Polychaetes from a southwestern shallow shelf Atlantic area ( Argentina , 38 ° S ) affected by sewage discharge

A benthos-sampling program was carried out in the coastal area of Mar del Plata city in order to know the composition and distribution of infaunal polychaetes living on a subtidal soft-bottom substrate affected by the sewage discharge. Sandy bottoms showed heterogeneous sediment composition and very low organic matter content. Thirty three polychaete species were identified in a range depth between 6.5 to 12.5 m. A comparison with data from 30 years ago from adjacent areas deeper than 12 m is herein presented. Polychaete distribution was related both to the type of sediment and to the content of organic matter. Oweniafusiformis Delle Chiaje was dominant and peaked at stations near the sewage effluent. Small-sized polychaetes were abundant in the sampling area. A possible explanation to the faunal composition of sites near sewage effluents is that hydrodynamic forces frequently disturb the area, and benthic fauna is permanently in an early stage of development. The effect of sewage appeared to be restricted to the shallower subtidal area around the outfall.


INTRODUCTION
Organic enrichment of sediments is perhaps the best-documented disturbance on marine environments (Pearson & Rosenberg 1978).To assess the impact of man-made changes, it is useful the use of benthic fauna, at the level of species, population or community.
One of the most neglected, major group of marine invertebrates may be the polychaetous Annelids that could be useful as indicators of varying degrees of marine pollution (Reish 1980).Polychaetes are used as monitors for toxic materials, as bioassay organisms, and as pollution indicators at the various levels for marine environmental quality (Pocklington & Wells 1992).Reish ( 1955) has early mentioned the relationship between Polychaetous Annelids and harbor pollution.After this, many papers have used these organisms in qualitative and/or quantitative studies about pollution (e.g., Reish 1957Reish , 1963)), including an Annelid Index of Pollution (Bellan 1980) to assess pollution at a site affected by municipal sewage (Bellan et al. 1988).Some species of the families Capitellidae, in particular Capitella capitata (Fabricius 1780), and Spionidae have become widely accepted as organic pollution indicators (Tsutsumi 1990, Pocklington & Wells 1992).
In Argentina, studies on the relationship between polychaetes and pollution are scarce, and are related to the community level.In Blanca bay (39° S), a mixohaline shallow environment, the presence of an infaunal community dominated by Polychaetes was found in relation to organic enrichment (Elias 1992, El as & Bremec 1995).
The Mar del Plata city has only a pre-treatment plant for sewage of 800,000 inhabitants, and the effluent discharges directly on an intertidal northern beach, with a continuous rate flow between 2.5 to 8.0 m 3 seg (in rainy days).However, the city is the greatest recreational place of Argentina, and receives more than 2,000,000 people during summer.The city also has a port that concentrates the catches and commercialization of coastal fisheries.Municipal government has the intention to construct a submarine pipe 3 km seaward, where sand bottoms are at 11 m depth.Previous benthic studies in the area of Mar del Plata were carried out in bottoms deeper than 12 m, almost 30 years ago (Olivier et al. 1968).Very scarce information was obtained till present about benthic communities of the shallow soft-bottoms.
A monitoring program was carried out in this area to know the composition and distribution of macrozoobenthos, to have reference data for comparing the present condition with the resulting changes during and after the pipe construction.In this contribution we describe the polychaete composition and distribution in subtidal bottoms between 6.5 to 12.5 m depth, in front of the domestic sewage discharge.The aim of this study is to detect a polychaete distribution pattern useful for assessing organic enrichment in the area.

Study area
The sampling area is an open coast subjected to the littoral current (south to north) and to autumnwinter storms (predominantly south-southeast).
Sand ridges dominate the northern areas, while sand ribbons characterize the southern part.Ridges are composed by fine sand with coarse material (mainly shells) in the swales, while sand ribbons are fine-sand bodies (Isla & Schnack 19861 ).The sewage flume is mostly directed to north, but with strong north winds it falls to the south (Isla & Ferrante 1997).Organic matter values in sediment range between 0.21 to 0.76 % (Isla et al. 1997).

