Polychaete assemblages in the Argentinean Biogeographical Province, between 34° and 38°S

Few studies on benthic polychaetes have been undertaken in the Southwestern Atlantic, especially given the wide geographical area involved. As a result, the group has been excluded from previous analyses of the structure and spatial distribution of benthic communities in the Argentine Sea (i.e. Bastida et al., 1992). Currently, the presence of polychaetes in southern regions is mainly monitored as part of the benthic assemblages in areas subjected to different anthropogenic impacts: trawling fisheries (e.g. Bremec and Lasta, 2002; Roux et al., 2002, 2005) or pollution (e.g. Elías and Bremec, 1994; Elías et al., 2004). Our study area comprises the Uruguayan and northern Argentinean shelves, both belonging to the Argentinean Biogeographical Province (Boschi, SCIENTIA MARINA 70S3 December 2006, 249-257, Barcelona (Spain) ISSN: 0214-8358


INTRODUCTION
Few studies on benthic polychaetes have been undertaken in the Southwestern Atlantic, especially given the wide geographical area involved.As a result, the group has been excluded from previous analyses of the structure and spatial distribution of benthic communities in the Argentine Sea (i.e.Bastida et al., 1992).Currently, the presence of polychaetes in southern regions is mainly monitored as part of the benthic assemblages in areas subjected to different anthropogenic impacts: trawling fisheries (e.g.Bremec and Lasta, 2002;Roux et al., 2002Roux et al., , 2005) ) or pollution (e.g.Elías and Bremec, 1994;Elías et al., 2004).

SCIENTIFIC ADVANCES IN POLYCHAETE
In this paper we analyse macrobenthic samples taken between 34°S and 38°S, in 1993, 2001 and 2002 during oceanographic surveys in mixohaline to oceanic waters.This contribution provides a preliminary characterisation of local polychaete assemblages and extends biogeographic knowledge of the polychaete fauna in the Argentinean Biogeographical Province.

MATERIALS AND METHODS
The material studied was collected at 49 stations, between 34 and 38ºS, in a depth range of 8-270 m (Fig. 1).Samples were obtained during 3 surveys: cruise EH-09-1993 (epibenthic sledge, 200 × 50 cm mouth opening, soft bottoms); cruise EH-09-2001 (Picard dredge, 65 × 20 cm mouth opening, in sandy bottoms and van Veen grab, 0.1 m 2 , in muddy bottoms); cruise CC-02-2002 (rectangular dredge, 2.5 × 0.5 m mouth opening, in gravel-sandy bottoms of a mussel bed and a lemon-fish feeding ground).As different gears were used to collect the samples from different types of bottoms, we present the results in terms of the relative abundance of the taxa found.
Benthic samples were sieved (0.5 mm mesh screen) and specimens were fixed in 4% formalin, preserved in 70% alcohol and identified to the lowest taxonomic level possible.In order to indicate the relative proportion of polychaetes within the benthic assemblages the total number of taxa of the major invertebrate groups was summed and the percentage of taxa of each group was estimated for the 3 surveys.
Cluster analysis (Bray-Curtis similarity index, UPGMA, Q mode) (Clarke and Warwick, 1993) and non-metric multidimensional scaling (MDS) (Clarke, 1993) were applied to the 4 th root transformed relative abundance (each species count divided by the total abundance per sample) of poly- chaetes.The SIMPER analysis (Clarke, 1993) was used to identify which species contributed most to the similarity between samples.Stations 48 and 49, where only one polychaete taxon was collected, were excluded from the data matrix.

Faunistic analysis
A total of 57 polychaete taxa distributed among 29 families was found (Table 2).The percentage of polychaetes which represent the fraction of the macrofauna larger than 0.5mm in the bottom samples was rather constant (20-24%) in the different habitats studied between 34ºS and 38ºS.Lumbrineridae and Onuphidae were the most diverse, with 8 and 6 taxa respectively.The highest frequency of occurrence was 44% (21 stations) for Onuphidae and 24% (13 stations) for Lumbrineridae.The other families were collected in 1-12 sampling stations.
The number of invertebrate species collected in soft bottoms was 128 and 141, during 1993 and 2001 respectively, and 118 in mussel and lemon-fish grounds during 2002.The faunistic composition of the benthic assemblages sampled in the 3 cruises showed a similar relative percentage of polychaete species.Molluscs (25-28%), Crustaceans (22-37%) and Polychaetes (20-24%) were the most diverse groups.

