused marked, short-term fall in the macrozoobenthos density [START_REF] Occhipinti-Ambrogi | Macrobenthos community structural changes off Cesenatico coast (Emilia Romagna, Northern Adriatic), a six-year monitoring programme[END_REF]. During the present study, the flow of the Po was very low with respect to historical data (Montanari et al., 2004[START_REF] Montanari | Eutrofizzazione delle acque costiere dell'Emilia-Romagna Rapporto annuale 2004. Agenzia regionale prevenzione e ambiente dell'Emilia-Romagna[END_REF](Montanari et al., , 2006). Yet, a series of stress events occurred during the autumn A. diadema at CEBEB highlighted that this species has a bivoltine cycle, with one winter-and one (or two) summer-generations [START_REF] Grandi | Distribuzione, ciclo biologico e comportamento dell'anfipode Ampelisca diadema, una possibile specie-test per saggi ecotossicologici su sedimenti marini[END_REF]. The increased temperature and the high primary productivity at the end of winter promoted the gametogenesis and the reproduction of the winter generation, which occurred between May and June. The descendants build up the short-living summer generation, which matured early and reproduced during the summer, when the maximum density were observed, and gave rise to the winter generation that survived until the next year [START_REF] Grandi | Distribuzione, ciclo biologico e comportamento dell'anfipode Ampelisca diadema, una possibile specie-test per saggi ecotossicologici su sedimenti marini[END_REF]. We have no experimental data on the effect of burial and hypoxia on this species, but some recent LC 50 96h test performed with several inorganic and organic toxicants highlighted that A. diadema is one of the most sensitive amphipods to stress and pollution [START_REF] Lera | Sensitivity of Ampelisca diadema (Crustacea: Amphipoda) to organic and inorganic toxicants in test of acute toxicity[END_REF] and many amphipod species are susceptible to oxygen deficiency [START_REF] Johansson | Behavioural response to gradually declining oxygen concentration by Baltic Sea macrobenthic crustaceans[END_REF][START_REF] Gray | Effects of hypoxia and organic. enrichment on the coastal marine environment[END_REF]. Moreover, Occhipinti Ambrogi et al. (2005) observed that the density of A. diadema declined after severe Po river outflows. It is possible that the stress condition that occurred between the first and second year would have caused a reduction in the number of adults in the winter generation and, subsequently, a lower recruitment into the summer generation in comparison to the previous year.

The water temperature and hydrological variables that varied according to season also affected the biological cycle of the bivalve Corbula gibba and the polychaetes Lumbrineris latreillii and Levinsenia gracilis. These species were characterized by seasonal variation with the same magnitude in the two years. Their recruitment took place in spring, probably as a consequence of environmental (temperature-photoperiod) stimuli, and then density declined during autumn. The stress events that took place during autumn 2005 and winter 2006 did not have an appreciable effect on these species, which can tolerate hypoxic conditions and high rates of sedimentation or sediment re-suspension [START_REF] Jensen | Increased abundance and growth of the suspension-feeding bivalve Corbula gibba in a shallow part of eutrophic Limfjord, Denmark[END_REF][START_REF] Diaz | Marine benthic hypoxia: a review of its ecological effects and the behavioural responses of benthic macrofauna[END_REF]Moodley et al, 1988;[START_REF] Makra | Benthic communities of the inner Argolikos Bay[END_REF].

