Distribution of Polychaeta in soft-bottoms of a Galician Ria ( NW Spain )

Macrobenthic polychaete distributions were studied along the soft-bottoms of the Ensenada de san simón (Galicia, nW spain). results suggest that the distribution and abundance of polychaetes in the inlet were highly dependent on depth, sediment characteristics (grain size, organic matter and calcium carbonate content) and bottom water temperature. in the inner area of the inlet, intertidal bottoms colonised by the seagrasses Zostera marina and Z. noltii were dominated by spionids and capitellids, and showed low species number and diversity. shallow muddy bottoms of central areas were mostly dominated by ampharetids, terebellids and cirratullids. These families along with paraonids, maldanids and syllids were the most abundant families in the deeper subtidal muddy bottoms at the mouth of the inlet. These sediments also showed the highest number of species, diversity and density of individuals.

Scientia Marina 72(4) December 2008, 655-667, barcelona (spain) issn: 0214-8358 doi: 10.3989/scimar.2008.72n4655inTroDuCTion The Galician rias (nW spain) are a particular kind of estuarine system and have been extensively studied due to their great socio-economic importance, especially with regard to fisheries, mussel culture on rafts and shellfish resources.This has led to numerous faunistic and ecological works on the macrobenthic communities of the Galician coasts in recent years (e.g. lópez-Jamar, 1982;Garmendia et al., 1998;olabarria et al., 1998).studying benthic faunas is particularly important because they are considered to be good indicators of marine bottom conditions (Grall and Glémarec, 1997).
benthic communities in the ría de vigo have been studied since 1886, when Hidalgo published a list of marine species of the nW spanish coast.later works focused on the distribution and composition of macrofaunal assemblages (e.g. anadón, 1980;lópez-Jamar and Cal, 1990;abella et al., 1996).Despite a number of scientific studies devoted to the Ensenada de san simón (nombela et al., 1995;Fernández-rodríguez et al., 1997), few studies have actually analysed patterns of spatial distribution of benthic fauna, and none have yet determined the structure of polychaete populations.Polychaete faunas have been extensively studied in other rias and in other areas of the ría de vigo, from either descriptive (viéitez, 1976;Moreira et al., 2006) or taxonomic points of views (Moreira et al., 2000a;Moreira et al., 2000b;Parapar et al., 2000).Consequently, improving our knowledge of benthic macrofauna in general, and of polychaetes in particular, is essential for ensuring correct resource management in the area, especially since it has been included in the nature 2000 network as a special Conservation Zone.Therefore, the aim of the present study is to describe the composition and distribution of polychaete assemblages and their relationships with a number of environmental variables.

Study area
The Ensenada de san simón is located in the inner part of the ría de vigo, between 42º17' and 42º21'n and between 8º37' and 8º39'W (Fig. 1).soft-bottoms of this inlet are mainly muddy with high organic matter content (vilas et al., 1995).intertidal and shallow subtidal areas have meadows of the seagrasses Zostera noltii Hornem.1832 and Zostera marina l.Culture of mussels on rafts is a common practice in large areas at the mouth of the inlet.There is a large freshwater input into the innermost part of the inlet which results in salinity fluctuations on a tidal and seasonal basis (nombela and vilas, 1991).

Sampling and sedimentary analysis
samples were collected during november and December 1999 from 29 sites (Fig. 1).Five replicate samples were taken at each site by means of a van veen grab (0.056 m 2 ).samples were sieved through 0.5 mm mesh and the retained material was fixed in 10% buffered formalin.Fauna was sorted from the sediment and preserved in 70% ethanol for identification.Temperature and pH were measured in situ for water and sediment.an additional sedimentary sample was taken at each site for later grain-size analyses and to determine calcium carbonate and total organic matter contents.sedimentary types were determined according to Junoy (1996).Median grain size (Q 50 ) and sort coefficient (s o ) were also determined for each sample.Calcium carbonate content (%) was estimated by treating samples with hydrochloric acid, and total organic matter content (%) was estimated from the weight lost (Parada et al., 1993) after samples were placed in a furnace for 4 hours at 450ºC.

