An Analysis of the Community Structure of Subtidal and Intertidal Benthic Mollusks of the Inlet of Bano (Ria De Ferrol) (Northwestern Spain)

18 Pages·1998·8.6 MB·English

by  C Olabarria

An analysis of the community structure of subtidal and intertidal benthic
mollusks of the Inlet of Bano (Ria de Ferrol) (northwestern Spain)
Celia Olabarria1 Victoriano Urgorri2 and Jesus S. Troncoso3
, ,
•Laboratoriode Invertebrados y Ecologi'adel bentos, Facultadde Cienciasdel Mar, UniversidadAutonomade Sinaloa, PaseoClaussen,
s/n, 82000 Mazatlan (Sinaloa), Mexico, [email protected] mx
^Departamentode BioloxiaAnimal, Facultade de Bioloxi'a, Universidade de Santiago, E-15706Santiago (A Coruna), Spain
-^Departamentode Medio Ambiente e Recursos Naturais, Facultade deCiencias del Mar, Universidade de Vigo, Vigo (Pontevedra), Spain
Abstract: A studyofthebenthiccommunitiesofthe InletofBano(Riade Ferrol)(northwestern Spain)wascarriedout,encompassingtheexaminationof
75 samplingstations (35 subtidaland40intertidal) Data weresubjectedtoclassificationandordinationtechniques Twomajorassemblageswereidentified,
anddividedintofivesubgroups in termsofdominanceofthespecies,constancy, andfidelity. The subtidal zone wascharacterizedbytheAbraalba(Wood,
1802) community. It wasstructuredinfadesas follows: TheNuculamtidaSowerby, 1833-Thyasiraflexuosa(Montagu, 1803)facies, was the innermost, in
theeastern areaoftheinlet; anotherfaciesexistedinthetransition towardthe Clausinellafasciata(daCosta, 1778)community intheouterinletarea. In the
intertidalzonethe Cerastodermaedule(Linne, [15%)-Scrobiculariaplana(daCosta, 1778)community wasdefined,whichwasalsostructuredinfacies:one
facies, locatedat the mouth ofthe river, was dominated by Hydrobia ulvae (Pennant, 1777); another facies, found in the inner inletareaassociated with a
meadow ofthe seagrass Zostera noltii Hornem.. 1832, characterized by Biniuin reticulatum (da Costa, 1778), Loripes lacteus (Linne, 1758), C. edule,
Venerupis senegalensis (Gmelin, 1791), and Rissostomia membranacea (Adams, 1800), and a third facies, situated at the border ofthe inlet, included
Gibbula umbilicalis (da Costa, 1778), Littorina littorea (Linne, 1758), L. obtusata (Linne, 1758), V. senegalensis, etc., as dominant species. The analyses
showedthatsedimentparameters(mainlygrainsize),organic matterin thesubtidal stations,andgrain sizeanddepthare the mostimportantfactorsgovern-
ingthedistributionandabundanceofthecommunities.
Key words: communitystructure,intertidal,subtidal, mollusks,Riade Ferrol, Spain
Research on the macrobenthic communities of the Quintino etal., 1986; Sauriau etal, 1989).
northern coast ofthe Iberian Peninsula has been carried out However, despite the plethora of faunistic studies
by several authors, who have taken both a faunistic and an involving Galician mollusks, there is a need for research on
ecological-descriptive approach. Their sources of informa- their sinecology. An exception to the lack of sinecological
tion have been primarily articles on the ecology ofthe mac- studies is the work by Cadee (1968) in the Ria de Arousa.
robenthos, and their ultimate goals have been to carry out Therefore the purpose of this work was to study the ecolo-
analyses of the faunistic community as a whole. This is gy of the benthic populations of mollusks living on soft
reflected by the great number of biocenotic studies on the substrata, both subtidal and intertidal, of the Inlet of Bano
benthos living on soft substrata in recent years on the (Ri'a de Ferrol, northwestern Spain). The aim was to define
Galician coasts (Vieitez, 1976, 1981; Anadon, 1980; Mora, the different communities living in the inlet and how they
1980; Penas and Gonzalez, 1983; Rodriguez Castelo and relate to physicochemical factors (granulometry, sorting
Mora, 1984; Planas and Mora, 1984 a, b; Laborda, 1986; coefficient, organic matter, carbonates, and nitrogen).
Planas, 1986; Lopez Serrano and Vieitez, 1987; Vieitez and
Baz, 1988; Junoy and Vieitez, 1989, 1990; Maze et al,
MATERIAL AND METHODS
1990; Palacio et al., 1991, 1993; Curras and Mora, 1991,
1992; PerezEdrosa and Junoy, 1993).
