Spatial distribution of benthic macrofauna in subtidal sediments of the Ría de Aldán ( Galicia , northwest Spain )

In the summer of 1997, 27 subtidal stations were quantitatively sampled in the Ría de Aldán. A total of 81770 individuals were collected, comprising 496 species. The distribution and composition of benthic assemblages was correlated to the sedimentary characteristics which, in turn, depend on the hydrodynamic features; those patterns are reflected in a sedimentary gradient present along the ria, which is characterized by an increasing grain size from the inner margins towards the mouth. Several faunal assemblages were determined through multivariate analyses and their composition is compared with that of several communities or facies previously described from similar sediments. The Venus fasciata community was present in clean coarse sediments of the outer ria, the Venus gallina and Tellina fabula-Tellina tenuis communities in the fine-sand bottoms at the centre and margins of the ria, a transition assemblage between the Venus fasciata and the Venus gallina communities in the medium-sand sediments, and a mix of species from the Syndosmya alba and the Amphiura communities in shallower and muddy sediments in the inner areas. In general, subtidal sediments of the Ría de Aldán showed a high benthic diversity which is related to the great sedimentary heterogeneity and the lack of significant anthropogenic alterations.


INTRODUCTION
Intertidal and shallow subtidal soft-sediment habitats constitute a small proportion of marine soft sediments (Ellis et al., 2000).They are, however, very productive and comprise a wide variety of habitats inhabited by a number of macrobenthic communities.Moreover, benthic faunas in coastal areas play important roles in providing food for humans, fish and birds (Ellis et al., 2000).In particular, macrofaunal activ-ity influences ecosystem processes such as nutrient cycles (carbon, nitrogen and sulphur), metabolism of pollutants and transport, and dispersion and burial of sediments (Snelgrove, 1998).The composition and diversity of macrofaunal assemblages must be studied in order to determine local and regional diversity patterns (Labrune et al., 2008).In addition, analysis of taxonomic diversity at the species level is a prerequisite in routine monitoring studies and helps to understand the functioning of a particular community because each species is characterized by an ecological role (Maggiore and Keppel, 2007).
During the last few years, there has been an ongoing interest in the Galician rias (NW Iberian Peninsula), because they are a special and complex kind of estuarine system with high primary productivity due to upwelling and regular input of nutrients (Nombela et al., 1995).The Galician rias have a great economic and social importance due to the presence of fisheries, bivalve culture and shellfish resources (Nombela et al., 1995;Figueiras et al., 2002).A number of activities resulting from the growing human population concentrating on the shoreline of the rias, such as mollusc harvesting, construction of artificial structures and sewage disposal, are heavily impacting marine sedimentary environments.For example, extensive culture on rafts of the blue mussel, Mytilus galloprovincialis Lamarck, 1819 in the rias of southern Galicia has resulted in alterations to the benthic environment in many areas (Abella et al., 1996).Indeed, mussels produce large amounts of faeces and pseudofaeces which are deposited on the bottoms right beneath them (López-Jamar, 1981), often producing organic enrichment, hypoxia and changes in granulometric composition.In general, the aforementioned perturbations result in changes in the composition of benthic assemblages and impoverishment of local biodiversity (López-Jamar and Mejuto, 1988).Therefore, studying the distribution and composition of benthic assemblages in the Galician rias is paramount in order to determine the status of the sediments and the degree of perturbation when they are subjected to anthropogenic activities (López-Jamar and Mejuto, 1986).
The composition and distribution of soft-bottom benthos are well-known in many areas of the Galician coast (e.g.López-Jamar and Cal, 1990;Mora et al., 1982;López-Jamar and Mejuto, 1985;Garmendia et al., 1998).There is, however, a lack of studies in some small rias such as the Ría de Aldán.In fact, the ecological catastrophe which affected most of the northern coast of Spain derived from the Prestige oil spill (November 2002) has revealed the lack of baseline data about benthic diversity and assemblages in many areas of the Galician coast; these data are needed in order to establish the true effect of these and other disturbances on the marine environment and determine whether the biota is recovering.
The Ría de Aldán is located at the mouth of the Ría de Pontevedra and shows a variety of subtidal sedi-ments, ranging from gravel to mud, at depths of up to 45 m.These physical features offer a good opportunity to study the patterns of distribution and composition of benthic assemblages across several sedimentary substrata at a relatively small spatial scale (<10 km).In addition, in the last years the Ría de Aldán has been subjected to bivalve culture on rafts in some areas.Therefore, the main aims of this paper are: i) to characterize the composition and distribution of the macrobenthic fauna (>0.5 mm) of the subtidal soft bottoms of the Ría de Aldán in order to provide baseline data for further comparative studies; ii) to determine the possible relation of several environmental variables to the distributional patterns of the benthic fauna; and iii) to compare the benthic biodiversity of the Ría de Aldán with that of other similar geographic areas in order to assess its ecological value.Ultimately, these data might serve to develop proper strategies for management and conservation of soft-bottom benthic habitats.