The population dynamics of the polychaete Aricidea claudiae and the bivalve Nucula nucleus were quite unusual, because they were very abundant only at the beginning or at the end of the investigation, respectively. Perhaps, the exceptionally low Po river discharge and the lack of severe flood events during the investigation could have had a negative effect on A. claudiae: in fact, [START_REF] Salen-Picard | Fluctuations of macrobenthic populations: a link between climate-driven river run-off and sole fishery yields in the Gulf of Lions[END_REF] observed that this opportunistic short-lived species exhibited high short-term peaks in density a few months after Rhone flooding events in the Gulf of Lion. Most information on the biology of N. nucleus referred to Northern European populations (see [START_REF] Holmes | The respiration and hypoxic tolerance of Nucula nitidosa and N. nucleus: factors responsible for determining their distribution?[END_REF]. This species spawned on alternate years and adopted an "exploitative function strategy, i.e. maturation is maximal in the absence of nutritional limits" [START_REF] Holmes | The respiration and hypoxic tolerance of Nucula nitidosa and N. nucleus: factors responsible for determining their distribution?[END_REF]. If we assume that also the Northern Adriatic population exhibits a similar strategy, the peak in the abundance of N. nucleus observed in spring and early summer 2006 could be associated with a recruitment event that occurred in the second year only, favoured by the peak of primary productivity recorded during winter 2006. Further analyses on the life cycle, population dynamic and secondary productivity will be carried out in order to confirm the speculation and hypotheses on the effects of intra-and inter-annual variability of the abiotic condition on the most abundant and characteristics species at the investigated area.

Among the biotic factors, cases of biotic interaction between species have been reported

for the North-western Adriatic Sea [START_REF] Prevedelli | Relationship of non-specific commensalism in the colonization of the deep layers of sediment[END_REF]. The occurrence of interspecific competition between A. diadema and C. gibba was suggested by [START_REF] Moodley | Benthic activity in sediments of the northwestern Adriatic Sea: sediment oxygen consuption, macro-and meiofauna dynamics[END_REF], who observed that C. gibba abundance may be negatively affected by the presence of large populations of Ampelisca. They consider the two species as suspension-feeders, which can compete for food supply; moreover, they suggested that dense stands of Ampelisca, which build tubes in the more superficial layers of sediment, would occupy space and influence the structure and density of the community deeper down [START_REF] Dauvin | Biologie, dynamique, et production de populations de crustacés amphipodes de la Manche occidentale. 1. Ampelisca tenuicornis (Liljeborg)[END_REF][START_REF] Dauvin | An overview of the amphipod genus Haploops (Ampeliscidae)[END_REF]. Also, Occhipinti Ambrogi et al.

(2005) reported that A. diadema replaced C. gibba as the dominant species, especially during and after periods of low river discharge. Our results did not support the existence of negative effects of A. diadema on C. gibba. In fact we observed a positive correlation between the densities of the two species (Table 3). In both field and laboratory conditions, A. diadema and C. gibba did not segregate in the space, occurring randomly mixed in the sediment surface, and exhibited generalist trophic behaviour, acting as facultative suspension-or sediment-surface feeders [START_REF] Simonini | Organic enrichment and structure of the macrozoobenthic community in the northern Adriatic Sea in an area facing Adige and Po mouths[END_REF]Grandi et al., 2007, unpublished data). Moreover, the density of Ampelisca diadema in the Northwestern Adriatic Sea did not appear to be so high as to allow the bio-construction of the dense carpets observed in the North-eastern Atlantic, where the ampeliscid amphipods can reach density of an order of magnitude greater (Bellan-Santini and [START_REF] Dauvin | Biologie, dynamique, et production de populations de crustacés amphipodes de la Manche occidentale. 1. Ampelisca tenuicornis (Liljeborg)[END_REF][START_REF] Poggiale | Long-term dynamics of three benthic Ampelisca (Crustacea-Amphipoda) populations from the Bay of Morlaix (western English Channel) related to their disappearance after the 'Amoco Cadiz' oil spill[END_REF]. Manipulative experimentations are required to establish the occurrence and the magnitude of the inter-specific competition between the two species.