Frequent studies have also been made on the mala- STUDY AREA
cological communities of different geographical areas of The Inlet of Bano (Fig. 1) is located on the south-
the European Atlantic coasts (Lande, 1975; Evans and ern border of the central channel in the Ria de Ferrol
Tallmark, 1976; Petersen, 1977; Glemarec, 1978; Tunberg, between the Punta do Faro da Palma (43°27'52"N;
1981; Dewarumez, 1983; Gentil et al., 1986; Cornet, 1985; 08°16'49"W) and Punta Piteira (43°27'57"N;
American Malacological Bulletin,Vol. 14(2)(1998):103-120
103
t
104 AMER. MALAC. BULL. 14(2) (1998)
maerlbed
Z.noltiimeadow
—143m
Esteiroriver
Fig. 1. Location ofthe 75 sampling stations in the InletofBano(Ri'a de Ferrol), northwestern Spain. A Spain. B. Galicia. C. Riade Ferrol. D. Inletof
Bano (I, intertidalstation; S,subtidalstation)
08°15'37"W), with an area of 0.5 km2 and a maximum inlet. These currents, which are strongerat the mouth ofthe
depth of 18 m (Fig. 2). The inlet is oriented in a NNE-SSW inlet than at more distal points, are the dominant factor
direction; the prevailing winds are southwesterly for most affecting sediment distribution within the inlet (Fig. 3).
ofthe year, except in summer when northeasterlies become The inlet is characterized by soft bottom bordered
dominant. The mean tidal range in the ria is 2.7 m, and tidal by a rocky strip. A maerl bed is present in four ofthe west-
effects give rise to strong currents (up to 1.5 m/s in the ria's ern stations, between 4.5 and 10.5 m depth. A seagrass
central channel). Outward movement of water from the ria meadow ofZostera noltii Hornem., 1832, is present in the
provokes flow to the southeast within the inlet, while central intertidal zone.
movement into the ria provokes flow to the SSW within the The distribution of different grain sizes suggests an
OLABARRIA ETAL.: BENTHIC MOLLUSKS OF INLET OF BANO 105
15.50 intertidal zone, samples were also taken every 100 m along
each transect (Olabarria et ai, 1996). In the subtidal zone,
samples were collected by scuba diving. At each point, a
0.5 m2 square sample was taken, to a depth ofapproximate-
ly 20 cm, using a rectangular shovel. All samples were sub-
sequently wet-sieved through a series of sieves with 10, 2,
mm
and 0.5 mesh. Finally the sieved samples were trans-
ported to the laboratory, and the living specimens were sort-
ed by the remounting technique (Ros, 1975). Sediment
samples were obtained for granulometric study. This con-
sisted of an analysis of grain size, organic matter, nitrogen,
and carbonates (Guitian and Carballas, 1976).
DATA ANALYSES
Data were organized into station by species matri-
ces. The Shannon-Wiener and Pielou's evenness indices
(Washington, 1984) were used to assess species diversity
and evenness.
The data for the population studies were processed
in two ways: (1) Analyses of qualitative data (presence-
absence) of the species in the sampling stations were per-
Fig.2.Depth(m)ofthesubtidal(-)andintertidal(+)stations. formed by applying the point correlation coefficient (O;
Daget, 1976), whereby a correlation matrix is created, fol-
lowed by classification into clusters through the clustering
enrichment gradient for fine fractions on the eastern part of algorithm UPGMA, by means of the NTSYS-pc program
the Inlet of Bafio. Sandy muds prevail in this area, while the
coarsest sediment fractions are prevalent in the outer part. On
the other hand, the percentages of organic matter are low,
with enrichment toward the eastern part (Table 1). Nitrogen
follows a variation pattern similar to that of organic matter.
Calcium carbonate does not present a high percentage, with
highest values found in the subtidal zone (Olabarria et ai,
1996).
The area has a temperate humid Atlantic climate and
the hydrographic conditions show thermal variations ofmore
than 5°C in the first 10 m. This variation decreases with
depth. Salinity fluctuates with the tides and seasons, due to
the fact that the ria has difficulty in draining because of the
narrow central channel, which creates water masses having
different salinities. The pH values are consistently between 7
and 9, and there are no anomalous data that would indicate a
source ofexcessive acidity or alkalinity (COTOP, 1987).
