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Estuarine, Coastal and Shelf Science 64 (2005) 658e670

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Spatial variability of intertidal rocky shore assemblagesin the northwest coast of Portugal

R. Araujo a,b, I. Barbara c, I. Sousa-Pinto a,d, V. Quintino b,*

a CIMAR, Centro Interdisciplinar de Investigacao Marinha e Ambiental, Rua dos Bragas, 289, 4050-123 Porto, Portugalb Departamento de Biologia, Universidade de Aveiro, CESAM, Centro de Estudos do Ambiente e Mar, Campus Universitario de Santiago,

3810-193 Aveiro, Portugalc Departamento de Bioloxia Animal, Bioloxia Vegetal e Ecoloxia, Campus da Zapateira, s/n. 15071 A Coruna, Spain

d Departamento de Botanica, Faculdade de Ciencias, Universidade do Porto, Rua do Campo Alegre, 1191, 4150-181 Porto, Portugal

Received 20 September 2004; accepted 18 March 2005

Available online 7 July 2005

Abstract

The spatial variability of rocky shore assemblages of the northwest Portuguese coast was studied in a total of 12 transects, visited

twice between March and August 2003. Each transect was positioned from the upper to the lower shore and four replicate samples(50! 50 cm) were taken at each visually identified assemblage. Multivariate analysis was used to test the null hypothesis of nosignificant differences among assemblages located on different heights along the transect and among assemblages located at the same

height on different transects. The distribution pattern of organisms along the height was not consistent across transects andsignificant variability could be found in assemblages located at the same height on the shore. Globally, the variability from the lowerto the upper shore (vertical axis) was larger than from transect to transect (horizontal axis) but the distribution pattern along height

was clearer in sheltered transects than in exposed ones. The heterogeneity in composition between the visually identifiedassemblages, within the same transect, was, however, the major source of variability in the study area. In the rocky intertidalnorthwest coast of Portugal, where tidal amplitude is broad and extremely wave-exposed sites are scarce, height above chart datum

was the most important factor determining the distribution and species composition of the assemblages. However, variability alongthe horizontal axis was also significant and should be considered in studies of spatial patterns of distribution of organisms withinthis area.� 2005 Elsevier Ltd. All rights reserved.

Keywords: macroalgae; spatial variability; intertidal; marine; NW Portugal

1. Introduction

Intertidal rocky coastlines are heterogeneous envi-ronments that support a wide variety of living forms. Inthese systems organisms are distributed in a particularway, occurring at specific levels along a height axis, from

* Corresponding author. Victor Quintino, Departamento de Biol-

ogia, Universidade de Aveiro, CESAM, Centro de Estudos do

Ambiente e Mar, Campus Universitario de Santiago, 3810-193 Aveiro,

Portugal.

E-mail address: [email protected] (V. Quintino).

0272-7714/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.ecss.2005.03.020

the lower to the upper shore (Underwood, 1981;Ballesteros, 1995; Thompson et al., 2002). The observa-tion of these distribution patterns led to the develop-ment of several models of vertical zonation of organismson rocky shores. These models usually consider threemain zones on the shore, in relation to a gradient ofemersion/desiccation, containing distinct organisms: theupper shore, the midshore and the low shore (Stephen-son and Stephenson, 1949; Lewis, 1964; Peres andPicard, 1964; Seoane-Camba, 1969). Nevertheless,some authors considered these general zonation schemes

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659R. Araujo et al. / Estuarine, Coastal and Shelf Science 64 (2005) 658e670

over-simplified, given that elevation above chart datumalone could not explain all the variability encountered indistribution patterns and abundance of benthic organ-isms (Underwood, 1981; Boaventura et al., 2002).

Causes underlying the distribution patterns of organ-isms in intertidal rocky systems have been approached bymany authors. Examples include the role of competition(Schonbeck and Norton, 1980; Kastendiek, 1982;Jenkins et al., 1999a,b), herbivory and predation(Moreno and Jaramillo, 1983; Underwood, 1998; Wil-liams et al., 2000), settlement and recruitment (Connell,1985; Kaehler and Williams, 1997; Jenkins et al., 1999c,2000), height above chart datum (Schonbeck andNorton, 1978; Underwood, 1978; Bockelmann et al.,2002) and gradient of wave exposure (Underwood, 1981;Menge et al., 1993).

More recently, the role of small-scale topographyheterogeneity in regulating species distribution andabundance was investigated (Bourget et al., 1994;Archambault and Bourget, 1996; Thompson et al.,1996; Lapointe and Bourget, 1999; Williams et al.,2000). Species distribution can change over very smallspatial scales if the complexity of the habitat is high(Benedetti-Cecchi and Cinelli, 1997). In fact, mosaics oforganisms are an obvious feature of intertidal habitats,mainly at low levels on the shore (Menge et al., 1993).