According with Occhipinti Ambrogi et al. ( 2005), the incidence of the 1 st and 2 nd order opportunistic species, which were dominant in the 1980s and early 1990s communities, seemed to be reduced in recent years. During the study period also, the average density of the bivalve C. gibba was three times lower relative to those observed from the 1980s until 2000, when dense populations with more than 1000-1500 ind. m -2 were extensively reported [START_REF] Crema | Long term eutrophication effects on macrofaunal communities in the Northern Adriatic Sea[END_REF][START_REF] Moodley | Benthic activity in sediments of the northwestern Adriatic Sea: sediment oxygen consuption, macro-and meiofauna dynamics[END_REF][START_REF] Prevedelli | Relationship of non-specific commensalism in the colonization of the deep layers of sediment[END_REF][START_REF] Simonini | Organic enrichment and structure of the macrozoobenthic community in the northern Adriatic Sea in an area facing Adige and Po mouths[END_REF]. On the other hand, the abundance of A. diadema, which was rare until the late 1980s (<5% of total abundances), seemed to increase starting from the mid 1990s until recent years, when densities of more than 2000-10000 ind.m -2 were observed (Simonini et al., 2005b;[START_REF] Occhipinti-Ambrogi | Macrobenthos community structural changes off Cesenatico coast (Emilia Romagna, Northern Adriatic), a six-year monitoring programme[END_REF]. If we assume that the contribution of inter-specific relationship between A. diadema and C. gibba is negligible, the decline of the opportunistic species in favour of the sensitive ones could be related to the variation of the Po and Adriatic Sea hydrographical regimes. A tendency towards a reduction in river inputs due to the decreases in rainfall and an increase in sea water temperature has emerged in recent years [START_REF] Russo | Decadal Climatic Anomalies in the Northern Adriatic Sea Inferred from a New Oceanographic Data Set[END_REF]Montanari et al., 2004[START_REF] Montanari | Eutrofizzazione delle acque costiere dell'Emilia-Romagna Rapporto annuale 2004. Agenzia regionale prevenzione e ambiente dell'Emilia-Romagna[END_REF]Montanari et al., , 2006)). This trend, which is generalized for the whole Mediterranean region and should be reinforced in the near future, as a consequence of climatic changes (4 th IPPC report, http://www.ipcc.ch/ipccreports/index.htm), could lead to two different scenarios. If no eutrophication events and hydrological disturbance occurred, the reduction of river flow could favour the occurrence of more stable physical conditions on the bottom and the establishment of benthic communities with a higher degree of structural complexity and/or a reduction of opportunistic-tolerant species in favour of more sensitive species [START_REF] Occhipinti-Ambrogi | Macrobenthos community structural changes off Cesenatico coast (Emilia Romagna, Northern Adriatic), a six-year monitoring programme[END_REF]. On the other hand, increased surface temperatures, altered circulation, and precipitation changes could increase the thermal stratification and amplify the frequency and extent of bottom water hypoxia or anoxia in late summer and autumn [START_REF] Russo | Decadal Climatic Anomalies in the Northern Adriatic Sea Inferred from a New Oceanographic Data Set[END_REF], with strong effects on community density and structure. If the beneficial impacts of the reduction of river flood prevailed, the establishment of communities at more mature successional stage would be favoured. Simonini et al., 2005b, modified) and Po river discharge in the period October 2004 -September 2006 (from Montanari et al, 2004[START_REF] Montanari | Eutrofizzazione delle acque costiere dell'Emilia-Romagna Rapporto annuale 2004. Agenzia regionale prevenzione e ambiente dell'Emilia-Romagna[END_REF]Montanari et al, , 2006)). In the Po river flow plot, the main storm events (wave height > 3 m) and the occurrence of hypoxic conditions are indicated by the grey bars and the asterisk, respectively.

  winter. During the period 1917-2006, the mean Po flow at Pontelagoscuro (44°55'16" N, 11°36'29"E) was about 1472 m 3 s -1 , with a peak of 9650 m 3 s -1 in October 2000. During the study period, the mean annual discharge of the Po river remained low (1 st year, October 2004-September 2005: 891 m 3 s -1 ; 2 nd year: October 2005-September 2006: 756 m 3 s -1 ) with exceptional minima of only 300-350 m 3 s -1 during the summers 2005 and 2006. The most important flow event was registered just after the beginning of the investigation (November 2004, max. 4600 m 3 s -1 ), and no other significant events were registered during the first year. During the second year, three minor discharge peaks were observed in autumn (October, November and December 2005, max. 2800 m 3 s -1 ) and there was an anomalous high flow event in winter 2006 (February-March

  assessed using a two factors nested design. We considered two factors: Year (two levels: first [1 st ] and second [2 nd ]); Season (nested in Year, with four levels: Autumn [Au 1 st , Au 2 nd ], Winter [Wi 1 st , Wi 2 nd ], Spring [Sp 1 st , Sp 2 nd ], Summer [Su 1 st , Su