Study area
The Ría de Aldán is located in Galicia, between 42°16'40' ' and 42°20'50''N and 8°49' and 8°52'W.This ria is located on the southern margin of the mouth of the Ría de Pontevedra (Fig. 1), and is 7 km long and 3.5 km wide.The Ría de Aldán has a maximum depth of 45 m, and its mouth is oriented northwards.The small Aldán River flows into the inner area, and there is an increase in salinity from the internal to the external part of the ria.The effect of this freshwater input is reduced by the strong oceanic swell and currents which reach the inner areas.Both margins of the ria are made up of rocky substratum which alternates with sandy beaches.There is a growing practice of bivalve culture on rafts in the inner parts of the ria.This activity is assumed to contribute to the increase of the content of silt/clay and organic matter in those areas, as occurs in other Galician rias.

Sample collection and processing
Quantitative sampling was done in the Ría de Aldán in July-August 1997 at 27 subtidal sites, thus covering the full extent of the subtidal domain of the ria.Samples were taken by means of a van Veen grab with a sampling area of 0.056 m 2 ; five replicates were taken at each site, accounting for a total area of 0.28 m 2 .Samples were sieved through a 0.5 mm mesh and fixed in 10% buffered formaldehyde solution.Fixed material was later taken to the laboratory for sorting and identification of the fauna.
Granulometric composition, calcium carbonate and organic matter content was also analysed from an additional sediment sample collected at each site.The granulometric fractions were considered following the Wentworth scale (1922) and sediment types were char-acterized according to Junoy (1996).Moreover, median grain size (Q 50 ) and sorting coefficient (S 0 ) (Trask, 1932) were also determined for each sample.Temperature, Eh and pH were measured in situ in the sediment, as was the temperature of surface and bottom water.Calcium carbonate content (%) was estimated by treating the sample with hydrochloric acid, and the total organic matter content (TOM, %) was estimated from the weight loss on combustion for 4 h at 450°C (Table 1).

Data analysis
Total abundance (N), total number of species (S), the Shannon-Wiener diversity index (H', as log 2 ) and Pielou's evenness (J') were calculated for each sampling station.At any given station, the species representing 4% or more of the total abundance were considered as numerically dominant (Field et al., 1982).
Faunal assemblages were determined through nonparametric multivariate techniques as described by Field et al. (1982) using the Plymouth Routines in the Multivariate Ecological Research software package (PRIMER; Clarke and Warwick, 1994).A similarity matrix between stations was prepared by means of the Bray-Curtis similarity coefficient after applying the fourth root transformation on the average abundance of each species (mean of five replicates) to reduce the contribution of the most abundant species.From the similarity matrix, a classification of the stations was done by cluster analysis developed from the groupaverage sorting algorithm and an ordination by means of non-metric multidimensional scaling (nMDS).The SIMPER program was then used to identify species that contributed highly to differentiating groups of stations determined by classification and ordination analyses.Species were classified according to the product frequency x dominance (Glémarec, 1964), which evaluates the constancy and the numerical importance of each species within a group of stations.
The potential relationship between faunal distribution in the ria and the estimated environmental variables was studied using the BIO-ENV procedure (belonging to the PRIMER package), and canonical correspondence analysis (CCA, using the CANOCO v4.02, Canonical Community Ordination package; Ter Braak, 1988).All variables expressed in percentages were previously transformed by log (x + 1), and then all of them were normalized.

Sediments
Sediments were chiefly sandy in most of the ria (Table 1), and their distribution followed a gradient in grain size from the mouth towards the inner areas.Muddy bottoms were confined to inner and sheltered areas, and coarser sandy granulometric fractions were more frequent at the mouth of the ria.The gravel content (0.1-52.7%) was higher at stations 18 and 3 (52.7 and 47.9%, respectively), while sandy fractions (26.6-98.0%)recorded high values at stations 7, 8, 9 and 13 (96.8-98.0%).The pelitic fractions showed the highest values at inner stations 27, 28 and 31 (59.2-69.4%),whereas the rest of the stations showed lower values, from 1.4% to 13.3%.There was an increase in total organic matter content from the outer to the inner areas of the ria (0.5-10.8%), whereas the calcium carbonate values were higher than 30% at all sampling stations (32.3-89.8%).Stations 33 and 34 were also characterized by the presence of the seaweed Ulva spp. on the surface of the sediment.