Polychaete assemblages
The cluster analysis produced 5 main groupings of stations (15% similarity level) and the corresponding MDS plot (Fig. 2) shows an ordination of samples according to the following areas: -the central

Environmental analysis
The matching of the environmental and faunal matrices (BIO-ENV analysis) is outlined in Table 3.No single variable provided the best correlation with biotic patterns.The abiotic variable that best explained the distribution pattern was the bottom salinity, while the best combination included 2 variables: bottom salinity and type of bottom.-Cluster analysis of stations (Bray-Curtis similarity index) according to presence-absence of polychaete feeding types in the study area (34ºS-38ºS).(cluster 1, deposit and filter feeders; cluster 2, omnivores; cluster 3, carnivores; cluster 4, mainly carnivores and depositfeeders with the addition of omnivores and suspension feeders).

DISCUSSION
As the sampled area included a variety of substrata ranging from consolidated sandy-gravel to soft sediments, it was not feasible to use the same sampling device.Consequently, due to this methodological constraint, the information presented here is based on the relative abundance and distribution of taxa.This contribution intends to compile information for a large area and future studies will attempt to obtain comparative quantitative samples from the different habitats, throughout the geographical range under study.
The dominance of polychaetes in a variety of macrofaunal assemblages and geographical regions is well documented, even though comparing data sets is difficult due to differences in sampling techniques, mesh size used and number of replicates (i.e Hutchings, 1998 and references).In previous studies in the region, in northern temperate latitudes (32ºS -34ºS), the number of polychaete species reaches 36% of the total number of species collected in a large variety of bottoms between the inner shelf and the upper slope, and the number of specimens represents 48% of the total number of macroinvertebrates (Capitoli, 2002).In southern areas of the southwest Atlantic, these estimates reach 9% in sandy bottoms on the Patagonian shelf and 35% in diverse substrata in the Magellan Strait (Gambi and Mariani, 1999;Bremec et al., 2000).
The multivariate analysis produced 5 different clusters of stations characterized by different suites of polychaetes in the study area.The spatial distribution of these sampling stations and the results of the SIMPER analysis clearly show that, in general, the polychaete fauna from the central estuary and mouth of the river (group 1) at almost 30m depth, is different from that collected at other coastal stations (groups 2, 4 and 5).Secondly, the deeper stations are clustered in group 3 and characterized by another suite of polychaetes.The BIO-ENV analysis indicates that species are distributed mainly according to a salinity gradient and bottom type.These variables serve to explain the patterns of faunal distribution in this study area that comprises an estuarine muddy sector and sandy or gravel-sandy bottoms of the adjacent marine waters, and a few stations on the open shelf.The granulometric type is the main abiotic variable that determines the spatial distribution of polychaete assemblages in other South American systems influenced by continental waters, like Lagoa dos Patos on the south Brazilian shelf (Capitoli et al., 2004) and Bahía Blanca estuary on the Argentinean shelf (Elías and Bremec, 1994).The close relationship between benthic fauna and sediment type has previously been recognised in classical papers regarding spatial distribution of benthos (e.g.Thorson, 1957;Sanders, 1958;1968;Lie and Kelley, 1970;Day et al., 1971) and sediment was considered as an indicator of the availability of feeding resources (Sanders et al., 1962;Rhoads and Young, 1970;Young and Rhoads, 1971).Recent investigations continue to show that the sediment characteristics are primary factors determining the spatial patterns of macrobenthos and polychaete assemblages (e.g.Muniz and Pires, 2000;Probert et al., 2001;Hernández-Arana et al., 2003;Rodriguez-Villanueva et al., 2003;Dauvin et al,, 2004;Díaz-Castañeda and Harris, 2004).
Mainly omnivores (Onuphidae) and carnivores (Polynoidae, Glyceridae, Lumbrineridae, Nephtyidae) characterize the different stations in the estuarine muddy areas.A variety of feeding types were found in the rest of the sandy coastal stations and in areas deeper than 23m.Studies on spatial distribution of macrobenthos in the study area show a higher biomass of deposit-feeders and carnivorous molluscs in the estuarine area and various feeding types widely distributed in the marine and open waters (Giberto et al., 2004).Polychaetes, as a group, reflect the trend observed for the benthic community as a whole.It was found that generally trophic diversity increased with salinity, meaning that a more even distribution of trophic structure is found at higher salinities (Gaston et al., 1998), which suggests differences in the availability of resources and in the interactions of food webs (Brown et al., 2000).The area studied comprises the Río de la Plata estuary and the adjacent marine coastal and open waters and little is known about trophic webs.The large estuarine zone (38,000 km 2 ) has both bottom and surface salinity fronts, important in fish reproductive processes and where high concentrations of zooplankton occur, and a turbidity front in the inner estuary (Mianzan et al., 2001a), with high biomass of deposit-feeding bivalves (Giberto et al., 2004).The whole study area is disturbed by trawling fisheries, many of the catches of which are dependent on benthic food resources.Thus future studies need to be conducted to assess the functional role of these benthic communities and in particular of polychaetes.
Our present records extend the distributional range of Sabellaria bellis from the mouth of the Río de la Plata estuary at 35°S (Bremec and Giberto, 2004) to 38°S (Station 7, 79 m depth), and the bathymetric range of Travisia amadoi from intertidal and shallow sandy bottoms (Elías et al., 2003a) to depths of 125 and 270 m (Stations 9 and 8 respectively) .Additionally, the southern distribution range of two more species is extended: Terebellides lanai, originally described from southern Brazil (Bremec and Elías, 1999), was collected at 35°S (Station 22) and Aglaophamus uruguayi, described for muddy bottoms at 33°S 51°W (Orensanz and Gianuca, 1974), was collected between 35°S and 37°S, at 24 m depth (Stations 45 and 12) and in deeper sandy substrata at 125-270 m depth (Stations 9 and 8).Our study material was collected in warm temperate waters of the Argentinean Biogeographical Province (Boschi, 2000a).It includes coastal waters from latitude 43-44°S (Patagonia) reaching 23°S (southern Brazil).The Río de la Plata (35°S) is the major hydrographical feature within this province (Boschi, 2000b), and considered a significant zoogeographic barrier.However, this barrier was found to be variable for plankton and fish species, according to climatic variation and discharge patterns of the river (Mianzan et al., 2001b).In the case of the decapod Ebalia rotundata (A. Milne Edwards, 1880), found in marine waters off Argentina and Uruguay, it was considered that both thermal and salinity tolerance could con-strain the dispersal of the species from south to north and within estuarine waters respectively (Giberto and Bremec, 2003c).Local patterns of larval transport could allow the colonization of new areas from Brazil to the south and the role of the Río de la Plata regime as a biological barrier to the distribution of coastal polychaete fauna between Uruguay and Argentina still has to be assessed.More frequent sampling and detailed examination of the polychaetes will almost certainly lead to new records and provide more detailed distributional patterns.

