Thalassas, 2004, 20 (2): 39-53 An International Journal of Marine Sciences

POLYCHAETE ASSEMBLAGES OF INTERTIDAL MIXOHALINE FLATS OF BAHÍA SAMBOROMBÓN (LA PLATA RIVER ESTUARY-ARGENTINA)

J. P. MARTIN(1,3), R. BASTIDA(1,2) & M. TRASSENS(1,2)

Keywords: polychaete, macrobenthos, tidal flats, estuaries, Bahía Samborombón , La Plata River.

ABSTRACT dominated by Laeonereis culveri and presented as secondary species Heteromastus similis, Neanthes During November 1998- December 1999 period, a succinea, Nephtys fluviatilis and Scolecolepides sp.. monthly benthic sample program was carried out along The other assemblage had Laeonereis culveri as the intertidal flats of the southernmost part of Bahía dominant species and Capitella capitata as Samborombón (external zone of La Plata River subdominant member, being secondary species estuary). The main objectives of this study were to Heteromastus similis, Neanthes succinea, Polydora define the spatial and temporal distribution patterns of cornuta and Scolecolepides sp.. The polychaete polychaete species in this mixohaline environment and assemblages of the studied area showed great affinities to determinate the main environmental factors related with other mixohalines coastal areas of Northern with the distribution of these invertebrates. Two low Buenos Aires Province and Southern Brazil diversity faunistic assemblages were identified in the studied area. One of them was related to muddy INTRODUCTION intertidal flats of San Clemente creek, characterized by its high clay and organic matter contents. The other The coasts of the Buenos Aires province are assemblage was related to the sandy bottoms of Punta characterized by two main environments: marine Rasa, an area with a higher marine influence, lower sandy beaches and mixohaline intertidal muddy organic matter content and fine sand as the sediment flats. This last environment is mainly distributed dominant fraction. The first of these assemblages was along La Plata River estuary and Mar Chiquita coastal lagoon. There are also other less important estuarial areas as Bahía Blanca, Quequén Grande and Colorado rivers. (1) Depto. de Ciencias Marinas, Universidad Nacional de Mar del Plata, CC 43 (7600), Mar del Plata, Argentina, e-mail: [email protected] The mixohaline intertidal flats play an important (2) Consejo Nacional de Investigaciones Científicas ecological role and are characterized by the presence y Técnicas (CONICET) of muddy-sandy sediments conforming beaches of (3) Centro de Investigaciones de Puerto Deseado, Universidad Nacional de la Patagonia Austral, low slope. The more extended ones are those present Puerto Deseado, Argentina in the southernmost area of Bahía Samborombón, in

39 J. P. Martin, R. Bastida & M. Trassens

the external zone of La Plata River estuary. These Benthic invertebrates of intertidal flats, mainly intertidal flats have low water energy but great polychaetes, are the most important trophic items of variability in relation with physical and chemical many resident and migratory fishes and birds of the factors, and are inhabited by infaunal communities mixohaline environments of the Buenos Aires province. of low diversity (Ieno & Bastida, 1998). Among the In this way, energy is transferred from low trophic levels few faunistic groups that conform these to higher ones (Olivier et al., 1972b; Ieno, 2000; Martin, communities it can be mentioned decapods 2002), being the top predators of Bahía Samborombón crustaceans and polychaetes. several marine mammal species (Rodriguez et al., 2002). These infaunal organisms exert also an important The first mixohaline polychaete studies of influence in the substratum characteristics through their Argentina were carried out in Mar Chiquita coastal biological actions, mainly related with sediment lagoon by Olivier et al. (l972 a, 1972 b), and followed particles transport and organic matter transformation by Ieno & Elías (1995), Schwindt & Iribarne (2000), (Palomo & Iribarne, 2000; Martin, 2002). Palomo & Iribarne (2000), among others. The coastal polychaetes of Bahía Blanca were studied by Elías Based on these aspects the main objectives of this (1992a, 1992b), Elías & Ieno (1993) and Elías & study were to characterize the temporal and spatial Bremec (1994). More recently, polychaete distribution pattern of polychaete fauna of the communities of Río Quequén estuary were studied by intertidal flats of Bahía Sambombón, and to know its López Gappa et al. (2001). relation with several environmental parameters.

Figure 1. Study area and sampling stations.