  The two-way nested ANOVA was performed to analyze the inter-annual and seasonal differentiation of several univariate measures: the mean Po river flow observed during the four weeks before the sampling, the chemical and physical variables of the bottom water (temperature, dissolved oxygen, transparency, Chl a), %(silt+clay) and % TOC of the sediment, the main ecological indices (number of taxa, abundance, Shannon-Wiener diversity [Log e]), the density of the three dominant species (Ampelisca diadema, Corbula gibba and Lumbrineris latreillii) and three very common species (Aricidea claudiae, Levinsenia gracilis and Nucula nucleus). The homogeneity of variances was verified a priori using the Cochran C-test, transforming the values (log-or angulartransformation) when necessary. The Spearman rank correlation test (n=24, r s crit =0.41) was used to investigate relationships between the diversity indices, the density of A. diadema, C. gibba, L. latreillii, A. claudiae, L. gracilis, N. nucleus and the environmental variables.

  Most of the seawater chemical-physical variables and the Po river discharge (ANOVA, log transformation: Year, F=0.36, p>0.56; Season, F=3.4, p<0.05) exhibited significant seasonal and low inter-annual variability (Fig.1, 2).The sea-bottom temperature showed marked cyclical variation during the year: significant differences were observed only at season level (ANOVA, log transformation: Year, F<0.01, p>0.95; Season, F=16.4, p<0.001, Fig.2). The highest temperatures were observed in summer (July 05-August 05: 24 °C; August 2006, 23.8°C); the lowest in winter (January 05-February 05: 6.5°C; February 06: 6.5°C).Anomalous high-water temperature (19.0°C) was observed in November 2005.Bottom water concentrations of dissolved oxygen and % saturation were closely related (r s =0.91): they exhibited significant seasonal variation with the same pattern in the two years (ANOVA on [O 2 ], log transformation: Year, F<0.01, p>0.95; Season, F=2.9, p<0.05, Fig.2). The dissolved oxygen was inversely correlated with temperature (r s =0.87): in fact, the highest oxygen concentrations (about 10 mg l -1 ) were observed in winter, the lowest at the end of summer or at the begin of autumn. In particular, shortterm hypoxic conditions were observed in the autumn of the 2 nd year (November 2005), in correspondence with the occurrence of the anomalous high-water temperature.The pH was correlated with dissolved oxygen (r s =0.61), but did not vary significantly at annual and seasonal level (ANOVA, untransformed data: Year, F=0.04, p>0.83; Season, F=1.15, p>0.37, Fig. 2). Most of the time, pH ranged from 8 to 8.25, with the exception of November 2005, when, in correspondence with the minimum of dissolved oxygen, the pH was 7.85. The concentrations of Chl a, considered as an estimate of the phytoplankton biomass, varied seasonally but not between the two years (ANOVA, log transformation: Year, F=0.89, p>0.38; Season, F=2.9, p<0.05, Fig. 2). Chl a concentrations > 10 g l -1 , which indicated eutrophic conditions, were observed during the winter months: in February 2006, a maximum value of 32 g l -1 Chl a was measured. The bottom salinity did not exhibit significant inter-annual or seasonal variation (ANOVA, log transformation: Year, F=0.31, p>0.59; Season, F=1.7, p>0.18, Fig. 2). It ranged between 36 and 38 psu, with minima of 35 and 34.5 psu in November 2004 and December 2005, in correspondence with the two peaks of the Po river flow. Nevertheless, the correlation with the Po river flow was rather low (r s =-0.34), probably because of the low discharge observed during the investigated period. The water-transparency also varied at seasonal level (ANOVA, log transformation: Year, F=0.3, p>0.60; Season, F=4.3, p<0.01, Fig. 2). During summer, when the meteomarine conditions are more stable, the Secchi-dish was visible at a depth of up to 8-9 m. Transparency started declining with the first storm events at the end of summer. Lower transparencies were observed during the second year winter (January-March 2006), when values of less than two meters were measured, because of the high concentration of suspended organic and inorganic particles in the water column due to storms and Po river discharge. The sediment composition was dominated by mud and silt (75 to 94 %) throughout the investigation (ANOVA on % mud-silt, angular transformation: Year, F=2.3, p>0.17; Season, F=1.1, p>0.4, Fig. 2). The % TOC showed significant seasonal variation: the highest values (1.5-2%) were observed at the end of winter and the begin of spring (ANOVA, angular transformation: Year, F=1.2, p>0.31; Season, F=3.2, p<0.05, Fig. 2). The period November 2005-March 2006, has been highlighted in grey in the plots of environmental (Fig. 2) and biotic variables (Fig. 4) because this featured several disturbance events (frequent storms, an anomalous winter flood and, overall, hypoxic events associated with the anomalous high temperatures of November 2005)