SAMPLE COLLECTION
The sampling program, which was designed to pro-
vide sufficient information on the distribution of the different
species of mollusks, consisted of35 subtidal and 40 intertidal
stations sampled from July 1991 to June 1992. Sampling
points were selected along 12 parallel transects across the
inlet at 100 m intervals, taking samples at the points that were £ 0.20-0.05mm
judged by visual examination to show a change in nature, tex- • <0.05mm
ture, or substrate covering. Intertidal samples were addition- Fig.3. Sedimentcharacteristicsofthe InletofBafio(Riade Ferrol). Mean
ally collected at the ends of each transect and, in the inner grainsizeinmm
1
106 AMER. MALAC. BULL. 14(2) (1998)
Table 1. Organic matter(MO), carbonates (CA), nitrogen (N) andsortcoefficient(So; Trask, 1932) in subtidal and
intertidalstations.
Station MO CA N So Station MO CA N So
S101 0.42 30.22 0.05 2.42 1103 0.59 19.64 0.08 >2.50
S102 0.51 31.11 0.04 2.69 1104 0.43 20.57 0.04 1.80
S103 0.42 33.33 0.03 1.76 1105 0.67 9.43 0.06 1.58
S104 1.03 18.22 0.06 6.06 1106 0.39 12.86 0.03 2.58
S105 0.70 34.67 0.08 480 1107 0.55 2.57 0.04 2.55
S106 1.04 8.89 0.01 8.36 1108 0.54 2 10 0.06 1.95
S107 1 99 32.44 0.13 6.44 1109 0.66 1.71 0.06 > 1.65
S108 1.16 34.67 0.10 2.18 1110 2.00 8.57 006 3.71
S109 1.53 29.33 0.10 9.60 1114 0.23 1.71 0.02 > 1.29
S110 0.69 32.89 0.05 2.75 1115 0.47 1.03 0.03 1.79
Sill 1.69 32.00 0.13 0.18 1116 1 41 0.34 0.10 5.07
S112 1.13 13.78 0.09 21.70 1117 0.63 26.67 0.08 3.77
S113 0.97 13.78 0.08 2.54 1118 0.34 16.29 0.05 2.92
SJ14 1.37 21.33 0.07 20.99 1119 0.59 0.86 0.01 1.63
S115 0.44 34.67 004 2.00 1120 094 0.34 0.02 1.49
S116 0.58 34.67 0.04 1.78 1121 0.91 0.86 0.07 >2.50
S117 0.32 4.89 003 2.08 1122 1.37 1.71 0.09 1.54
S118 1.55 24.89 0.09 9.57 1123 064 026 0.03 1.89
S119 0.44 17.33 0.03 2.63 1124 0.83 0.17 0.04 1.45
S120 0.59 18.40 004 3.39 1125 0.83 0.26 0.04 1.37
S121 0.87 27.56 0.08 2.54 1126 0.43 1 03 0.02 1.27
S122 0.60 30.22 0.07 7.50 1127 0.5 0.86 0.04 2.29
S123 0.45 16.44 0.04 2.35 1128 0.82 0.34 002 2.23
S124 0.69 30.22 006 >3.30 1129 1.04 0.86 0.09 1.38
S125 097 35.56 0.14 10.04 1130 1.23 0.26 0.05 0.32
S126 1.31 22.58 0.05 >390 1131 0.48 0.86 0.04 4.31
S127 1.83 2.22 0.12 >1.84 1132 0.72 0.17 0.03 1.41
S128 0.26 6.29 0.04 >3.50 1133 0.91 0.34 0.02 1.43
S129 1.80 17.33 0.12 8.48 1134 0.91 0.26 0.03 1.32
S130 1.51 25.78 0.11 9.00 1135 0.49 1.03 0.02 1.50
S131 2 16.27 0.10 8.50 1136 0.44 0.17 0.35 1.77
S132 0.61 1.33 0.04 1.61 1137 0.95 0.17 0.04 1.46
S133 2.04 6.22 0.11 5.21 1138 0.80 0.17 0.05 3.77
S134 0.93 19.02 0.03 >3.53 1139 0.76 0.86 0.07 3.92
S135 0.42 089 0.03 1.70 1140 0.63 0.86 0.05 7.17
1101 0.17 12.00 0.02 1.13 1141 0.69 4.29 0.06 3.08
1102 0.38 1.42 0.01 1.78 1142 0.38 24.89 0.03 1.58
1143 0.78 14.23 0.06 3.65
(ver. 1.60; Rohlf, 1990). the polynomial method, by means ofthe CANOCO-pc pro-
gram (ver. 3.10; Ter Braak, 1988). The data were trans-
0,= ad- be formed according to the formula, x = logio (x + 1). This
-
i(a + b)(a + c)(b + d)(c + d) technique plots the stations along the axes according to
similarity in species composition. The axes are often inter-
where a = number of species in both stations; b = number preted as environmental gradients whose identity can be
of species in the first station; c = number of species in the analyzed by means of statistical correlations between the
second station; and d - species absent in both stations. 4> location ofthe stations on the axes and their environmental
values ranged from -1 to+l;p = a + b + c + d, %2 = p <t>2- characteristics (Eleftheriou and Basford, 1989; Junoy and
(2) Analyses ofquantitative data of the species pre- Vieitez, 1990). The environmental variables measured
sent in at least 10% of the stations were performed, omit- were: depth (positive values for intertidal stations; negative
ting those which could create "noise" because of their nar- values for subtidal stations), percentages of the different
row distribution and scarcity. The Detrending granulometric fractions, organic matter, silt-clay ratio, car-
Correspondence Analysis (DCA) was applied according to bonates, nitrogen, and sortcoefficient. Theseenvironmental