40 Polychaete A ssemblages of Intertidal Mixohaline Flats of Bahía Samborombón (La Plata River Estuary-Argentina)

MATERIALS AND METHODS water influence. The studied zone can be classified as a typical mixohaline (oligohaline and polihaline) Study area environment (Urien, 1972).

La Plata River estuary, together with its oceanic Sample collection and analysis front, forms a wide ecosystem or "ecosystem complex" of great ecological significance, probably one of the Between November 1998 and December 1999 a main natural phenomena that regulate, together with monthly sample program was developed in the the Malvinas-Falklands and Brazil currents, the intertidal flats of the southernmost sector of Bahía general dynamics and biological production of the Samborombón. Four sampling stations were Argentine biogeographical province (Ieno & Bastida, established at different intertidal levels of the San 1998). The present study was conducted in the external Clemente creek (36º 19' 30" S; 56º 46' 30" W) and sector of La Plata River estuary at the extreme Punta Rasa (36º 20' S; 56º 45' W): stations SC1 and southeastern section of Bahía Samborombón, in the SC2 correspond to the higher and lower levels of the estuarine zone located between Punta Rasa and San intertidal zone of San Clemente creek, and stations Clemente harbor (Fig. 1). PR1 and PR2 correspond to the same levels at Punta Rasa (Fig. 1). The studies were carried out in an environment characterized by soft bottoms of fine sediments with One sample and two replicates were obtained at high percentage of mud and organic matter. In the each station during low tide, with a 12 cm diameter southern tip of Bahía Samborombón there is a sandy core to a depth of 30 cm. The invertebrates were coastal bar, oriented NW, known as Punta Rasa. This separated from the sediments using different mesh size formation is generated as a result of the coastal drift, sieves. The first 10 centimeters of the sediment core and in its internal bay margin, there is an important were sieved in the laboratory under stereoscopic sector dominated by intertidal flats. These are microscope using a mesh of 0.1mm that allowed to characterized by fine sand sediments, over which retain both macro and meiobenthos organisms. The important mud sedimentation processes takes place rest of the sediment core was sieved in the laboratory due to the low water energy of the environment and using a 0.5mm mesh. All benthic organisms were fixed the great extension of the very low slope beach in 10% formalin neutralized solution, and some (lower than 0,4%, according to Bértola & Morosi, polychaetes were preserved afterwards in 70% ethanol. 1997). All organisms were identified to species level and counted (Holme & Mc Intyre, 1984). The margins and bottom of the tidal creeks that flow into Bahía Samborombón are mainly dominated Simultaneously, a seasonal sampling was carried by silt and clay sediments. The dominant vegetation of out in the intertidal level of San Clemente creek this mixohaline marsh area is composed of Spartina (stations SC2, SC3 and SC4) and Punta Rasa (stations densiflora Brong, Spartina densiflora Loisel, PR1, PR2 and PR3) (Fig. 1). During each sampling, Salicornia ambigua Michaux and Scirpus sp., while sediments were also obtained for granulometric the most conspicuous invertebrate is the mud crab studies, organic matter content and depth of the redox Chasmagnathus granulata Dana, 1851 who makes big layer determination. In all of them, standard burrows in the soft bottom sediments (Botto & sedimentological methods were used (Ingram, 1971; Irigoyen, 1979; Ieno & Bastida, 1998). Galehouse, 1971). Sediment fractions were classified using the Wentworth (1922) scale, and the The tidal amplitude is below 2 meters, with spring granulometric parameters considered were mean grain tides between 0.37 and 1.46 m and ebb tides between size (Mz) in phi units (- log2 of grain diameter) and the 0.58 and 1.25 m (Bértola & Ferrante, 1996). standard deviation (Sd), which indicates the selection Superficial water temperature ranges from 9 to 25° C, grade of the sediment. Organic matter determinations while the salinity ranges between 0.5 and 25 ‰, were based on the oxidation method proposed by depending of the tidal level and the fresh and marine Walkley & Black (1965).