Sampling procedures and analysis
During 1996 we collected 12 samples from subtidal bottoms (between 6.5 to 12.5 m depth) using a benthic dredge (600 x 250 x 200 mm) during 10 min each (Fig. 1 ).The sediment was sieved (I mm) and the retained polychaetes were identified and counted.Sediment analysis (by dry sieving) was performed for subsamples of sedi-

Linkage distance
Fig. 2: Cluster analysis (1-Pearson r Index, UPGMA) performed on the abundance of 22 polychaete species having a frequency of 20% or more (data transformed to Iog 10 X+ 1), which were found at 12 subtidal stations in front of the Mar del Plata's sewage effluent.
ments, inc I uding the determination of grain-size parameters in phi units ( -log 2 of particle diameter) and organic matter content (Isla et al. 1997).
Polychaete species were ranked according to their dominance (number of individuals/total number I 00).The frequency  1).All specimens were juveniles or smallsized polychaetes.
Owenia fusiformis reached higher dominance in three stations, two of them closest to the sewage effluent ( 4 and 65) with abundances up to 400 ind sampJe• 1 and also associated to low values of both species number and diversity (Table 2).In Cluster and MDS (Fig. 2 and 3), these stations were grouped around a mean phi of 3.0 ± 0.41 and they had the highest organic matter content of sediments (between 0.48 and 0.76 %).Diversity, especially near the effluent, was low.The highest dominance at these stations was achieved by Owenia fusiformis, and, secondarily, by Ampharete sp.The Station 63, the richest and most diverse, was dominated by Aglaophamus uruguayi, Glycera Americana, and Prionospio sp.(with moderate dominances between 19 to 14 %), exhibited a phi of 3.17, an organic matter content of 0.48 %, and it was associated in cluster analysis with the Oweniadominated stations.
Stations 1, 2, 3, 9 and 62 on the one hand, and stations 10 and 11 on the other, were grouped (in opposite sides) Graphically, the two major groups were distributed in fringes parallel to coastal line (Fig. 4).Dimension I seemed influenced by grain-size parameters.Dominance (%) and frequency (% ), and feeding guilds of polychaete species found in the shallow shelf area in front of Mar del Plata city affected by domestic sewage.Feeding guilds follow Fauchald & Jumars ( 1979); the three-letter code denotes feeding type (F for filterfeeders, C for carnivores, S for superficial deposit-feeders, and B for subsuperficial depositfeeders), motility (M for motile, D for discretely motile, and S for sessile), and feeding structures (J for jawed, P for pumping, T for tentacled, and X for other) Dominancia (% ), frecuencia (%) y gremios tr6ficos de poliquetos hall ados frente a Ia plataforma somera de Ia ciudad de Mar del Plata afectada por efluentes cloacales.La asignacion de gremios sigue el criteria de Fauchald & Jumars (1979); el codigo de tres letras indica tipo de alimentacion (F para filtradores, C para carnfvoros, S para alimentadores de deposito superficial y B para alimentadores de deposito subsuperficial), movilidad (M para moviles, D para discretamente moviles y S para sesiles) y presencia de estructuras alimentarias (J para mandibulados, P por bombeo, T para tentaculados y X para otras estructuras) Fig. 3: Multi-dimensional scaling analysis of 12 subtidal stations located in front of Mar del Plata sewage, which included 22 polychaete species whose frequency was 20 % or more.
Analisis de escalamiento multidimensional de 12 estaciones submareales frente a Ia descarga cloacal de Mar del Plata con Ia inclusion de 22 especies de poliquetos cuya frecuencia fue de 20 % o superior.