OLABARRIA ETAL.: BENTHIC MOLLUSKS OF INLET OF BANO 107
Table 2. Species classifications according to constancy and fidelity bivalves were slightly more dominant than gastropods, due
indices. (Cam, addition ofconstancies ofspecies A within each popula- to the high number of individuals of Mysella bidentata, in
tion; Nai, number ofstations within population where species A exists; addition to Papillicardium papillosum, and several species
N|,totalnumberofstationswithinpopulation). of Anomiidae. The bivalves Mytilus edulis and Venerupis
Constancyindex Fidelityindex senegalensis were the most abundant species in the inter-
CAi =(Nai/N,)x 100 (CAi/CAii)x 100 tidal zone. [Note: The species Littorina littorea, L. obtusa-
accidental < 10% ta, L. mariae, Lepidochitona cinerea, and M. edulis are nor-
rare < 12% occasional 11-33%- mally associated with hard bottoms. In some cases during
notverycommon 13-25% accessory 34-50%
common 26-50% preferential 51-66% this study, small stones were included in the soft sediment
verycommon 51-75% elective 67-90% samples collected by shovel from the Inlet ofBano. These
constant 76-100% exclusive 91-100% small stones provided hard substratum for the chitons, and
also for attached algae, thus providing food for the periwin-
kle species.]
factors were correlated with the axes through Spearman's The values of the Shannon-Wiener diversity index
correlation analysis. (Table 4) fluctuated between 0.00 (station 1134) and 4.35
Based on the group of stations defined using the (station SI12), and evenness ranged from 0.06 (station
point correlation coefficient, the species were classified 1135) to 0.94 (station S124). These two parameters were
according to the criteria ofconstancy and fidelity, based on positively correlated (r= 0.57; p < 0.001).
number of species (Dajoz, 1971; Table 2), and fidelity- Diversity correlated positively with carbonate con-
dominance product (FxD; Glemarec, 1964), including only tent, percentage ofcoarse sand, and sorting coefficient, and
species that appeared in at least two stations (Glemarec, negatively with depth and percentage offine sand. Both rel-
1964; Cabioch, 1968; Lastra et al, 1990; Junoy and ative diversity and evenness showed positive correlations
Vieitez, 1990; Curras and Mora, 1991). with the percentage of coarse sand, percentage of gravel,
and the sort coefficient, and a slightly negative correlation
with the percentage offine sand (Table 5).
The lowest population density was recorded at sta-
RESULTS
tion 1135 with 32 ind/m2 and the highest density at station
,
Sill (6,040 ind/m2). The mean density in the subtidal zone
FAUNISTIC ANALYSIS, SPECIES DIVERSITY, AND (1,294.6 ind/m2) was higher than that in the intertidal zone
ABUNDANCE
(931.9 ind/m2).
The 75 samples analyzed yielded a total of 20,647 Species richness varied widely, with a greater num-
individuals belonging to 148 species, in which gastropods berofspecies in the subtidal than in the intertidal zone. The
were the most abundant (62.4%; Table 3), followed by lowest values for species richness were found at the inter-
bivalves (36.7%); chitons and scaphopods were present in tidal stations near the mouth ofthe river, due to high abun-
small numbers. The gastropods numerically dominated the dance of the dominant species Hydrobia ulvae. Species
intertidal zone, outnumbering the bivalves by more than richness correlated negatively with the percentage of fine
twofold. This is probably caused by the large number of sand and positively with carbonate content, and percentages
individuals belonging to abundant species such as Hydrobia of coarse sand, gravel, and silt-clay, which explains the
ulvae and Rissoa par\>a. The latter was also present in the increase in species richness at the outer subtidal stations
subtidal zone in a high numberofsamples along with Hinia with coarser grain size and higher silt-clay percentages.
reticulata, H. incrassata, and Onoba semicostata. There,
Table3. Numberofindividuals(ind)andspecies(sp),andtotalpercentage(%)ofindividualsinthesubtidalandintertidalzonesand
inboth.