  diadema was positively correlated with the density of L. latreillii, C. gibba, A. claudiae and L. gracilis. With regard to the environmental variables: 1) a positive correlation was found between transparency, temperature and the density of A. diadema, L. latreillii and C. gibba; 2) the transparency was negatively related to diversity, oxygen concentration and the Po flow; 3) the dissolved oxygen was inversely correlated with the number of taxa, total abundance and densities of A. diadema and C. gibba (Tab. 3).The number of taxa and Shannon diversity did not vary significantly between the two years and among the different seasons (ANOVA for the number of taxa, log transformation: Year, F=1.5, p>0.26; Season, F=1.5, p>0.24; ANOVA for Shannon diversity:Year, F=4.6, p>0.08; Season, F=2.1, p>0.11). Yet, both indices exhibited strong variation on a short temporal scale, among dates of the same season: the number of taxa and diversity ranged from 14-35 and 1.5-2.7, respectively (Fig.4).The total abundance varied on both the inter-annual and seasonal scales, but most of the variability was found between the two years (ANOVA, log transformation: Year, F=8.8, p<0.05; Season, F=3.0, p<0.05). The highest densities were observed during the first year, peaking in July 2005 with 12900 ind.m -2 ; in the second year, the macrozoobenthic assemblages showed lower densities, with a minimum of about 400 ind. m -2 in February 2006 (Fig.4). Despite the difference in the absolute values, the trends in abundance were similar in the two years: it increased starting from the end of spring, peaking in summer and then declining. These trends can be associated with the population dynamic of the dominant species, which exhibited different patterns of temporal variability.The total abundance was primarily related (r s =0.97, Table3) to the density of the dominant amphipod A. diadema (mean density= 1840 ind. m -2 ± 2560 S.D.), which varied on a seasonal level and, overall, between the two years (ANOVA, log transformation: Year, F=7.9, p<0.05; Season, F=3.1, p<0.05). During the first year, between October 2004 and April 2005, the density of A. diadema population was 600-1800 ind.m -2 (Fig.4). Starting from May 2005, a rapid demographic increase was observed: the density peaked in July 2005, with 11000 ind.m -2 . Then the density declined rapidly: in August and September it was about 3200 ind. m -2 , and in October 2005 it was similar to those observed at the begin of the investigation (1100 ind.m -2 ). During the second year, from November 2005 and May 2006, a further decrease in the density was observed, with a minimum of 25 ind.m -2 in March 2006. Between the end of spring and the begin of summer, new recruitment events were observed. In fact, between May and July 2006, density increased by an order of magnitude, from 270 ind. m -2 to more than 2000 ind. m -2 . Subsequently, the density of A. diadema was reduced to about 500 ind. m -2 (Fig. 4). The density of two other dominant species, C gibba (mean density= 334 ind. m -2 ± 218 SD) and L. latreillii (mean density= 230 ind. m -2 ± 138 SD) varied only at seasonal level (ANOVA for C. gibba density, log transformation: Year, F=0.5, p>0.52; Season, F=7.3, p<0.05; ANOVA for L. latreillii density, log transformation: Year, F=0.3, p>0.59; Season, F=3.6, p<0.05). Between October 2004 and March 2005, the density of C. gibba was 250-330 ind.m -2 (Fig. 4). Starting from May 2005, a rapid demographic increase was observed that resulted in a peak density in September 2005 (750 ind.m -2 ). Subsequently, during autumn and winter, density decreased: a minimum of 25 ind.m -2 were found in March 2006. Starting from April 2006, density increased, peaking in July 2006 with 730