TABLE2
Biotic parameters of polychaete benthic assemblages of sampling stations in the Mar del Plata shallow shelf affected by sewage outfall.Parameters include maximum polychaete dominance, total abundance (A, ind•sample-1 ), species richness (S), species diversity as measured by the Shannon index (H' ), sediment grain size (Phi = -log 2 of sediment particle diameter), and the content of organic matter of sediments in each station Parámetros bi6ticos de los ensambles de poliquetos en estaciones de muestreo ubicadas en Ia plataforma somera de Mar del Plata afectada por vertidos cloacales.Los parametros incluyen dominancia maxima de poliquetos, abundancia total (A, ind muestra•'), riqueza de especies (S), diversidad de especies de acuerdo al fndice de Shannon (H'), tamafio de partfcula del sedimento (Phi= -log 2 del diametro de las partfculas), y contenido de materia organica del sedimento en cada estaci6n

DISCUSSION
The only available information about polychaetes in the area of Mar del Plata is referred to bottoms deeper than 12 m and is represented by records cited in Olivier et a!. ( 1968), and taxonomic reports by Orensanz (1973Orensanz ( , 1974)).
The onuphids Onuphis eremita Audouin & Milne Eduards 1833 and 0. setosa Kinberg 1865 were found in waters shallower than previous records, more than 40 m in the former, and 13 to 25 m the latter, while Diopatra viridis was frequent in the subtidal sandy bottoms of the area (Orensanz 1974 ).In Talcahuano bay (Chile), an environment subjected to sewage outfall, Diopatra chilensis Quatrefages 1865 is the third species in terms of biomass (Oyarzún et a!. 1987).Two lumbrinerids were recorded, but only Lumbrineris tetraura was the most frequent.This species has been cited for sandy bottoms of the area, being a common predatory polychaete (Orensanz 1973, Carrasco & Oyarzún 1988).Oyarzún eta!. ( 1987) found a very important biomass of L. tetraura in the nearest station in front of the Talcahuano port affected by sludge from factories and municipal sewage.However, it has been suggested that if the genus Lumbrineris is absent, and the diversity is low, the chance that the site is impacted is great (Rygg 1985).Other frequent predatory polychaete species were Glycera americana, common in the area at all depths, Aglaophamus uruguayi, Syllidae, and Phyllodoce sp.
The pattern of polychaete distribution reported herein agrees with previous analyses carried out at a community level in the study area (Elfas eta!. 1997 3 ), and in which sampling stations formed three groups after conducting principal component and cluster analyses.One of them contains the "rich" stations (mean species number S = 34, mean abundance A= 544 ind sample-1 ) in fine and very fine sand (phi= 3).An association characterized by juveniles of the bivalve Amiantis purpuratus (Lamarck 1835), tanaidaceans and amphipods inhabits these stations.Other group cluster of "poor" stations (meanS= 19, mean A= 128), located in coarse sand and gravel with shell debris (phi between 1 and 2), was dominated by the crustacean Corophium sp., the polychaetes Armandia sp.andAmpharete sp., and amphipods.A third group, with the high loading stations 4 and 65 (the closest to the effluent), is dominated by the polychaete Owenia fusiformis.The similarity of these results reveals the important role of polychaetes in structuring the subtidal community.
On the other hand, other benthic associations occupied bottoms deeper than 12m.The shallowest one is characterized by the anthozoan Renilla sp., the echinoderm Encope emarginata (Leske 1778), and the amphioxus Branchiostoma platae Hubbs 1922 (Olivier et a!.1968).Owenia fusiformis was widely distributed in the stations deeper than 12m, but rare (Olivier eta!. 1968).The occurrence of a relatively high and stable population abundance of 0. fusiformis (45 % of the total biomass of the assemblage) was also reported in relation to domestic sewage in the Mediterranean Sea, which is supported by the increased possibilities of flow changes at the mouth of the harbor (Pinedo et al. 1997).In the Talcahuano bay, in the vicinity of a sewage discharge, 0. collaris (= 0. fusiformis) is also frequent (Oyarzún eta!. 1987) Gambi 1989) that alternates its feeding mechanisms in relation with the environmental conditions.When high planktonic inputs are produced and flow conditions change, 0. fusiformis can behave as a filter feeder (Gambi 1989).Desroy et a!. ( 1997) show that filter feeding predominates at the adult population level, while juveniles alternate between deposit and filter feeding activities under still water conditions at similar frequencies.In the Mar del Plata area, the great south to north current with the high suspended matter due to the domestic sewage allows this species to be enhanced at intermediate distances from the effluent.