Benthic fauna
Sampling yielded a total of 81770 specimens belonging to 496 species.Malacostracan crustaceans (162 species and 16851 individuals) and polychaetes (145 species and 28878 individuals) were the dominant groups in terms of number of species and individuals.Some taxa such as Nematoda, Nemertea and Harpacticoida were not identified to lower taxonomic levels.
Values of univariate measures are shown in Table 1.The lowest abundance values were recorded at station 8 (fine sand; 977 specimens), while the highest ones were recorded at station 34 (muddy sand; 12181 specimens).The number of species varied between 55 (St.8) and 153 (St.18).In general, total number of species increased towards the inner areas of the ria, showing the highest values at stations 27, 28, 29, 32, 33 and 34.The lowest number of species was recorded at the mouth of the ria, in the northeast area next to the shoreline (stations 8, 9 and 14).In number of species, polychaetes were the dominant group at 18 stations, while crustaceans dominated at 9 stations (mainly coastal stations with fine and medium sands).At station 34, polychaetes and molluscs showed a similar number of species.
In general, diversity (H') decreased from both the western and eastern margins of the mouth of the ria towards the central area.Maximum values were recorded at stations 17, 26, 12 and 18 (5.55-5.77).Minimum values were determined at stations 21, 30 and 23 (1.89-3.11);this was due either to the low number of species present or because of the high numerical dominance The most widespread taxa in the ria, apart from the suprageneric taxa Nematoda spp., Nemertea spp.and Harpacticoida spp., were Thracia papyracea (found at 24 stations), Spio decoratus, Heteromastus filiformis (23

Multivariate analysis
The dendrogram obtained by cluster analysis showed the presence of two major groups of stations at a similarity level of 30% (Fig. 2).Group A was further subdivided into group A1 (medium-sand) and group A2 (coarse sand) at a similarity level of 40%.Group B was subdivided into B1 (fine-sand stations close to the shoreline), B2 (muddy sand), B3 (fine sand) and B4 (mud) at a similarity level of 40%.nMDS ordination showed similar results to those of the dendrogram (stress: 0.13).The summary of characteristics for each association is shown in

Species affinities
Cluster analysis done on the abundance data of the dominant species showed the existence of five major groups at the similarity level of 30% (Fig. 3).Group 1 comprised species mostly found in gravel, group 2 comprised species found in muddy sands (cluster group B2), while group 3 comprised species with higher abundance in mud (cluster group B4).Group 4 was composed of nine species mostly found in coarse sand (cluster group A), and three species mostly found in muddy sand.Finally, group 5 was subdivided into group 5a (fine-sand species), 5b (a number species found in several types of sediment) and 5c (species found in fine and muddy sands).

Relation of benthic fauna with environmental variables
The BIO-ENV procedure showed that the combination of gravel, coarse sand, fine sand, very fine sand, depth, redox potential (Eh) and total organic matter content had the highest correlation with faunistic data (ρ w : 0.615).Very fine sand was the variable that alone showed the highest correlation with the faunistic data (ρ w : 0.462).
Axes I and II of the forward selection of CCA were the most important in the CCA ordination, accumulating 24.1% of species variance and 31.1% of speciesenvironment variance.Cluster groups with higher content of coarser granulometric fractions were distributed on the left of the ordination, while assemblages distributed along fine-sand and muddy sediments were distributed on the right, following a gradient defined by a decrease in median grain size (Fig. 4).