  2005 and winter 2006: the hypoxic event associated with the anomalous high temperature of November 2005, and the anomalous winter flood of February 2006 could be related to the generalized reduction of density observed in the second year. The most important species exhibited different patterns of temporal variation, in response to seasonality and hydrological stress. The population number of the dominant amphipod A. diadema was characterized by marked seasonal and inter-annual variations. The preliminary analyses on the sex-and stage-structure of the population of

Fig. 2 .

 2 Fig. 2. Chemical-physical variables of bottom sea water, % (silt+clay) and %TOC of sediment from CEBEB in the period October 2004 -September 2006. Several disturbance events during November 2005-March 2006 are highlighted in grey.

Fig. 3 .

 3 Fig. 3. 2D-nMDS ordinations of macrozoobenthic communities at CEBEB site during the period October 2004 -September 2006 (2 2d -root transformed abundance data). Symbols: Sampling year (circles=1 st year; Triangles=2 nd year); Season (white=winter; light grey=spring; black=summer; dark grey=autumn).

Fig. 4 .

 4 Fig. 4. Main ecological indices (total abundances, number of taxa and Shannon diversity) and density of the dominant species Ampelisca diadema, Corbula gibba and Lumbrineris latreillii, and the three common species Aricidea claudiae, Levinsenia gracilis and Nucula nucleus from CEBEB during the period October 2004 -September 2006. Note the different scales for the total abundance and species density plots. Several disturbance events during November 2005 -March 2006 are highlighted in grey.

Table 1 .

 1 Within-season Bray-Curtis similarities (diagonals) and between-season dissimilarities (under the diagonals), estimated for the two years of sampling (2 nd root transformed data) by means of SIMPER analysis. The dissimilarity values were given together with the corresponding R statistic value, obtained from the pair-wise one-way ANOSIM comparison test between season. Significant R values are highlighted in bold (p<0.05 for the global test, R pairwise test < R global test for pair-wise comparison between season of the same year, Clarke and Warwick, 2001).

		1 st year (ANOSIM among Seasons, R=0.50) Au 1 st Wi 1 st Sp 1 st	Su 1 st
	Au 1 st Wi 1 st Sp 1 st Su 1 st	72 31; R=0.15 37; R=0.56 38; R=0.96	71 36; R=0.15 38; R=0.74	65 36; R=0.44	71
		2 nd year (ANOSIM among Season, R=0.52) Au 2 nd Wi 2 nd Sp 2 nd	Su 2 nd
	Au 2 nd Wi 2 nd Sp 2 nd Su 2 nd	78 52; R=0.63 38; R=0.56 45; R=0.93	50 46; R=0.26 55; R=0.67	65 41; R=0.67	64

Table 2 .

 2 Seasonal density (mean± standard deviation) of the species that contributes 1) at least 3% to the total abundances observed at each season; 2) to more than 60% of the dissimilarity between sampling years and season (SIMPER, 2 nd root transformed data). Class (C.):