41 J. P. Martin, R. Bastida & M. Trassens

During monthly sampling, other data was also was composed by fine sands (5 to 30%) and medium obtained as: air, water and sediment temperature; water sands (0.3 to 2.5%) (Table 1). and interstitial sediment salinity. Both parameters were obtained in-situ using a digital thermometer and a The mean grain size (Mz.) varied between 7 and 9 pocket refractometer. phi, while high values in standard deviation of the sediment (Sd. 1 to 2.7) indicated a poor selection. No Statistical analysis seasonal significant differences were observed in granulometric values of sediments, indicating that the Polychaete abundance and density variations in substratum is stable along the whole year. relation to localities and intertidal levels were analyzed through ANOVA unifactorial test. The normal The sediment of the intertidal flats of Punta Rasa distribution of data was checked through by (stations PR1, PR2 and PR3) was composed mainly by Kolmogorov-Smirnov test, while the analysis of fine sands (68 to 89%), with higher values than those homogeneity in the variances was made using Levene of San Clemente creek (PR: mean= 82.36%, n= 4; SC: test (Zar, 1984; Dytham, 1999). When data did not fit mean= 22.58%, n= 4; U-Test; P= 0.021). Medium and the normality and homogeneity of variances, coarse sand was present in lower percentages (5 to differences in the density of the organisms were 24% and 0.6 to 2.8% respectively) (Table 1). analyzed using the Mann-Whiney non parametric test (U-Test). The different sediment fractions percentages Sediments of the intertidal flats of Punta Rasa (mud -silt+clay -, fine sand, medium sand) and organic differentiate clearly from San Clemente creek because matter were compared using the Mann-Whitney (U- of its low mud content (Table 1) (PR: mean= 4.93%, Test) (Zar, 1984; Dytham, 1999). n= 4; SC: mean= 76.36%, n= 4; U-Test; P= 0.021). Mean grain size (Mz.) reached between 2.5 and 2.7 phi For those statistical analysis the program Statistica and the standard deviation (Sd. 0.5 to 0.9) indicated a (edition 1999) was used, and for diversity aspects the moderate selected sediment. Shannon-Wiener (H') index (Pielou, 1966). Punta Rasa sediments are influenced by near Statistical multivariate analysis (Redundancy marine sandy beaches and the values obtained were analysis or RDA) (Manly, 1994) was used to know the similar to those found by Bértola et al. (1993) in the grade of association between distribution and southernmost area of Bahía Samborombón. These data abundance of polychaete and the main environmental indicate that silt and clay values tend to be higher in parameters. Among the last ones, it can be mentioned relation with ocean distance, reaching the highest granulometric fractions of sediments (mud, fine sand, values in the tidal creeks. Sand transport is produced medium sand), organic matter percentage, interstitial by the coastal drift from the southern marine beaches, salinity and sediment temperature. All these values and is deposited afterwards on the intertidal flats of were matched with polychaete species abundance Punta Rasa due to the low energy of the system

(transformed as log10 X+1).The statistical analysis was (Bértola et al., 1993). carried out using Brodgar multivariate program version 1.8. Organic matter: The organic matter content of superficial RESULTS AND DISCUSSION sediments of San Clemente creek ranged high values between 6.5 and 8.4%. The highest values were Sediment characteristics obtained in the stations close to San Clemente harbor (stations SC2 and SC3), near the urban sewage. Granulometry: Anoxic sediment layer was observed at 6-8 cm Intertidal flats of San Clemente creek (stations sediment depth. SC2, SC3 and SC4) were characterized by fine sediments, mainly mud, composed by 55 to 85% of Values obtained at Punta Rasa were lower than clay and 9 to 11.5 % of silt .The rest of the sediment those found in San Clemente creek (PR: mean= 1.35%,