TOTAL SUBTIDAL INTERTIDAL
% % %
ind sp ind sp ind sp
Polyplacophora 177 6 0.85 110 6 0.99 67 4 0.72
Gastropoda 1,288 89 62.39 5,206 82 45.95 7,676 41 82.37
Scaphopoda 9 1 0.04 9 1 0.07 0 0 0.00
Bivalvia 7,579 52 36.70 6,003 52 52.99 1,576 32 16.91
TOTAL 9,053 148 100.00 11,328 141 100.00 9,319 77 100.00
51 1 1 233311 1
108 AMER. MALAC. BULL. 14(2) (1998)
Table 4. Faunistic parameters for each subtidal and intertidal station: abundance, species richness (R), Shannon-
Wienerdiversityindex(H'),Pielou'sevennessindex(J'),anddensity(ind/m2).
Station Abundance R H' J' ind/m2 Station Abundance R H' J' ind/m2
S101 273 31 4.25 0.86 1,092 1103 530 29 3.17 0.62 2,120
S102 194 29 3.61 0.74 776 1104 249 24 3.46 0.75 996
S103 158 20 3.09 0.72 632 1105 162 12 2.28 063 648
S104 112 1') 3.43 081 448 1106 165 13 2.60 0.70 660
S105 204 27 3.85 0.81 816 1107 481 23 3.11 0.69 1924
S106 924 37 3.74 0.72 3,696 1108 154 1 2.37 0.68 616
S107 461 36 3.82 0.74 1,844 1109 387 17 3.48 0.85 1,548
S108 1,188 39 3.26 0.62 4,752 1110 56 8 2.11 0.70 224
S109 658 38 3.15 0.60 2,632 1114 152 17 2.68 0.66 608
S110 260 32 4.07 0.81 1,040 1115 12 5 2.12 0.91 48
Sill 1,510 45 2.81 0.51 6,040 1116 56 16 3.08 0.77 224
S112 365 42 4.35 0.80 1,460 1117 274 25 3.21 0.69 1,096
S113 802 49 4.19 0.75 3,208 1118 79 18 3.24 0.77 316
S114 553 43 4.24 0.78 2,212 1119 10 4 1.85 0.92 40
S115 218 33 4.1 0.81 872 1120 24 9 2.43 0.76 96
S116 523 29 2.87 0.59 2,092 1121 140 8 2.14 0.71 560
S117 264 19 2.95 0.69 1,056 1122 271 17 2.43 0.59 1,084
S118 231 2 2.24 0.5 924 1123 143 1 3.06 0.88 572
S119 127 24 3.39 0.74 508 1124 343 14 2.1 0.55 1,372
S120 48 1 3.38 0.86 192 1125 363 1 1.78 0.48 1,452
S121 85 24 3.00 065 340 1126 442 1 1.61 0.43 1,768
S122 369 3—3 2.87 0.57 1,476 1127 62 1 2.92 0.81 248
S123 112 3.51 0.78 448 1128 53 9 2.59 0.82 212
S124 29 15 3.69 094 116 1129 734 9 0.44 0.14 2,936
S125 96 21 2.46 0.56 384 1130 204 1 1.72 0.46 816
S126 337 29 3.58 0.37 1,348 1131 199 9 1.38 0.44 796
S127 40 8 2.39 0.79 160 1132 388 4 0.20 0.10 1,552
S128 70 16 3.25 0.81 280 1133 802 8 0.30 0.09 3,208
S129 82 21 3.84 0.87 328 1134 45 1 0.00 180
S130 379 33 2.82 0.56 1,516 1135 8 3 1.06 0.07 32
S131 126 21 3.57 0.81 504 1136 171 1 0.00 684
S132 110 10 1.58 0.47 440 1137 746 2 0.06 0.06 2,984
S133 205 26 3.28 0.69 820 1138 400 11 2.49 0.72 1,600
S134 131 18 2.50 0.59 524 1139 56 9 2.83 0.89 224
S135 84 8 1.94 0.65 336 1140 20 6 1.94 0.75 80
1101 142 24 3.99 0.87 568 1141 146 13 2.21 0.59 584
1102 333 24 3.41 0.74 1,332 1142 131 20 3.41 0.79 524
1143 186 21 3.48 0.79 744
COMMUNITY STRUCTURE which are subjected to stronger hydrodynamism.