Data analysis
Total abundance (n), number of species (s), shannon-Wiener's diversity index (H', as log 2 ) and Pielou's evenness index (J) were determined for each site.Polychaete assemblages were determined through non-parametric multivariate techniques using the Plymouth routines of the Multivariate Ecological research software package (PriMEr; Clarke and Warwick, 1994).a similarity matrix was carried out using the bray-Curtis coefficient after applying the fourth-root transformation to species abundance.Classification and ordination of sites and species were carried out with cluster analysis using the uPGMa algorithm and non-metrical multidimensional scaling (MDs) respectively.The siMPEr analysis was used to identify species that contributed to dissimilarity among groups of sites determined by classification and ordination analyses.species were classified according to the constancy index (Dajoz, 1971) into five categories, according to the number of sites in which any given species was found in relation to the total number of sites: constant (>76%), very common (51-75%), common (26-50%), uncommon (13-25%) and rare (<12%).species were further classified according to the fidelity index (Cabioch, 1968) into accidental (<10%), occasional (11-33%), accessory (34-50%), preferent (51-66%), elective (67-90%) and exclusive (>91%).species were also classified according to the Frequency-Dominance product.
relationships between polychaete abundance and environmental variables were studied by means of the bioEnv procedure (PriMEr package).En-vironmental variables expressed in percentages were previously transformed by log (x+1) and all of them were normalised.

rEsulTs
The soft bottoms of the Ensenada de san simón were characterised by a predominance of muddy sediments with high total organic matter content and low calcium carbonate content (Table 1).sandy sediments were present in tidal channels in the inner inlet where total organic matter values were low.sediments became increasingly muddy towards the deeper bottoms at the centre and mouth of the inlet.The areas around the outer part had muddy sands with a large gravel fraction composed of mussel shells.
sampling yielded 24581 polychaetes belonging to 123 species in 38 families (appendix 1).spionidae was the family best represented in terms of number of individuals (32.13% total abundance), with Pseudopolydora paucibranchiata accounting for 22.33% of all polychaetes.in general, spionid species were more abundant in outer areas, except for Streblospio shrubsolii, which reached the highest densities in internal sandy areas.other dominant families were These species, along with Paradoneis lyra and Microphthalmus pseudoaberrans, accounted for more than 65% of the total abundance of polychaetes.The families best represented in terms of number of species were spionidae and syllidae (14 species each), Phyllodocidae (11 species) and Dorvilleidae and Cirratulidae (8 species each).Three species were found to be constant (P.paucibranchiata, Capitella cf.capitata and Heteromastus filiformis), 11 very common, 25 common, 29 uncommon and 55 rare.
The lowest densities were recorded at sites 11, 1, 4, 29 and 24 (31 to 92 individuals per 0.28 m 2 ) (Table 2).The greatest polychaete numbers were recorded at sites 22, 26 and 27 (2683-3025 ind.per 0.28 m 2 ) due to the high abundance of P. paucibranchiata, A. finmarchica, P. lyra and A. marioni.These sites also showed the largest number of species (60-69).only 7 species were found at sites 1, 2 and 29, which were located at the mouth of the oitabén-verdugo and alvedosa rivers.The shannon-Wiener's diversity index fluctuated between 1.17 (site 20) and 3.99 (sites 21 and 26), (2.89 ± 0.89; mean ± standard deviation) and evenness ranged from 0.27 (site 20) to 0.83 (site 25).The greatest diversity values (H') were recorded at sites along the mouth of the inlet while the lowest values were found in intertidal sites near the mouth of the rivers.
Cluster analysis and MDs based on abundance data of the dominant species (dominance>1%) showed four main groups (Fig. 3).Group 1 was composed of the species S. shrubsolii and M. pseudoaberrans, which were numerically dominant in the coarser sediments of the intertidal bottoms (Group a of the dendrogram).Pseudopolydora paucibranchiata and C. cf.capitata (Group 2) were distributed along the entire inlet, and appeared in high densities in a variety of bottoms which differ in stability and   granulometric composition.Group 3 was formed by H. filiformis and Cirriformia tentaculata; the former was present in the coarse sands of the inner part of the inlet and both species were present in deeper muddy sediments.Group 4 comprised the largest group of species, which were mostly found in muddy bottoms with high total organic matter content located at the middle and outer part of the inlet (Groups b and C of the dendrogram).
The combination of bottom water temperature and medium sand, very fine sand, total organic matter and calcium carbonate contents showed the highest correlation with polychaete abundance data (bioEnv: spearman's rank correlation p w : 0.318).