DISCUSSION
This study shows that the Ría de Aldán has a diverse soft-bottom benthic fauna.In total, 496 different taxa were found in a variety of sediments ranging from very coarse sand to mud.The multivariate analyses revealed the presence of several benthic assemblages whose distribution along the ria is related to that of sediment types.In addition, the faunistic composition of the assemblages mostly agrees with those of several communities or facies, as described by Petersen (1918) or Thorson (1957).Thus, the assemblage of group A2 of cluster can be included among the different varieties of the Branchiostoma lanceolatum-Venus fasciata community (Thorson, 1957) present on clean coarse sediments with a high content of biogenic carbonates (Table 5).The medium-sand assemblage corresponding to group A1 showed intermediate features between the Venus fasciata community and the Venus gallina community (fine sand); this group has a low number of exclusive species (3), suggesting that these sites are a transition zone between different assemblages, i.e. the coarse-sand assemblage (group A2) and the fine-muddy sand assemblage (group B).The fine-sand sites of group B1 are characterized by species which are typical of the Venus gallina community (Thorson, 1957).The muddy sands of group B2 are located in the inner ria, into which the Aldán River flows; this group is characterized by the presence of seaweeds (Ulva spp.) that may favour the appearance of epifaunal species such as Bittium reticulatum.Group B3 (the fine-sand assemblage) shows similarities with the Venus gallina community described by Thorson (1957); coastal and shallow sites also have typical species from the Tellina tenuis-Tellina fabula community (Thorson, 1957).Group B4 is characterized by muddy sediments showing the highest content of organic matter in the ria.The species found there are characteristic of the Syndosmya alba community and the Amphiura community.In general, the distribution of assemblages and sediments is similar to that found in other Galician rias with similar orientation and exposure to oceanic swell such as the Ría da Coruña (López-Jamar and Mejuto, 1985), the Ría de Ares-Betanzos (Garmendia et al., 1998) and the Ensenada de Baiona (Moreira and Troncoso, 2008).
Soft bottom faunas are often structured by abiotic factors (Labrune et al., 2008).Several studies have highlighted sedimentary features among the most important factors influencing the composition and distribution of benthic assemblages in marine sediments (e.g.Pearson and Rosenberg, 1978).In the Ría de Aldán, there is a correlation between the spatial distribution of the benthic fauna and the granulometric composition of the sediment.Thus, there is a conspicuous grain-size gradient in the study area, which is defined by an increase in finer sandy fractions from the mouth of the ria towards the inner margins.When studied separately, the distribution of the molluscan, polychaete and peracarid assemblages in the Ría de Aldán also follows this sedimentary gradient (Lourido et al., 2006(Lourido et al., , 2008a,b),b).Although hydrodynamic features of the Ría de Aldán were not measured, hydrodynamism is known to act as a "superparameter" which has a major influence on sedimentation of particles and organic matter, stability of sediments and patterns of sedimentary distribution, and therefore on faunal composition (Rosenberg, 1995).In general, the particular overall granulometric composition of the sediments in the Galician rias is a consequence of the hydrodynamic regime found there (López-Jamar, 1981).Thus, the greater hydrodynamism due to exposure to oceanic swell in the outer areas of the rias does not allow deposition of finer particles and results in the presence of coarser sediments (Troncoso et al., 1993); finer sediments occurs in sheltered, inner areas of low energy.The effects of hydrodynamism thus result in a gradation of sediment grain size along the rias, which translates into a presence of assemblages typical of coarse sediment in the outer areas and those of fine sand or mud in the inner areas.Those patterns can be altered by the construction of artificial structures on the shoreline, which results in changes in current dynamics (López-Jamar and Mejuto, 1985), or by the deposition on the sediment of large amounts of pseudofaeces produced by mussels cultured on rafts (Abella et al., 1996).
The total number of species has been used to describe biodiversity in many studies (Olsgard et al., 2003) and therefore will be used here as a simple measure to establish comparisons in benthic diversity among different areas.In general, the total number of species found on the subtidal soft bottoms of the Ría de Aldán (496) was high taking into account the spatial scale of sampling (<10 km) and in comparison with other Galician rias and with estuaries, fjords and bays across the European Atlantic (see Table 6).However, accurate comparisons among different geographic areas according to total number of species and other biological parameters are limited by differences in local physical conditions, taxonomic knowledge, sampling procedures (e.g.mesh size, sampling gear, sample size) and time of the year of sampling (De Grave et al., 2001).The benthic biodiversity of the Ría de Aldán mostly agrees with that reported from other similar Galician rias, such as the Ría de Ares-Betanzos and the Ensenada de Baiona.This high biodiversity might be related to the variety of sediments present in these rias, ranging from gravel to mud.In short, this sedimentary heterogeneity provides a large variety of ecological niches across a small spatial scale, thus supporting a greater diversity of species than more homogenous sediments do (Pearson and Rosenberg, 1978).In addition, the high primary production related to upwelling events occurring in the rias usually twice a year (Figueiras et al., 2002) might grant a priori an important food supply for the benthic fauna which, in turn, might benefit the presence of richer faunas there than in other areas.On the contrary, previous work done in other Galician rias of similar or greater size (e.g.Ría de Vigo, Ría de Pontevedra) shows that these have a smaller number of species (Mora et al., 1982;López-Jamar and Cal, 1990).This fact might be related to the greater sedimentary homogeneity found there, with a predominance of the silt/clay fractions; the greater presence of mud is greatly related to anthropogenic alterations such as those reported above.
In conclusion, the subtidal sediments of the Ría de Aldán show a rich benthic fauna whose diversity and distribution is linked to the hydrodynamic and sedimentary features, as occurs in other Galician rias.The overall situation suggests that most of the benthic environment of the Ría de Aldán has not yet been greatly altered by phenomena of organic enrichment in spite of the presence of a number of rafts devoted to mussel culture in the central part of the ria.Nevertheless, our results emphasize, again, the need for baseline studies to obtain a basic knowledge of the environment in order to develop proper short-and long-term strategies for conservation that will avoid impoverishment of benthic assemblages and loss of biodiversity.(Glémarec, 1973); 2, Assemblage of Goniadella-Spisula (Salzwedel et al., 1985); 3, Branchiostoma lanceolatum of gravel and sandy gravel facies (Toulemont, 1972); 4, Gravel and coarse sand assemblage (Troncoso and Urgorri, 1993); 5, Coarse-sand sediments (Besteiro et al., 1987); 6, Facies of Zostera noltii (Olabarria et al., 1998); 7, Syndosmya alba community (Petersen, 1918); 8, Other facies from Syndosmya alba community; 9, Heterogeneous sandy gravels and heterogeneous muddy sands (Chassé and Glémarec, 1976); 10, Boreo-mediterranean Amphiura community (Petersen, 1918); 11, Maldane glebifex-Amphiura filiformis-Amphiura chiajei subcommunity (Toulemont, 1972); 12, Mud assemblage (Moreira et al., 2006).