The classification (Fig. 4) that was obtained after A non-parametric Kruskal-Wallis test proved that
applying the point correlation coefficient for 99% signifi- there were significant differences between assemblages A
cance (<£> - 0.29) showed two large assemblages (A and B) and B in terms of the percentages offine sand, carbonates,
which belong to the intertidal and subtidal stations, respec- and silt-clay, the sort coefficient, and diversity (p < 0.001),
tively. These assemblages were divided into five sub- as well as the percentages of coarse sand, gravel, and
groups: Al, A2, A3, Bl, and B2. Subgroup Al (five sta- organic matter (p < 0.05). Assemblage B stations had high-
tions) located in the innerpart ofthe inlet near the mouth of er mean percentages of gravel, coarse sand, silt-clay, and
the river; subgroup A2 (18 stations) included a group of carbonates than assemblage A. It also surpassed assem-
intertidal stations located along the border ofthe inlet; sub- blage A in the mean percentage of organic matter, mean
group A3 (17 stations) included the intertidal stations ofthe diversity value, and sort coefficient. However, assemblage
central zone, mostly covered by Zostera noltii, which also A stations exhibited higher percentages of fine sand than
comprised two subtidal stations; subgroup Bl (ten stations) those ofassemblage B. The same analysis was later applied
located in the middle-eastern subtidal zone; subgroup B2 to the different subgroups and a multiple comparison test
(23 stations) consisted of the outermost subtidal stations (Conover, 1971) was carried out a posteriori, to reveal the
OLABARRIA ETAL.: BENTHIC MOLLUSKS OF INLET OF BANO 109
Table 5. Spearman's rank correlations between ordination axes and community index, and environmental
variables in InletofBano. Depth (measured in relation tozero tidal level): subtidal stations (-m) andinter-
tidal stations (+m). (H', Shannon-Wienerdiversity index; J', Pielou's evenness index; R, species richness;
*,p<0.05; **.p<0.01; NS,notsignificant ).
Subtidal Intertidal
Environmental variables R J' H' AxisI Axis II AxisI Axis II
Gravel 044** 033** 051** -0.29NS -0.14NS -0.20NS -0.52**
Coarsesand 0.43** 0.35* 0.52** -0.39* 062** -0.13NS -0.64**
Mediumsand -0.15NS 0.04NS -0.09NS -0.05NS 0.I0NS -0.08NS -0.04NS
Finesand -0.69** -0.24* -0.69** 0.66** -0.25NS 0.27NS 0.67**
Carbonates 0.78** 0.13NS 0.66** -0.59** -040** -0.05NS -0.67**
Organic matter 0.14NS -0.19NS -COINS 0.32NS -0.52** -0.21NS 0.12NS
Nitrogen 0.41** -0.03NS 0.25* 0.22Ns -0.47** -0.04NS -0.22ns
Silt-clay 0.45** -0.04NS 0.27* 0.18ns -0.17NS 0.29NS 0.05NS
Depth -0.68** -0.13NS -0.56** 0.05NS -0.06NS -0.39* 0.32*
Sortcoefficient 0.44** 0.30* 0.50** -0.41* -0.16ns -009NS -0.41**
groups that significantly differed from one another in the age ofcarbonates and nitrogen (Table 5). In this analysis, a
different parameters (Table 6). group of stations and species defining subgroup Bl can be
The ordination analysis (DCA) was carried out on seen in the upper and lower right-hand quadrants, located
the subtidal and intertidal stations as a whole. However, toward the positive end of axis I. On the negative side of
because of the excessive concentration of points in the axis I, in both the upper and lower quadrants, there is a
upper and lower quadrant of the negative part of axis I, a group of stations and species that has been defined by the
separate analysis for the subtidal and intertidal samples was similarity index (O) and the constancy and fidelity indices
performed. These analyses were based on the groups as subgroup B2. In the vicinity of this group there is a
derived from the application ofthe point correlation coeffi- group of stations (S126, S122, S112, S118, S130, S134)
cient fora 99% level ofsignificance. which are transition stations, grouped in the same cluster in
In the analysis ofthe intertidal stations, the first two the point correlation analysis (subgroup B2). These are sta-
axes accounted for 36.4% of the total variance (Fig. 5). tions with modified bottoms, although originally they most
Axis I had a slightly negative correlation with the depth likely had bottoms similar to the stations in subgroup Bl.