Recent studies report on the variability in verticalpatterns of distribution of organisms among sites and inthe species composition of assemblages within the samelevel on the shore (Benedetti-Cecchi and Cinelli, 1997;Menconi et al., 1999; Kelaher et al., 2001). InMediterranean rocky systems, for example, Benedetti-Cecchi (2001) found more vertical than along-shorevariations at small scales but this tendency was notconfirmed at larger spatial scales.

Qualitative descriptions of intertidal rocky shoreassemblages for the Portuguese coast have been pro-vided by some authors (Boaventura et al., 2002).However, quantitative data on spatial patterns ofdistribution of organisms are scarcer. Examples includethe work of Boaventura et al. (2002), who reported onsignificant differences between the upper and the lowerparts of the midshore zone and between midshoreassemblages of the northern and southern coasts ofPortugal. No study ever addressed which of thesevariability axes is strongest on the Portuguese coast:the vertical axis, from the upper to the lower shore,versus the horizontal axis, across the shore, at the sameheight. The main objective of this work was toquantitatively analyse the distribution patterns of in-tertidal organisms along horizontal and vertical gra-dients, in a 60 km stretch of the northwestern coast ofPortugal. Different scales of variability were analysed:global horizontal and vertical variability along the studyarea, variability among and between different heights onthe shore and variability within transects.

2. Material and methods

2.1. Study area

The tidal regime along the Portuguese coast is semi-diurnal with the largest tidal range during spring tides of3.5e4 m. The northwest rocky shores are typicallygranite and the coast is exposed to the prevailingnorthwest oceanic swell, which can reach values over5 m in the winter. Sea surface temperature variesbetween 13 �C and 20 �C.

The northern Portuguese rocky intertidal ecosystemsare divided into three major zones, as described ingeneral zonation schemes by Stephenson and Stephen-son (1949), Lewis (1964), Peres and Picard (1964) andSeoane-Camba (1969). The uppermost zone of the shoreis dominated by incrustant lichens such as Verrucariamaura, Lichina pygmaea and Lichina confinis and by thegastropod Melaraphe neritoides. In the colder seasons,this level is also colonized by Porphyra linearis. Sessilefilter feeders such as Patella sp., Chthamalus sp. andMytilus sp. are the most common organisms at midlevels on the shore of exposed zones, brown algae suchas Fucus spiralis, Fucus vesiculosus, Ascophyllum nodo-sum and Pelvetia canaliculata being the predominantorganisms in sheltered midshore environments. Thelower littoral is characterised by the presence ofa considerable diversity of turf forming algae andcanopy species like Saccorhiza polyschides, Laminariaochroleuca, Laminaria hyperborea, Bifurcaria bifurcata,Chondracanthus acicularis and Himanthalia elongata.

2.2. Sampling

Sampling occurred in 12 sites, placed betweenMoledo (41 �50#5$N, 8 �52#18$W) and Apulia(41 �29#14$N, 8 �46#58$W) (Fig. 1). At each site thesampling points were positioned along a transectextending from the upper to the lower shore. Eachtransect was visited twice, between March and August2003. The positioning of transects was chosen accordingto the presence of extensive rocky shore systems and wasrestricted to bedrock. Rock pools were not considered inthis study. The study area comprehends exposed,moderately exposed and sheltered habitats.

In all transects, four replicate samples were taken ateach visually identified assemblage (homogeneous mac-roalgal assemblages) (Schils and Coppejans, 2003). Theunit sample, each replicate, corresponds to a quadrateof 50! 50 cm, divided into 25 sub-quadrates of10! 10 cm. The quadrates were placed randomlywithin each assemblage (Dethier et al., 1993). Percentcover was assessed by means of non-destructivemethods, by counting the number of sub-quadratesoccupied by each species and giving 4% cover toeach sub-quadrate. Whenever necessary, sampling was

660 R. Araujo et al. / Estuarine, Coastal and Shelf Science 64 (2005) 658e670

Moledo

Cepães

Apúlia

Amorosa

Montedor

Forte do Cão

Viana do Castelo

Vila Praia de Âncora

S. Bartolomeu do Mar

9°5'0"W 9°0'0"W 8°55'0"W 8°50'0"W 8°45'0"W

41°30'0"N

41°35'0"N

41°40'0"N

41°45'0"N

41°50'0"N

0 4 8 12 Km

Fig. 1. Study area. Transects distanced less than 50 m are marked in the same locality. The transects studied were: Moledo, Vila Praia de Ancora 1

and 2, Forte do Cao 1 and 2, Montedor 1 and 2, Viana do Castelo, Amorosa, S. Bartolomeu do Mar, Cepaes and Apulia.

stratified in different layers and the upper visible layer ofmacroalgae was distinguished from substrate cover,being estimates of percent cover made for each layerseparately (Dethier et al., 1993). Four replicates weretaken per assemblage on each sampling occasion.