Description of assemblages
Three different assemblages were determined according to the dendrogram and MDs (Groups a, b, C; Table 3).Group a was located in the innermost part of the inlet, in intertidal sediments subjected to strong salinity variations close to the mouth of the rivers oitabén-verdugo, Xunqueira and alvedosa.sampling sites differed greatly in granulometric composition, with sedimentary types ranging from mud to very coarse sand.Diversity values were low due to the low number of species and also due to the high dominance in numbers of P. paucibranchiata, S. shrubsolii and C. cf.capitata, where average dominance was 23.48%, 20.04% and 24.13% respectively.The seagrasses Zostera marina and Z. noltii were spread across most of these intertidal and shallow subtidal bottoms, at depths of 1.6 to 2.9 m. other species with high abundance were M. pseudoaberrans and H. filiformis, with densities higher than 450 ind m -2 at some sites.The aforementioned species were the most characteristic according to frequency-dominance product values (FxD, Table 4).siMPEr analysis showed that C. cf.capitata, P. paucibranchiata and S. shrubsolii were the species that made the greatest contribution to similarity (56.78%) within Group a.The average similarity for this group was 42.21%.
Group b was present along muddy bottoms in shallow sediments at the centre of the inlet, from intertidal areas to 4.7 m depth (site 28).sediments were mainly composed of silt and clay (>50%), and ranged from mud to sandy mud.species numbers generally increased from sites in inner areas towards those located at the mouth.average individual density was the lowest in comparison to Groups a and C. Evenness in this group showed the highest values for the inlet.FxD and dominance values indicated that the most characteristic species were M. palmata,   Group C was distributed along the mouth of the inlet at a maximal depth of 28.2 m. sediments were predominantly composed of silt and clay that showed high total organic matter content.This group showed the highest number of species, densities and diversity.The species with the highest values of FxD were P. paucibranchiata, A. finmarchica and M. palmata.a total of 52 species were exclusive to this assemblage and 31 were constant.These species contributed 64% of the similarity within the group (average similarity of the group was 66.75%); most of these species (Table 4) were also responsible for dissimilarities among dendrogram groups.