parameter, and axis II correlated positively with the per- However, due to dredging operations, sediments from other
centage of fine sand and negatively with the percentage of
carbonates and coarse sand (Table 5). In this analysis there
Table 6. Multiple comparison test (Conover. 1971) between the sub-
are three groups of stations; on the positive side of axis I, groups derived from the point correlation analysis (Al, A2. A3, Bl, B2)
both in the upper and lower quadrants, there is a group of indicatinggroupsthatshowednosignificantdifferences(p<0.01).
stations that in the similarity (O) and constancy-fidelity
analyses distinguished subgroup A3. In the upper left-hand Gravelpercentage Coarsesandpercentage
quadrant, there was a group of closely-linked stations for Al <A3<B1 <B2<A2 Al <A3<B1<A2<B2
which the most characteristic species was Hydrobia ulvae.
This species reached densities of 2,960 ind/m- (station
1137). This group defined subgroup A in the similarity
analysis. Below, in the upper left-hand quadrant which con- Finesandpercentage Silt-claypercentage
tinued onto the lower quadrant, there was a group of sta- B2<A2<B1 <A3<Al Al <A2<A3<B1 <B2
tions and species that characterized what has been defined
as subgroup A2 using the point correlation and the constan-
cy-fidelity indices.
In the analysis of the subtidal stations, the coordi-
nates ofthe first two axes accounted for 33.4% of the total Carbonatespercentage Diversityindex
variance (Fig. 6). Axis I had a high positive correlation Al <A3<A2<B1 <B2 Al <A3<A2<B1 <B2
with the percentage offine sand and carbonates, and axis II
displayed a correlation with the percentage of coarse sand
and organic matter, and to a lesser extent with the percent-
— — —
110 AMER. MALAC. BULL. 14(2) (1998)
— — —
0.00 0.25 0.50 0.75 1.00
i i i 1
sioi
S102
•sua
•S115
S105
SI10
•S123
S106
Sill
S107
S108
S109
S126
S112
S122
S130
-S113
SU4
•
Sll?
•S118
S134
S103
S116
S120
•S125
•S129
-S131
-S133
-S127
-S128
S135
S121
S124
-S104
-1110
•SI32
1108
1102
1120
1123
1130
1128
1124
1125
•1126
1129
1116
111?
1118
1115
1119
1133
1101
1104
1103
1105
1106
1131
1114
1141
1142
1121
1127
1138
1107
1109
1122
1139
1143
1140
1132
1135
11134
11136
1137
Fig.4.Clustersofthesamplesbasedonthepointcorrelationindexandsampleofthefivemaingroups.
areas oftheria weredeposited there. found. The results pointto the presence offive groups (Fig.
After applying the entire series of statistical analy- 7) characterized as follows:
ses, a clear coherence can be seen among them, with very Subgroup Al was located in the intertidal zone
similar results obtained from the different methods. A cor- close to the mouth ofthe river. It consisted offive stations
respondence between the classification analysis (similarity in which 13 species were found (Table 7). This area had
coefficient, <t>) and the ordination analysis (DCA) was predominantly fine sands (62.43% ± 6.04%) and the lowest
OLABARRIA ETAL.: BENTHIC MOLLUSKS OF INLET OF BANO 111
II
\
I1361134y
A
lfS5
"29 Rissmem
"Chryobt
I1J5
Hydrulv / ™° ii*V
Lorilac =n23
I 1124\\
1139 1119 \
'Cardedu II »Elysvir 1126 X
1140
A2 1131 \ .Rlsspar -Cardexl \
Venesen\\ . Hintret "S132 \\1
" °M22 \ 1109° \.
