Distributional Patterns of Polychaeta in the Alfaques Inlet (Ebro River Delta; Western Mediterranean): Faunistic and Coenotic Analysis of an Estuarine System

BULLETIN OF MARINE SCIENCE, 48(2): 369-375, 1991

DISTRIBUTIONAL PATTERNS OF POLYCHAETA IN THE ALFAQUES INLET (EBRO RIVER DELTA; WESTERN MEDITERRANEAN): FAUNISTIC AND COENOTIC ANALYSIS OF AN ESTUARINE SYSTEM

R. Capaccioni-Azzati, S. Villora-Moreno, A. M. Garcia-Carrascosa and F. J. Torres-Gavila

ABSTRACT Polychaetous annelids associated with soft bottoms in the Alfaques inlet (Ebro river Delta, western Mediterranean) were studied to evaluate species composition and spatial distribution by cluster classification and Principal Component Analysis (PCA). Spatial distribution by PCA explains more than 63% of the variance through seven axes. Cluster classification using physicochemical parameters distinguished two main assemblages, one extending through central inlet's corridor, deeper than 2 m with less than 60% sand, and a second shallower with sand content greater than 60%. Cluster classification identified four distinct groups. The first group comprised sampling stations in the outer sector with only infaunal species ubiquitous in soft bottoms. The second group can be considered as a subgroup of the previous assemblage with infaunal species typical of polluted sediments. A third set presented a diverse fauna and with could not be characterized from the coenotic standpoint. The fourth group possessed more than 20 species, none of which was found in stations belonging to the former groups.

The Alfaques inlet an estuarine system with an area of 50 km2 and a maximum depth of 10 m opens to the sea by a narrow (3 km wide) opening to the SW (Fig. I). Although essentially marine in nature with a mean annual salinity of 350/00 (Camp et al., 1985), it often exhibits salinity values down to 20-300/00 because of fluvial discharge (Perez and Camp, 1986). These values are low in relation to the typical Mediterranean salinity of 38.20/00. The sediments of the inlet vary from mud to fine and medium sand. Sandy sediments are found along the SW shore (south littoral bar). The muddy sediments are found mainly in the deepest central area of the inlet. The major aquatic macrophytes are Caulerpa prolifera in deep muddy sediments, and Cymodocea nodosa and Zostera nana in shallow sandy sediments along the south littoral bar. Two shallow coastal lagoons (2 m maximum depth) in the north shore, "La Encaiiizada" and "La Tancada," supply slightly brackish water « 30/(0) to the inlet through channels. In a previous report we presented preliminary data regarding the macrobenthos of the Alfaques inlet (Capaccioni-Azzati et al., 1990). Here we present results of a coenotic study of its polychaete fauna carried out through a quantitative analysis to obtain a distributional pattern based on both faunistic and environmental parameters.

MATERIALS AND METHODS

Sampling. - Twenty two samples were taken with an anchor dredge, collecting 20-22 liters, or by hand at shallower stations (I m2 of surface sediment to 25 em depth) during two benthic surveys in September 1984 and April 1985. For fauna separation, samples were washed through a I-mm mesh screen. The material retained on the sieve was fixed in 5% formalin for subsequent analysis in the laboratory. Environmental parameters measured at each station included salinity, depth, percent sand, percent mud, percent silt, organic matter, and percent macrophyte cover.

369 370 BULLETIN OF MARINE SCIENCE, VOL. 48, NO.2, 1991

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Figure I. Alfaques inlet showing position of the sampling stations and spatial distribution of polychaete assemblages according to coenotic analysis,

Data Analysis.-For the statistical analysis, data were arranged in two matrices: (a) 22 stations x 93 species and (b) 21 stations x 7 environmental parameters. Species occurring only once were excluded from the first matrix to provide a more solid baseline for subsequent analyses. The abundance values of the species were standarized to obtain relative data (percent abundance of each species in each station) (Campbell, 1978) and transformed, to obtain normalized data, using the double root method (Field et aI., 1982). Two kinds of analyses were performed from the normalized data. A cluster analysis (UWPGMA) was made by calculating Pearson's correlation coefficients between stations. In addition, relative species abundance data were subjected to spatial ordination through PCA, plotting stations on the basis of the gradients indicated by the axes. Calculations were made by means ofSPSS/PC+ statistical integrated package.