TABLE 1 .
-Depth, bottom temperature, bottom salinity and bottom type of the sampling stations between 34ºS and 38ºS.
SCI. MAR., 70S3, December 2006, 249-257.ISSN: 0214-8358 estuary and mouth of Río de la Plata from 8 to 27 m depth, reaching the Uruguayan coast at 39 m depth (group 1), -the coastal shelves of Uruguay (15-39 m) and Argentina (20-74 m) and the transect off the mouth of Río de la Plata, between 24 and 270 m depth (groups 2 and 3).Other coastal stations from both countries are clustered in groups 4 and 5.The results of the SIMPER test show that group 1 clusters stations (average similarity 21.40) where Polynoidae , Onuphidae and Glyceridae are the most representative taxa.Group 2 (average similarity 29.95) represents areas with Ampharete sp. and Serpulidae and group 3 (average similarity 15.21) with Lagisca sp., Eunice argentinensis, Lumbrineris sp., Diopatra viridis, Phyllochaetopterus socialis, and Maldanidae and Cirratulidae, as the most rep-resentative taxa.The species that contribute most to the average similarity of group 4 (35.00) are Nephtys sp. and Glycera americana, and Onuphis tenuis to the average similarity of group 5 (51.41).Stations 32 and 47 are characterized by Nephtyidae.

TABLE 3 .
-Combinations of n variables giving the largest rank Spearman correlations between biotic (4 th root transformed relative abundance data, Bray-Curtis similarity) and environmental (Euclidean distance) similarity matrices (BIOENV analysis).Bold type indicates overall optimum.