/ Littlit \ °I107 Blttret I
11118 Lepicl.n11 1118 Turbacu 1120 j
Mysebid Abraalb /
ii2i y-iiM /
Uttobt"\ '127 / 1117 ^>^»noz ^>x/r
°S104
Venerho"0i .RetLtru <^081110
Monotin °,142 MusemarVenuver
Qlbbumc
aLfttmar 1116
* Patevul lAoanorl lGibbcjn
1101
.,H,latarci.H.l.nl1IAncnomeph
1105
Kellsub
Fig.5. Stations andspecies arrangedaccording toaxes I and II ofthe correspondencesanalysis (DCA) forthe intertidal stations, showing the division into
threemaingroups,confirmedbytheconfidenceellipsismethod(SokalandRohlf, 1979). SpeciescodesasinTable7.
mean diversity values (0.53 ± 0.79). Evenness was between Table 2), Scrobicularia plana (exclusive common),
0.91 (station 1115) and 0.06 (stations 1135, 1137), even Cerastoderma edule (common occasional), and Rissoa
though this index was undetermined (H' = 0) in two sta- parva (very common occasional). The FxD value and the
tions by the exclusive presence of the gastropod Hydrobia dominance indices indicated that H. ulvae is most charac-
ulvae, which reached very high densities in this subgroup teristic ofthis subgroup (Table 7).
(2,960 ind/m2). There was a small number ofspecies which Subgroup A2 was located in the intertidal zone on
varied from 5 (station 1115) to 1 (stations 1134, 1135). In the border of the inlet. It comprised 18 stations in which a
this subgroup, the presence of a freshwater bivalve total of29 species were found (Table 7). It had higher mean
Pisidium casertanum was detected. This species' occur- percentages of gravel and coarse sand than of medium and
rence was probably due to the proximity of the mouth of fine sands. The mean percentage of carbonates (7.22% ±
the river. 0.86%) and the mean diversity value (2.77 ± 0.86) were the
This subgroup was represented primarily by highest in the area. The evenness values fluctuated from
Hydrobia ulvae (accessory constant; see classification in 0.09 (station 1139) to 0.43 (station 1131) and the number of
112 AMER. MALAC. BULL. 14(2) (1998)
II
CaecImp
Obtuint
°S103
Moerdon
Mangnab
Thyafie
v}' Onobserr "Nucunit
S101
OI
Alvapun X?„
Jujuexa •
Leptcan Manzcra Cythcos-
"Hlatarc°S106 °SI27
Vodosqu
"Acmavir °S1Q7
Fig.6.Stationsandspeciesarrangedaccordingtoaxes IandIIofthecorrespondenceanalysis(DCA)forthesubtidalstations,showingthedivisionintotwo
maingroups,confirmedbytheconfidenceellipsismethod. SpeciescodesasinTable 8
species varied from 29 (station 1103) to six (station 1140). membranacea (elective common), and Retusa truncatula
The characteristic species of this subgroup were (very common accessory). The FxD index indicated that
Gibbula umbilicalis, Littorina obtusata (exclusive con- the most characteristic species of this assemblage were P.
stants), L. littorea (elective constant), L. mariae (very com- exiguum, T. acuta, C. terebellum, and R. membranacea
mon exclusive), Lepidochitona cinera (very common elec- (Table 7). Bittium reticulatum was also present and showed
tive), Mytilus edulis (very common occasional), Monodonta a high dominance and FxD index, making it an important
lineata, and Patella vulgata (exclusive common). species in the characterization ofthese bottoms, although it
According to the values ofFxD and the dominance indices, also had a high dominance in subgroups A2 and B2.
the most characteristic species of this subgroup were G. Cerastoderma edule, Venerupis senegalensis, and
umbilicalis, L. littorea, L. obtusata, L. mariae, and L. Hydrobia ulvae also had high FxD index values in sub-
cinerea (Table 7). group A2. This shows that there were three important
Subgroup A3 was established in the mid-inner area species in the characterization ofthese intertidal bottoms.
of the inlet, along with two subtidal stations (SI32, SI04). Subgroup Bl was located in the mid-outer subtidal
It encompassed a total of 17 stations which account for 24 zone of the inlet. It was comprised of ten stations with a
species. This group had high mean percentages offine sand total of 31 species. It had relatively high mean percentages
(45.92% ± 25.60%) and medium sand (21.89% ± 14.63%). of gravel (22.47% ± 1.99%) and fine sand (21.29% ±
The diversity had a lower mean value than in the A2 sub- 20.01%). The percentage of carbonates was lower than in
group (2.35 ± 0.82), evenness varied from 0.09 (station subgroup B2. The mean diversity value was high (3.07 ±
1133) to 0.92 (station 1119), and species richness varied 0.62) and evenness fluctuated from 0.94 (station S124) to
from 24 (station 1102) to4 (station 1119). 0.56 (station S125). The number of species varied from 26
The most noteworthy species were Parvicardium (station S133) to 8 (station S135).
exiguum (preferential constant), Turbonilla acuta, This subgroup was primarily composed of Thyasira
Chrysallida obtusa (preferential common), Rissostomia flexuosa (preferential constant), Hinia reticulata (occasion-