42 Polychaete A ssemblages of Intertidal Mixohaline Flats of Bahía Samborombón (La Plata River Estuary-Argentina)

n= 4; SC: mean= 7.89%, n= 4; U-Test; P= 0,021), with The salinity in Punta Rasa sector showed the same mean values between 0.6 and 2.9%. The redox variation pattern as San Clemente creek, although discontinuity layer was deeper, around 15 cm in the values were slightly higher as a consequence of the sedimentary core. Its higher depth is related with a less marine influence. Interstitial water sediments values organic matter concentration and a bigger grain size of ranged between 15 and 20 ‰, similar to those obtained sediments, allowing a better access of oxygen to in the water column. deeper levels of substratum and the subsequent oxidation of the organic matter. Species composition of the intertidal flats community Temperature: Superficial temperature of San Clemente creek Among the macrobenthic organisms, the sediments, during low tide, showed the same seasonal polychaete were the dominant group of the infaunal pattern of air temperature. Temperature data was community with a total of seven species: Laeonereis obtained during highest radiation hours (around noon); culveri (Webster, 1880), Neanthes succinea (Frey & values varied between 10 and 24°C, reaching lowest Leuckart, 1847), Nepthys fluviatilis Monro, 1937, temperatures during July and the highest during Heteromastus similis Southern, 1921, Capitella February. capitata (Fabricius, 1780), Polydora cornuta Bosc, 1802 and Scolecolepides sp. The dominant species of Minimum superficial sediment temperature the community was L. culveri, followed by C. capitata (12.2°C) was obtained during July in Punta Rasa, and N. succinea, while N. fluviatilis, C. capitata and while the maximum value (25.8°C) was obtained Scolecolepides sp. are new zoological records for the during February. Here it was also observed the soften Bahía Samborombón area. effect of water in sediment temperature. During summer air temperatures were higher than in sediment, The decapod crustaceans Chasmagnathus whereas during winter air temperatures were lower granulata Dana, 1851 and Uca uruguayensis Nobili, than those of the sediment. 1901 - together with the mentioned polychaete species- formed the typical fauna of the intertidal Salinity of interstitial sediment water: flats, in both Punta Rasa and the San Clemente creek. Salinity of sediment interstitial water of San Because of its high mobility these crustacean species Clemente creek ranged between 9 and 19 ‰. These were not sampled during this study, but its density data were obtained during low tide and at highest sun was calculated through its caves count. Mean caves radiation hours, although the values were similar to density was around 50 ind/m2. U. uruguayensis those registered in the water column, not detecting showed a patch pattern distribution, covering the increment in salinity by evaporation effects. upper intertidal level were the sediment is more

Table 1. Sediment characteristics based on seasonal sampling. Mz.: mean of grain size; Sd.: standard deviation.

43 J. P. Martin, R. Bastida & M. Trassens

compact. Whereas Ch. granulata was distributed The Punta Rasa macrobenthos showed a higher along the whole intertidal area, mainly in places were diversity than San Clemente creek, with H' mean value the substratum is water saturated, showing a more of 0.974. In this sector, the highest diversity value regular space distribution than that of U. (H'=1.5) was observed during spring in the lower level uruguayensis. of the intertidal zone. These diversity values were related with a decrease of Laeonereis culveri Other benthic invertebrates of the area, but less abundance and a simultaneous increment in the abundant, were mollusks represented by the snail abundance of other associated species. Similar H' Littoridina australis (d'Orbigny, 1835), the razor values were observed in other communities dominated clam Tagelus plebeius (Lightfoot, 1786) and the by polychaete as those of the inner sector of Bahía gammarid amphipod crustacean Bathyporeiapus Blanca estuary (38º 40´ S; 61º 40´ W) and the intertidal bisetosus Escofet, 1970 and Melita sp.. L. australis flats of Mar Chiquita lagoon (37º 46´ S; 57º 27´ W), showed higher densities at Punta Rasa during near its mouth (Elías, 1992a; Ieno & Elías, 1995). autumn, with values up to 3,500 ind./m2, while T. plebeius was only present in intertidal lower level The San Clemente creek most conspicuous of Punta Rasa, forming reduced banks of low meiobenthos species were the ostracod Cyprideis cf. density. salebrosa, the copepod of the genus Harpacticus and an unidentified oligochaete species. C. cf. salebrosa The intertidal macroinfauna was characterized by was present along the whole year in very high its low species richness and diversity values, similar to densities, although maximum values were registered the rest of the mixohaline communities of the Buenos during autumn and spring (values higher than 130,000 Aires Province coastal area (Olivier et al., 1972a; ind./m2). Oligochaete worms were abundant at the Elías, 1985, 1995; Elías & Ieno, 1993; Ieno & Bastida, beginning of spring, when densities up to 20,000 1998; López Gappa et al., 2001). ind./m2 were observed.

As a result of the huge dominance of Laeonereis The Punta Rasa meiobenthos was composed by the culveri, the intertidal flats of San Clemente creek ostracods Cyprideis multidentata Hartmann, 1955 and showed an extremely low diversity, with H' mean C. cf. salebrosa, undetermined nematod worms and the value of 0.072, much lower than those observed in copepod Harpacticus sp.. C. multidentata was the other similar environments of the region. Maximum dominant species reaching its highest densities during diversity values were found during spring (Fig. 2), as a autumn and spring, with values up to 32,000 ind./m2, consequence of an increment in the abundance of the while C. cf. salebrosa reached maximum densities, up rest of the species. to 10,000 ind./m2, during spring.