DisCussion
Polychaete fauna distribution was primarily determined through sediment characteristics and bottom water temperature in the Ensenada de san simón.The spatial distribution of polychaetes has generally been related to these and other factors, such as depth, sediment characteristics, temperature, salinity and hydrodynamism (Hutchings, 1998;simboura et al., 2000).a predominance of muddy sediments in the Ensenada de san simón is a consequence of the hydrodynamic regime in the area: lack of strong currents in most of the inlet (maximum of 18 cm s -1 in the bottom layers; vilas et al., 1995), and the presence of seagrass meadows in the inner parts.These features result in the deposition of finer fractions brought in by rivers; bivalve raft culture also contributes to this deposition by means of the mussel faecal pellets. in the Galician rias, coarser sediments are generally located at the mouth while the finer fractions are deposited in greater amounts in the inner areas.However, the opposite effect is seen in the Ensenada de san simón, where the inner and marginal areas are regularly subjected to river currents and therefore mean grain-size is greater in these areas than in deeper bottoms, where finer particles are deposited.
Total organic matter content found in san simón fluctuated between 0.95 and 36.93%.Maximum values are higher than in other muddy sediments of the Galician rias (e.g.rodríguez-Castelo and Mora, 1984;lópez-Jamar and Mejuto, 1986;Mora et al., 1989;lópez-Jamar and Cal, 1990;Parra et al., 2002).organic matter content in the rias of arousa, Muros, the harbour area of Ferrol, vigo and Pontevedra, ranged from 2.32 to 21.31%. in san simón, the high organic matter content might be due to the contribution of detritus from the dense seagrass meadows located in the intertidal and shallow subtidal areas and also from detritus produced by mussels cultured on rafts.a lack of strong currents also results in gradual ongoing organic enrichment in the area.Thus, vilas et al. (1995) found maximal organic matter content of 10% while four years later maximal contents were found to be about three times higher (this work).The anoxic conditions present in the inner areas of the southern Galician rias (rías baixas) may affect benthic populations through a decrease in macrofaunal diversity and by favouring colonisation of opportunistic species such as Spiochaetopterus costarum (rodríguez-Castelo and Mora, 1984).surprisingly, this species, which is characteristic of organically enriched muddy sediments in the rías baixas, appears in low densities in san simón.
intertidal sediments in the san simón area are subject to salinity changes due to freshwater input from several rivers (vilas et al., 1995).such salinity fluctuations may greatly influence the number and types of species that can survive there and/or numerically dominate the community (Planas and Mora, 1987).Thus, the dominant species in these sediments were the deposit-feeder Streblospio shrubsolii and the carnivorous Microphthalmus pseudoaberrans.Moreover, coarser sediments provide a greater diversity of interstitial spaces to organisms than finer sediments (olabarria et al., 1998), and thereby facilitate habitats for small-sized species such as M. pseudoaberrans.on the other hand, intertidal sediments colonised by Z. noltii and Z. marina showed low diversities.seagrass meadows are known to provide a complex habitat that may be colonised by many species (somerfield et al., 2002). in the Ensenada de san simón, however, these meadows are located in areas subjected to wide salinity changes which are a major limiting factor for many species (Planas and Mora, 1987;Junoy, 1996).
The greatest values of species diversity and numbers were found in muddy sediments at the mouth of the inlet, where there is a greater stability of salinity values and hydrodynamic conditions (nombela et al., 1987). in fact, the highest diversity value was recorded at site 26, which was characterised by a greater grain-size heterogeneity and was the deepest site.Heterogeneous sediments with biogenic fragments which allow high diversity of microhabitats are mostly associated with greater faunal diversity (nicolaidou and Papadopolou, 1989;lastra et al., 1990;villora-Moreno, 1997;simboura et al., 2000;Moreira et al., 2006).