	A = amphipods; B = bivalves; G = gastropods; P = polychaetes.
	C. taxa	Au 1 st	Wi 1 st	Sp 1 st	Su 1 st	Au 2 nd	Wi 2 nd	Sp 2 nd	Su 2 nd
	P Aricidea claudiae	931±238	581±246	358±36	300±33	239±25	69±71	206±38	69±73
	P Euclymene oerstedii	44±41	36±34	22±25	19±13	0±0	11±10	11±13	0±0
	P Glycera tridactyla	31±38	11±10	8±8	11±10	3±5	6±10	14±13	8±8
	P Lumbrineris latreilli	242±169	192±167	303±46	417±90	78±84	158±93	267±123	331±84
	P Magelona alleni	75±43	56±5	31±5	11±13	42±0	17±17	17±14	17±17
	P Monticellina dorsobranchialis	331±200	628±427	789±740	142±130	47±10	14±5	56±10	0±0
	P Neanthes succinea	0±0	0±0	28±35	0±0	0±0	6±10	0±0	0±0
	P Nephtys hystricis	33±14	36±34	56±32	33±17	47±10	25±8	64±38	78±41
	P Levinsenia gracilis	106±47	144±167	247±32	147±67	72±13	31±29	203±43	28±13
	P Prionospio cirrifera	122±71	119±64	44±55	78±46	33±0	14±24	6±10	0±0
	P Sigambra tentaculata	69±70	22±10	19±17	22±13	0±0	8±8	6±5	3±5
	A Ampelisca diadema	972±263	1522±211	3961±3755	5747±4362	647±483	125±145	564±551	1186±1007
	A Pseudolirius kroyerii	0±0	3±5	0±0	3±5	0±0	14±17	6±5	361±465
	B Abra alba	14±13	53±25	3±5	61±63	31±25	47±21	106±67	78±27
	B Corbula gibba	361±142	278±49	156±92	700±50	297±100	56±27	361±321	464±64
	G Hyala vitrea	22±38	3±5	3±5	14±24	14±10	8±14	6±5	83±117
	B Mysella bidentata	131±91	22±38	17±14	119±145	56±59	36±43	3±5	0±0
	B Nucula nucleus	56±17	56±38	42±30	33±17	92±71	31±27	225±276	203±142
	B Paphia aurea	3±5	3±5	0±0	72±86	44±13	0±0	3±5	3±5
	B Scapharca inaequivalvis	19±13	0±0	3±5	92±112	192±171	22±32	31±17	25±8

Table 3 .

 3 Spearman correlation between the main univariate indices, the density of most important species and the environmental variables that gave the high correlation between the environmental and biological similarity matrix (BIOENV). Significant correlations were highlighted in bold (n=24, r s crit =0.41). Diversity: Shannon Index; Tot Ab: Total Abundance; Density of 6 main species: Ampelisca diadema, Corbula gibba, Lumbrineris latreilli, Aricidea assimilis, Levinsenia gracilis, Nucula nucleus; Transparency: Secchi disk. Location of the sampling station CEBEB (Cesenatico-Bellaria Bianco) (from

		Taxa	Tot Ab Diversity A. diadema C. gibba L. latreilli A. assimilis L.gracilis	N. nucleus	Transparency Temperature	[O2]
	Total Abundance	0.36
	Diversity	0.23	-0.71
	A. diadema	0.24	0.97	-0.83
	C. gibba	0.34	0.61	-0.37	0.56
	L. latreilli	0.18	0.66	-0.39	0.55	0.60
	A. assimilis	0.54	0.61	-0.15	0.53	0.13	0.13
	L.gracilis	0.41	0.55	-0.13	0.41	0.11	0.49	0.58
	N. nucleus	0.38	0.08	0.15	0.02	0.48	0.27	-0.09	0.01
	Transparency	0.24	0.58	-0.41	0.54	0.52	0.36	0.23	0.41	0.21
	Temperature	0.28	0.50	-0.37	0.46	0.77	0.51	0.01	0.07	0.32	0.58
	[O2]	-0.42	-0.47	0.20	-0.48	-0.67	-0.37	-0.25	-0.09	-0.13	-0.42	-0.87
	Po Flow	0.09	-0.31	0.38	-0.37	-0.32	-0.39	0.27	-0.12	-0.20	-0.41	-0.29	0.01

Acknowledgements

The authors thank the crew of the oceanographic M/S ''Daphne II'' (Struttura Oceanografica Daphne, ARPA Emilia-Romagna), Mr. Marco Papperini and Mr. Dino Pagan, and Dr. Katie Henry for English language corrections. We would like to thank the Editor and the three anonymous reviewers of the paper for their helpful advice.