Figure 2. Macrobenthos diversity (H') at different intertidal levels of San Clemente creek and Punta Rasa.

44 Polychaete A ssemblages of Intertidal Mixohaline Flats of Bahía Samborombón (La Plata River Estuary-Argentina)

Polychaete assemblage of the intertidal flats of San densities than in San Clemente creek. Mean density of Clemente creek L. culveri decreased in relation with San Clemente creek values (PR: mean= 5758.51 ind./m2, n= 55; SC: Intertidal macrobenthos of the muddy flats of San mean= 8185.35 ind./m2, n= 54; U-Test; P= 0.0024), Clemente creek (stations SC1 and SC2) was clearly whereas a significant increase in densities of Neanthes dominated by the polychaete Laeonereis culveri with succinea (PR: mean= 519.26 ind./m2, n= 55; SC: mean densities over 8,000 ind./m2. The highest density mean= 16.37 ind./m2, n= 54; U-Test; P< 0.0001) and was 13,700 ind./m2 corresponding to the high intertidal Heteromastus similis (PR: mean= 170.41 ind./m2, n= level during December (Fig. 3). No significant 55; SC: mean= 19.65 ind./m2, n= 54; U-Test; P< differences were observed between several intertidal 0.0001) was observed. Nevertheless, values of levels of San Clemente creek (Table 2). Other Scolecolepides sp. densities decreased (PR: mean= polychaete species of the assemblage, as 11.25 ind./m2, n= 55; SC: mean= 63.86 ind./m2, n= 54; Scolecolepides sp., Heteromastus similis and Neanthes U-Test; P= 0.0017) (Fig. 4). succinea, were present in the area in much lower densities than L. culveri (Fig. 3). Laeonereis culveri, Polydora cornuta and Scolecolepides sp. did not show significant differences Scolecolepides sp., Heteromastus similis and in the mean density in several intertidal levels of Punta Neanthes succinea had its maximum densities during Rasa, although differences in densities were observed spring, whereas H. similis reached maximum density in the rest of polychaete species (Table 3). Capitella values during spring and summer. Also these species capitata had a greater affinity for higher intertidal didn't show significant differences in its spatial levels, were it is the second species in abundance. On distribution and mean density at several levels of the the contrary, Heteromastus similis and Neanthes intertidal zone (Table 2), allowing us to conclude that succinea demonstrated greater affinity for lower the community is homogenous along the whole intertidal levels. intertidal zone of San Clemente creek. Nephtys fluviatilis was another polychaete species of the Laeonereis culveri in Punta Rasa had its highest assemblage but with sporadic presence along the study densities during autumn and winter (fig.5), while and in low number. Capitella capitata had its highest density during autumn in the lower intertidal levels and during winter Table 2. in the higher levels. Heteromastus similis had a more Mean density of polychaete species at different intertidal levels of San Clemente creek. ANOVA test for differences between levels. variable pattern with highest density during summer, autumn and spring, whereas Neanthes succinea and Polydora cornuta had its highest densities during winter and spring.

Basically, polychaete species showed low densities during summer, probably related with an increase in the trophic activities of fishes and sea birds in the intertidal flats during the warm seasons (Ieno, 2000). Subsequently an increment phenomena of polychaete populations was observed during autumn and winter. Benthic organisms of the intertidal zone of Punta Rasa Polychaete assemblage of the intertidal flats of are predated during summer by a high number of Punta Rasa migratory birds that use Bahía Samborombón as a refuel station (Botto et al., 1998; Ieno & Bastida, 1998; Laeonereis culveri was the dominant organism of Ieno, 2000). These bird species apply a high predation the infaunal macrobenthos of the intertidal flats of pressure on polychaete of the intertidal flats, which Punta Rasa (stations PR1 and PR2). The associated promote an important decrease in density of these species of L. culveri were present here in higher invertebrate preys.

45 J. P. Martin, R. Bastida & M. Trassens

Figure 3. Density of polychaete species at the intertidal region of San Clemente creek.

Figure 4. Mean density of polychaete species ( 1*Standard error and 1,96*Standard error) at San Clemente creek and Punta Rasa.