Fig. 1 .
Fig. 1. -Location of the Ría de Aldán (Galicia, NW Spain), and position of the 27 sampling stations with the spatial distribution of faunal assemblages as determined by multivariate analysis.

Fig. 4 .
Fig. 4. -Canonical correspondence analysis (CCA) ordination of stations and environmental variables relative to axes I and II for the Ría de Aldán.Gravel, G; very coarse sand, VCS; coarse sand, CS; medium sand, MS; fine sand, FS; very fine sand, VFS; median grain size, Q 50 ; sorting coefficient, S 0 ; bottom temperature, Bottom temp; sea surface temperature, Surf temp; sediment temperature, Sed temp; carbonate content, Carb; total organic matter content, TOM.

Table 2 .
-Summary of abiotic and biotic characteristics for each assemblage defined by multivariate analyses.TOM: total organic matter; S:

Table 4 .
Table2and the first ten species according to frequency x dominance values of each group of stations are shown in Table3.Group A was located in the outer part of the ria.-Results of SIMPER analysis.Species were ranked according to their average contributions to dissimilarity (AD) between assemblages in the Ría de Aldán.Average abundance (AA), ratio value (R: dissimilarity/standard deviation) and percentage of cumulative dissimilarity (%Cum) were also included.andthemolluscsCaecum trachea and Goodallia triangularis.Group B was located in the sheltered area of the ria.The species that most contributed to similarities in B1 were Pontocythere sp., Exogone hebes and Thyasira flexuosa.SIMPER analysis (Table4) showed that Protodorvillea kefersteini, Streptosyllis websteri, Spio decoratus and Pisione parapari explained most of the dissimilarity between groups A1 and A2.Pisione parapari, Pontocythere sp. and Pisione remota contributed greatly to the differentiation of A1 from B1. Siphonoecetes kroyeranus and Polygordius lacteus differentiated group A2 from B1. Group A2 differed from B3 due to Apseudes latreillii and Polygordius lacteus.Prionospio pulchra and Paradoneis lyra differentiated group A2 from B4, whereas Apseudes latreillii and Caulleriella alata differentiated group B1 from B3. Myodocopida sp. 3, Gammarella fucicola and Parvicardium exiguum explained most of the dissimilarity between groups B2 and B3, while Paradoneis lyra and Apseudes latreillii greatly contributed to separating B3 from B4.

Table 5 .
-Characteristic species of the benthic assemblages of the Ría de Aldán determined by multivariate analyses; other assemblages in which those species were previously reported are also indicated.1, Venus fasciata community