The muddy bottoms of Group b had polychaete fauna which was mostly dominated by ampharetids, terebellids, cirratulids and spionids.These families were also present in Group C. Group C was located in the deepest muddy bottoms at the mouth of the inlet and showed the highest number of species and densities.other deposit feeders, such as paraonids and maldanids, were numerically dominant in these bottoms.according to the polychaete fauna present in Groups b and C (Melinna palmata, Chaetozone gibber, Cossura pygodactylata, Ampharete finmarchica, P. paucibranchiata, Nephtys hombergi, C. cf.capitata), these groups can be considered as two different facies of the Abra alba community defined by Petersen (1918).This community, also determined by other invertebrates such as the molluscs Hydrobia ulvae (Pennant, 1777) and Cerastoderma edule (linnaeus, 1758), has been reported along European and Galician coasts in a number of muddy sediments (lastra et al., 1988;lópez-Jamar and Cal, 1990;Carpentier et al., 1997;Thiébaut et al., 1997;olabarria et al., 1998;sánchez Mata and Mora, 1999;Moreira et al., 2005;lourido et al., 2008).several species, such as C. pygodactylata, have been linked to muddy bottoms with Zostera leaves showing decomposition (bachelet and laubier, 1994;Moreira et al., 1999). in fact, C. pygodactylata is known to be an opportunistic species which appears in anoxic habitats such as those present in san simón.
species numbers, diversity and evenness values were lower in the intertidal bottoms than in the subtidal areas of the inlet.salinity fluctuations coupled with effects from a number of human activities may be responsible for the scarce polychaete fauna.Thus, the sites with the lowest number of species and densities were found along bottoms close to the mouth of freshwater channels and in the main harbour area at the inlet (site 29).For instance, the deposit-feeder C. cf.capitata, usually recorded in organically enriched environments (Méndez et al., 1997), was the dominant species at site 29.Deposit feeders are, in general, numerically dominant in fine grain-size sediments (sanders, 1968; levinton, 1977).This was also true for muddy bottoms of the Ensenada de san simón, where surface and subsurface deposit-feeder polychaetes (spionids, cirratulids and capitellids) were the most abundant polychaetes.Dominance by spionids has been considered to be an indicator of polluted environments; nevertheless, their reproductive and feeding plasticity make them good colonisers of different kinds of environments (Herrando-Pérez et al., 2000), and therefore they should not always be considered to be a symptom of anthropogenic perturbation (Moreira et al., 2006).average diversity and number of species is generally greater in ocean-exposed rias with sandy and more heterogeneous sediments, such as the Ensenada de baiona (total number of species: 132; Moreira et al., 2006) and the ría de aldán (total number of species: 145; lourido et al. 2008). in fact, high organic matter content is related to low diversity and species richness (lópez-Jamar and Cal, 1990).nevertheless, polychaete diversity values (2.89 ± 0.89) and number of species (27.1 ± 19.1, 123 species in total) in the Ensenada de san simón were higher than in other Galician rias with similar sedimentary conditions.For example, the Spiochaetopterus costarum community described by lópez-Jamar and Mejuto (1986) in the rias of arousa and Muros was only composed of 12 polychaete species.lópez-Jamar (1978) found a total of 52 species in the ría de Pontevedra, while rodríguez Castelo and Mora (1984) cited 12 to 27 species per site for the same area.organic enrichment of sediments in the san simón area is mostly due to mussel culture and detritus originating from seagrass meadows.This organic richness, promoted by high hydrodynamical and substrate stability, benefits depositivore polychaetes, which reach maximum densities in the subtidal bottoms where there is no tidal stress or influence from rivers.aCknoWlEDGEMEnTs We gratefully thank the team at the Marine animals adaptations laboratory for their invaluable help with sample collection and comprehension of statistical methods.Two anonymous referees provided useful comments which contributed to improving the final version of the manuscript.This work is a contribution to the research project PGiDT 00PXi30119 Pr. rEFErEnCEs abella, F.E., J.M. Parada and J. Mora. -1996. relationship Fig. 1. -location of the Ensenada de san simón (ría de vigo) and position of the 29 sampling sites.

Fig. 2 .
Fig. 2. -Dendrogram (i) and MDs (ii) using bray-Curtis similarity coefficient showing the classification and ordination of sites.

Table 1 .
-Depth (those under 2 m are intertidal sites sampled during high tide) and physical characteristics of sediments at each site: Q 50 , median particle size; bt, bottom type (M, mud; sM, sandy mud; Ms, muddy sand; vCs, very coarse sand; Cs, coarse sand); ToM, total organic matter and Co 3 , calcium carbonate content.seagrass presence is cited in the last column.

Table 3 .
-summary of characteristics for each association.-i.Ecological features of the polychaete assemblages determined in the Ensenada de san simón, indicating average and standard deviation of biotic and physical characteristics.Q 50 , median grain size; bt, bottom type (M, mud; Cs, Coarse sand; sM, sandy mud, Ms, muddy sand); ToM, total organic matter content; Co 3 , calcium carbonate content; H', shannon Wiener diversity index; J, Pielou's evenness; s, number of species; D, density (ind per 0.28 m 2 ).