46 Polychaete A ssemblages of Intertidal Mixohaline Flats of Bahía Samborombón (La Plata River Estuary-Argentina)

Table 3. Mean density of polychaete species at different intertidal levels of Punta Rasa. ANOVA test and Mann-Whitney test (U-Test) for differences between levels (n high level = 27, n low level = 28). * significant differences (p < 0.05).

Environmental factors related with polychaete Laeoneresis culveri showed a negative correlation with species spatial distribution the mentioned species and a very low correlation with the sedimentological factors considered. It seems that There are numerous abiotic factors that determine Scolecolepides sp. was not associated with other the temporal-spatial distribution and diversity of polychaete species, nevertheless, the abundance of this benthic organisms in estuarial environments. These polychaete was positively correlated with mud and factors can be divided in three main groups: those of organic matter content in the sediment, and negatively global scale affecting biogeographical distributions; correlated with the interstitial water salinity. those of medium scale, related with estuaries, as water salinity, oxygen saturation, wave action, pollution The high percentage of the total variance in the conditions, etc. and finally those of small scale as RDA (81.55%, Table 5), indicates that the variation of water depth, current velocity, sediment characteristics, polychaete abundance is strongly related with the etc. (Wolff, 1983). environmental factors considered during the present study. In small scale, sediment characteristics are the Redundancy analysis (RDA) was applied in order abiotic factors with greater capacity controlling the to define the relation between environmental factors ecological process of the community (Zajac & and polychaete species abundance (Tables 1 and 4). Whitlach, 1982), and, close to salinity, they seem the Based on this analysis different patterns were main factors determining the existence of one or observed, being possible to explain the spatial another assemblage in Bahía Samborombón. distribution of several polychaete species of the southernmost area of Bahía Samborombón. Based on RDA analysis, two groups of samples were clearly defined. On the one hand, those that Heteromastus similis, Polydora cornuta and reflect the environmental factors of tidal creeks Neanthes succinea showed a high association in their (samples SC), with high percentages of mud and distribution and abundance, while Capitella capitata organic matter, mainly associated with the presence of was less associated to the mentioned species (fig. 6). Laeonereis culveri and Scolecolepides sp. On the other This polychaete group was highly correlated with the hand, another group of samples was identified and interstitial water salinity and the content of fine and related with the typical environmental factors of the medium sand in the sediment. In other way, intertidal flats of Punta Rasa (samples PR), as high

47 J. P. Martin, R. Bastida & M. Trassens

Figure 5. Density of polychaete species at intertidal region of Punta Rasa. content of fine and medium sand, low percentages of Laeonereis culveri and Scolecolepides sp. seem less mud and organic matter and higher interstitial salinity correlated with sedimentological aspects and show a values. This samples group was characterized by the greater adaptability to different salinity concentrations. presence of L. culveri, C. capitata, N. succinea, H. similis and P. cornuta. Although the presence of Laeonereis culveri seems to be independent of the organic matter contained in The highest abundance of eurihaline marine sediments, the high densities of this species in San species as Capitella capitata, Neanthes succinea and Clemente creek indicate its high tolerance to organic Polydora cornuta in the macroinfauna of Punta Rasa matter concentrations. This fact has been mentioned by indicates a great influence of marine waters, increasing Ieno & Bastida (1989) as they found high densities of the salinity values as well as the percentages of fine L. culveri in San Clemente harbor, highly impacted by and medium sand in sediments. Estuarial species as urban sewage and artisanal fisheries activities.

48 Polychaete A ssemblages of Intertidal Mixohaline Flats of Bahía Samborombón (La Plata River Estuary-Argentina)

Table 4. Mean abundance of polychaete species in seasonal sampling.

The benthic communities of Bahía Samborombón and its relation with other regional mixohaline environments

Intertidal infaunal communities dominated by Laeonereis culveri (=L.acuta, = L. pandoensis) are a common characteristic shared by many mixohaline environments of the Buenos Aires Province and part of the Atlantic coast of South America. These communities have been mentioned from San Pablo, Brazil to Golfo Nuevo, Argentina (Olivier et al., 1972a; Orensanz & Gianuca, 1974; Bemvenuti et al., 1978; Capitoli et al., 1978; Escofet et al., 1978; Escofet, 1983; Elías, 1985; Bemvenuti, 1987; Elías & Ieno, 1993; Ieno & Bastida, 1998; Elías et al., 2001; López Gappa et al., 2001). Although associated species and its relative abundances may change from one locality to another, the basic community structure seems to be the same for all those environments, mainly characterized by its low specific diversity (Ieno & Bastida, 1998).