RESULTS AND DISCUSSION A total of 124 species of polychaetes belonging to 32 families were collected. The most abundant families were Cirratulidae (11 species), Capitellidae, Phyl- lodocidae, Syllidae (10 species each), Paraonidae (9) and Maldanidae (7). A list of species collected can be found in Capaccioni-Azzati et al. (1990). Table 1 shows the main environmental factors for each sampling station. Two kinds of descriptors, physicochemical parameters and faunistic inventories, were used for the classification of stations through cluster analysis. The results obtained using physicochemical parameters showed the presence of two major affinity groups designated I and II (Fig. 2a). Group I extends through the central inlet's corridor, from the inner zone to the outer one, with depths greater than 2 m, always less than 60% sand and high proportion of organic matter (maximum 9.4%). Within this group, two subgroups can be detected. The first (Ia) includes stations located in three patches of deep muddy sediments away from the littoral coast and with an increasing degree of shelter (Fig. 1). The second subgroup (Ib) AZZATI ET AL.: MEDITERRANEAN POLYCHAETA DISTRIBUTION PATTERNS 371

Table I. Physicochemical parameters for sampling stations [Abbreviations: salinity (Sal), organic matter contents (OM), percent macrophytic cover (MACR)]

Sal. Depth Sand Mud Silt OM MACR. <%.) (m) (%) <%) <%) (%) <%) C-5 38.4 10.0 9.4 74.8 15.8 6.8 0 B-5 38.3 7.0 37.3 47.7 12.5 5.8 0 D-5 38.0 8.0 18.7 70.6 10.6 4.4 0 C-4 38.0 6.5 7.3 79.0 13.7 5.7 0 D-4 38.0 7.0 19.1 74.5 6.4 5.0 50 D-3 37.9 5.5 6.8 83.2 10.0 6.7 0 F-2 37.9 5.5 7.5 63.9 28.6 3.1 10 E-3 37.6 2.0 35.5 47.8 16.6 7.3 10 E-2 38.3 4.5 8.3 75.4 16.3 8.2 10 F-3 37.9 5.5 6.2 66.8 26.4 9.4 10 G-2 36.7 5.6 15.3 55.7 29.1 8.5 0 H-2 36.9 5.0 9.8 61.7 28.5 1.5 0 1-1 36.8 3.0 56.2 28.0 15.8 9.4 0 1-2 36.7 5.0 30.1 52.4 17.5 9.0 0 K-I'" 36.6 2.0 99.3 0.5 0.2 3.5 0 K-IAFRC 36.5 0.3 99.5 0.5 0.1 1.3 0 K-IZ 36.5 0.3 94.4 4.5 1.1 2.9 30 K-IC 36.5 0.3 99.0 0.9 0.1 0.5 20 H-3 38.0 0.2 96.3 3.1 0.6 1.9 70 E-4AFRC 38.2 0.4 97.6 2.1 0.2 1.5 0 E-4C 38.2 0.4 97.6 2.1 0.2 1.5 20 comprises stations closer to the shore with relatively high percentage (30-56%) of sand. Station D-4 is similar to Ia except that it possesses abundant macrophytes. This probably accounts for its separation by the classification procedure. Group II includes shallow stations with high sand content (>90%) and low proportion of organic matter (maximum 1.25%). Within this assemblage, one subgroup of stations (IIa) corresponds to the inner sector of the inlet, whereas the rest of the stations, subgroup lIb, are located both in the inner zone and in the outer one. Cluster analysis based on species profiles shows four relatively well-characterized groups (Fig. 2b). The first one (A), includes all the stations located in the outer sector of the inlet. Dominant species include: Diopatra neapolitana, Laonice cirrata, Poecilochaetous serpens, Aricidea assimilis, Notomastus aberans, N. for- mianus, N. latericius, Ampharete acutifrons, Chone duneri and Terebellides stroe- mi. All are typical and exclusive infaunal species, ubiquitous in soft bottoms with occasional occurrences on hard bottoms. They are mostly surface and subsurface deposit feeders with one suspension feeder (c. duneri) (Fauchald and Jumars, 1979). The second group (B), could be considered as a subgroup of A. Like the previous assemblage, it consists of infaunal species of muddy sediments, but the most abundant species are characteristic of polluted areas: Chaetozone setosa, Cirratulus filiformis, Tharyx marioni and Cossura soyeri. Some of them (T. marioni and C. filiformis) are associated with hard substrata (photophilic algae, biogenous con- cretions in hard substrata, and others). Most of these species are motile surface detritivores and motile subsurface burrowers (Fauchald and Jumars, 1979). The third set of stations (C) belongs to a heterogeneous sector with different depths, and a diversity of bottoms with or without macrophytic cover. High environmental instability is caused by flow from rice field irrigation canals that 372 BULLETIN OF MARINE SCIENCE, VOL. 48, NO.2, 1991