Capitella capitata, a classical indicator of organically enriched sediments, appeared once and at low abundance.However, it is frequent and abundant in the interstitial sediments of intertidal mussel beds developed around the effluent (Elías et a!.1999 4 ).
It is remarkable that the shallower shelf was never studied faunistically, and several species were new records for Argentine waters, like Magelona riojai (Elías & Bremec 1997), and the genus Armandia (Elías & Bremec unpublished results).Besides, at least two species (Aricidea and Prionospio) seem to be either new for the area or new species.The fringe between 6.5 to 12.5 m depths seems characterized by small-sized polychaetes, like Owenia fusiformis, Prionospio sp., Magelona noJal, Scoloplos (S.) sp., Paraprionospio pinnata (Elhers 1901 ), Armandia sp., and Glycera americana.Most of these polychaete species are deposit feeders or carnivores, preying upon other polychaetes or small invertebrates, while suspensivores are scarce (Table 1).
A similar association of small polychaetes was observed in the northern shallow shelf of Chile and Perú, characterized by species of the genus Aricidea, Tharyx, Prionospio, Owenia, Magelona and Leitoscoloplos, and also by the mediumsized Paraprionospio pinnata (Carrasco 1997).The frequent dominance of the macrobenthos by small-sized forms has been already reported (Beukema 1988).A model proposed by Rumohr et a!. (1996) for benthic communities at the Baltic sea predicted a intermediate successional stage (III) dominated by small polychaete worms (or their ecological equivalents, like amphipods) in relation to a gradient of bottom organic enrichment.
Other possible explanation for the presence of small-sized polychaetes is that hydrodynamics forces frequently disturb the area, and benthic fauna is permanently in an early stage of development.Recolonization following complete defaunation due to disturbances caused by winds and storms is frequently reported on benthic environments (see Probert 1984 ).Small, opportunistic, tube-dwelling polychaetes are the first faunal components to colonize new or newly disturbed bottoms (Rhoads & Boyer 1982).In the shallow shelf of southern Brazil, it is suggested that after the winter defaunation a recolonization period by opportunistic species (like spionid Polychaetes) takes place (Paiva 1993).In Talcahuano bay (Chile), the small opportunistic spionid Cossura chilensis Hartmann-Schroeder, 1965 dominated bottoms affected by domestic sewage (Oyarzún et al. 1987).
Physical disturbances could be responsible for the low diversity, the small size of dominant fauna, and for the great variability in species number and relative abundance of these assemblages.Sedimentary pattern found in this study agrees with a high-energy environment (Isla eta!. 1997).Other studies carried out on macrobenthos of the Argentine shelf also revealed a great bottom heterogeneity, even in relatively small areas, and probably due to hydrodynamic forces (Olivier eta!. 1968, Roux et al. 1993 ).These facts suggest that the main structuring factor in the shallow shelf is the physical disturbance due to strong hydrodynamic forces.However, it is suspected that between 0-6 m depth sewage discharge will be also an important structuring factor of both polychaete populations and macrobenthic communities.

AKNOWLEDGEMENTS
To the crew of Coast Guard CG-64 for logistic assistance.The student Julia Desojo and Lie.J.P. Martin participated in sampling and sorting.We are indebted to Dr. F.I. Isla for including us in the sampling program and for advice in sediment analisis.This work was funded by the Obras Sanitarias Sociedad de Estado (Mar del Plata).Stuart Anderson and two anonymous reviewers improved the english version.This paper was read in the Sixth International Polychaete Conference, 2-7 August of 1998, at Curitiba, Brazil.This study corresponds to the scientific contribution 108 from Departamento de Ciencias Marinas, and to the scientific contribution 1147 from

Fig. 1 :
Fig. 1: Location of sampling area and sampling stations around sewage outfall in the shallow shelf area in front of the Mar del Plata city (38° S, Argentina).Ubicaci6n geogr fica del rea y de las estaciones de muestreo alrededor del etluente cloacal en Ia plataforma somera frente a a ciudad de Mar del Plata (38° S, Argentina).