Two polychaete assemblages can be identified in the intertidal flats of Bahía Samborombón: one of Figure 6. Redundancy analysis (RDA) between abundance of polychaete them is related with muddy flats of tidal creeks with species (transformed to log10 X+1) high clay and organic matter content; the other and environmental variables at study area. assemblage is related with muddy-sandy intertidal SC: San Clemente creek sampling stations; flats with marine influence and less organic matter PR: Punta Rasa sampling stations; S= summer, A= autumn, content and a fine sand dominant fraction in W= winter, Sp= spring. sediments.

49 J. P. Martin, R. Bastida & M. Trassens

Table 5. Capitella capitata densities observed at Punta Redundancy analysis (RDA). Percentage of total variance explained Rasa are much lower than those of high polluted by first three axes. environments. Grassle & Grassle (1974) mentioned that in pristine estuarial environments this species can be found in low stable densities along the whole year. This indicate that the studied sector of Bahía Samborombón can be considered as a low impact environment, and for that reason this research can be used as a base line study in order to evaluate future environmental changes, of both natural or anthropic origin.

Intertidal communities dominated by Laeonereis The first one is clearly dominated by Laeonereis culveri (=L. acuta) has been also mentioned for culveri, having as associated polychaete species intertidal flats of Bahía Blanca estuary, south of Heteromastus similis, Neanthes succinea, Nephtys Buenos Aires Province (Elías, 1985, 1995; Elías & fluviatilis and Scolecolepides sp. This assemblage is Ieno, 1993; Elías et al., 2001). Associated fauna is very similar to the one mentioned for Mar Chiquita integrated by Littoridina australis, Eteone sp., coastal lagoon (Orensanz & Estivariz, 1971; Olivier et Cyrtograpsus altimanus Rathbun, 1914, al., 1972a; Ieno y Elías, 1995; Palomo & Iribarne, Scolecolepides sp. and Priapulus tuberculatospinosus 2000). Baird, 1868, and together with L. culveri, seems to constitute the only macroinfauna of the mixohaline The second assemblage has Laeonereis culveri as sector of Bahía Blanca (Elías, 1985, 1995; Elías & dominant species and Capitella capitata as Ieno, 1993). Therefore, polychaete fauna of the subdominanant one, being the rest of the species southern sector of Bahía Samborombón seems to have Heteromastus similis, Neanthes succinea, Polydora more affinity with Mar Chiquita and southern Brazil cornuta and Scolecolepides sp. C. capitata is communities, than those of southern Buenos Aires mentioned for the first time in the studied area, and its province (Table 6). The presence of common species relative high density is a distinctive ecological sign of for both environments, as Littoridina australis, this community. In Buenos Aires province this species Tagelus plebeius, Bathyporeiapus bisetosus, Melita sp. has been mentioned previously for the intertidal and Chasmagnathus granulata indicates more community of Río Quequén estuary (38º 36' S; 58º 40' similarities with the infaunal community of Lagoa dos W) (López Gappa et al., 2001) and in the internal area Patos (Brazil). Benthic infaunal communities of Bahía of Bahía Blanca (Elías et al., 2001). The genus Blanca are more diversified and similar- in certain Capitella has also been mentioned for Mar Chiquita aspects- to those of northern Patagonia gulf, coastal lagoon sandy bottoms (Orensanz & Estivariz, characterized by fauna of magellanic biogeographical 1971), while in the estuarial communities of southern origin (Escofet et al., 1978; Escofet, 1983; Elías, 1985, Brazil C. capitata is a frequent species (Lana et al., 1992a, 1995). 1989; Netto & Lana, 1996; Attolini et al., 1997). The abundant ostracod Cyprideis multidentata is The presence of Capitella capitata in the another species present in both Buenos Aires and intertidal communities of Río Quequén estuary, southern Brazil benthic communities. This species has together with the dominance of Laeonereis culveri been found in Mar Chiquita coastal lagoon (Martin, (=L. acuta) (López Gappa et al., 2001), makes this 2002) and also in Lagoa dos Patos (Capitoli et al., community much similar to those of Bahía 1978; Bemvenuti et al., 1978). Samborombón; although the absence of Heteromastus similis, Neanthes succinea and Based on all the benthic studied species, we can Scolecolepides sp. (op. cit.) makes a clear difference conclude that the infaunal community of the between both environments. southernmost sector of Bahía Samborombón has great