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Table 2. Factors obtained through principal component analysis

Factor Eigenvalue % variation Cum %

I 4.267 19.4 19.4 2 2.552 11.6 31.0 3 2.119 9.6 40.6 4 1.745 7.9 48.6 5 1.213 5.5 54.1 6 1.116 5.1 59.1 7 1.029 4.7 63.8

discharge directly into the inlet. Further environmental disruption is due to ir- regularities in fluvial discharge created by the fiShery mandated closing (from September or October to March or April, Demestre et al., 1977) of channels that connect the two coastal lagoons, Encaiiizada and Tancada, with the inlet (Fig. 1). This sector cannot be characterized from the coenotic point of view. It possesses a diverse fauna including Genetyllis rubiginosa. Eunice vittata, Magelona equi- lamellae, Cirriformia filigera. Aricidea fragilis mediterranea, Heteromastus filiformis. Euclymenecollaris, E. oersteedi, Oweniafusiformis, and Melinnapalmata. These species have different feeding guilds: carnivores, omnivores, suspension feeders, and surface detritivores. This heterogeneous environment supports a high biological diversity (Gambi and Giangrande, 1985). The last group (D) includes the same stations associated in the first analysis with group II, indicating that they represent a characteristic group from both faunistic and environmental points of view. The dominant species are Malacoceros fuliginosus. Capitella capitata, Exogone verugera, and Amphiglena mediterranea. The Principal Component Analysis explains more than 63% of the variance on the basis of seven axes (Table 2); the first three contributing more than 40%. The analysis of the distribution of stations by means of axis I shows a clear correlation between F-3, E-3, E-2, and F-2 (Fig. 3); all are influenced by a high organic matter content and located in the center of the inlet. This assemblage corresponds to group B obtained by cluster analysis. Stations K-l Z, K-l C, H-3, E-4 C (Fig. 3) are sheltered, shallow, with macrophytic cover, high sand percentage and low organic matter content, and are correlated with axis II. These stations correspond to the biocenosis of Fine Sands in Very Shallow Areas (Peres and Picard, 1964). The remaining axes or principal components are difficult to characterize because the percentage of variance is very low (Table 2) which is probably due to interaction of multiple variables. Axes III and IV combined are probably related to a group of variables associated with the outer environment of the inlet. Their stations D-4, B-5 and D-5 are correlated mainly with axis III, whereas K-l*, C-5, C-4 and D-3 are highly correlated with axis IV. These results divide Alfaques inlet into four coenotic regions (Fig. 1) on the basis of granulometric characteristics, organic matter, depth, macrophytic cover and salinity. These factors appear to have the greatest influence on the distribution of the polychaete population in the inlet. Comparison of the polychaetes obtained in Alfaques with other littoral Med- iterranean semi-enclosed coastal lagoons and estuaries, shows greater diversity of species (124) with high abundance values for some species. No more than 50 species are reported for other similar areas (Casabianca et al., 1972-1973; Gue- 374 BULLETIN OF MARINE SCIENCE, VOL. 48, NO.2, 1991

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Figure 3. Ordination models of the sampling stations in a plane formed by the first two axes obtained through Principal Component Analysis. lorget and Michel, 1979a; 1979b; Colombo et aI. 1981; Ambrogi and Bedulli, 1981; Gravina, 1985; Capaccioni-Azzati et at, 1987). On the other hand, these results could relate to the hypothesis suggested by Barnes (1974) that there is a decline in species numbers with regard to the open sea in deltas, estuaries and lagoons which are under physiological "stress" due to large variations of environmental variables. However, our data suggest that, in spite of environmental instability, the Alfaques inlet functions as a relative stable or buffer region, where the polychaetes have a sufficient variety of niches where its specific richness dominates over the environmental pressure. It would be interesting to compare several of these integrated systems in the "domaine paralique" (Guelorget and Perthuisot, 1983) to characterize the factors affecting the biotic composition as well as to develop a basic understanding of how they function in hope of preventing negative environmental impacts by human activities.

ACKNOWLEDGMENTS

We are grateful to Dr. E. Font for critically reading the manuscript.

LITERATURE QTED

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DATEACCEPTED: November 8, 1990.

ADDRESS: Department oj Zoology, Faculty oj Biological Sciences. University oj Valencia, Burjasot £-46100, Valencia, Spain.