50 Polychaete A ssemblages of Intertidal Mixohaline Flats of Bahía Samborombón (La Plata River Estuary-Argentina)

coincidences with other mixohaline communities of by Universidad Nacional de Mar del Plata, Agencia the Argentine Biogeographical Province (Balech, Nacional de Promoción Científica y Tecnológica 1954; Bernasconi, 1964), specially with Mar Chiquita (BID1201/OC-AR-PICT 0107111), Fundación Mundo coastal lagoon (Buenos Aires) and Lagoa dos Patos Marino and Consejo Nacional de Investigaciones (southern Brazil). Científicas y Técnicas (CONICET).

One of the more conspicuous differences of Bahía BIBLIOGRAPHY Samborombón- Lagoa dos Patos communities with Attolini FS, Flynn MN & Tararam AS (1997). Influence of Spartina those of Mar Chiquita coastal lagoon is the presence, densiflora and tide level on the structure of polychaete in this last one, of the polychaete species Ficopomatus associations in an euryhaline salt marsh in Cananeia Lagoon estuarine region (SE Brazil). Revista Brasileira de Oceanografia enigmaticus Hove & Weerdenburg 1978, which forms (Brasil) 45: 25-34. extended underwater reefs with its calcareous tubes. Balech E (1954). División zoogeográfica del litoral sudamericano. This cosmopolitan polychaete was introduced by man- Rev. Biol. Mar. Valparaíso, 4:184-195. transport in Mar Chiquita, and although the species is Bastida R & Brankevich G (1980). Estudios ecológicos preliminares sobre las comunidades incrustantes de Puerto Quequén original from the Indian Ocean, nowadays inhabits (Argentina). V Congreso Internacional de Corrosión Marina e many geographical areas of the world. During the first Incrustaciones (España) Biología Marina:113-138. half of the XXth. century it invaded estuarial and Bastida R, L'hoste SG, Spivak E & Adabbo HE (1977a). Las incrustaciones biológicas de Puerto Belgrano. I. Estudio de la harbor areas of Buenos Aires Province (Orensanz & fijación sobre paneles mensuales. Período 1971/72. Corrosión y Estivariz, 1971; Bastida et al., 1977a, 1977b, 1980; Protección (España) 8: 1-23. Bastida & Brankevich, 1980; Obenat & Pezzani, 1994; Bastida R, L'hoste SG, Spivak E & Adabbo HE (1977b). Las Schwindt, 2001). Although this species is present in incrustaciones biológicas de Puerto Belgrano. II. Estudio de los procesos de epibiosis registrados sobre paneles mensuales. other coastal sectors of the Buenos Aires Province and Corrosión y Protección (España) 8: 24-33. mainly in harbor areas, its absence is complete in Bastida R, Trivi De Mandri M, Lichtschein De Bastida V & Stupak M Bahía Samborombón, probably because of the very (1980). Ecological aspects of marine fouling at the port of Mar del Plata (Argentina). V Congreso Internacional de Corrosión scarce hard bottoms in the area. Marina e Incrustaciones (España) Biología Marina: 299-320. Bemvenuti CE (1987). Macrofauna bentónica da regiao estuarial da ACKNOWLEDGEMENTS Lagoa dos Patos, R.S., Brasil. Simposio sobre Ecosisstemas da Costa Sul e Sudeste Brasileira. Publicaciones ACIESP (Brasil) 54(1): 428-459. The authors would like to thank Mundo Marino Bemvenuti CE, Capitoli RR & Gianuca NM (1978). Estudos de ecologia Foundation members for its cooperation during the bentonica na regiao da Lagoa dos Patos. II- Distribucao quantitativa sampling campaigns in the study area, and to Lic. do macrobentos infralitoral. Atlantica (Brasil) 3: 23-32. Bernasconi I (1968). Distribución geográfica de los equinoideos y Nilda Manolidis for its contribution in meteorological asteroideos de la extremidad austral de Sudamérica. Bol. Inst. data gathering. This study was financially supported Biol. Mar., 7: 43-50.

Table 6. Distribution of studied polychaete species in several mixohaline environments of Buenos Aires and southern Brazil.

51 J. P. Martin, R. Bastida & M. Trassens

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(Received: August, 10, 2004. Accepted: September, 13, 2004)

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