The specimens which could not be identified in thefield were collected and identified in the laboratory. Allmacroalgae were identified to species level except for thegenus Ulva. Only sessile animal species were consideredin this study and were identified to the genus level. The

lichens Verrucaria maura, Lichina pygmaea and Lichinaconfinis were also considered.

Littoral level was calculated in relation to chartdatum. Height on the shore of the sampled communitiesvaried between 0 and 6 m above chart datum.

The concept of community used in this study is basedon the knowledge that species aggregate in a particularway in relation to biological factors and environmentalgradients, making it possible to recognize mosaics ofcommunities whose dominant species lead to the notion


of homogeneous macroalgal assemblages (Turner andLucas, 1985; Menge et al., 1993; Ballesteros, 1995).

In this study the number of assemblages sampled inthe lower shore was higher than in the other levels, dueto the wider diversity of assemblages found in thatzone.

2.3. Data analysis

Data analysis was performed with non-parametricmultivariate techniques using the PRIMER software(Clarke and Gorley, 2001).

Samples similarity was calculated with the BrayeCurtis coefficient, after log (xC 1) data transformation.Non-metric multidimensional scaling (nMDS) was usedto produce two-dimensional ordination plots.

One-way ANOSIM (Clarke and Warwick, 2001) wasused to test the null hypothesis of no significantdifferences between: (1) sampling occasion; (2) assemb-lages located on different heights along transects evertical gradient; (3) assemblages located at the sameheight on different transects e horizontal gradient and(4) visually identified assemblages within the sametransect.

ANOSIM test produces a statistic (R-statistic) thatlies in the range (�1;1). Values of RZ 1 are obtainedonly when all replicates within groups are more similarto each other than any replicates from different groups(Clarke and Warwick, 2001).

To test the null hypothesis 1, samples were groupedinto a sampling occasion factor and the comparison wasmade between each visually identified assemblage withinthe same transect, and between each height class, alsowithin the same transect. This test also included thecomparison of the whole transect, sampled on the twooccasions. Only the global R-value for each height classand for the whole transect will be presented, given theelevated number of assemblages under test. To test forthe null hypothesis 2, a height factor was established,with the classes: below 1 m, 1e2 m, 2e3 m and above3 m. All the unit samples (each individual50 cm! 50 cm quadrate) obtained within each heightclass were regarded as replicates of the same heightfactor, independently of belonging or not to the samevisually identified assemblage. This analysis was con-ducted separately for each transect and for the wholegroup of transects. This test was conducted with thedata collected on the first sampling occasion. To test thenull hypothesis 3, a transect factor was considered. Allunit samples within the same transect and height classwere regarded as replicates. This test was conductedseparately for each of the four height classes. For thistest, only the global R-value (Clarke and Warwick,2001) for each height class will be presented, given thevery large number of possible pairwise comparisonsbetween transects (66). To assure data independence

from the previous test, this analysis was conducted withthe data collected on the second sampling occasion. Totest the null hypothesis 4, the unit samples were groupedinto an assemblage factor. The assemblages werevisually identified in each transect. Not all the assemb-lages are present in all transects. This analysis wasconducted separately for each transect, comparingpotentially distinct assemblages located on the sametransect. This test was conducted with data from six outof the 12 transects visited on the first sampling occasion,chosen randomly. Only the global R-value for eachtransect is presented. A similar test was conducted tocompare the same assemblage located on distincttransects. This analysis was conducted with datacollected on the first sampling occasion, from theremaining six transects.

The Primer Routine Relate was also used to test therelationship between the succession of communitiesalong height and a seriation matrix, representing thesampling sites according to this factor. This testproceeds by calculating and testing the significance ofa Spearman rank correlation between the two matrices(Clarke and Warwick, 2001).

3. Results

Concerning the variability between samples taken atthe same sites on different occasions, no significantdifferences were found between each transect (Table 1)or between some of the height levels (Table 2). However,significant differences were found between some of thevisually identified assemblages on the two samplingoccasions. In spite of presenting similar percent cover ofthe dominant species, these assemblages showed highvariability in the species composition and percent coverof the accompanying species. Higher on the shore, nosignificant differences could be found between the

Table 1

One-way ANOSIM. Global R-values and associated probability for

the comparison between the same transect sampled in the two

occasions. In this test, the null hypothesis was never rejected

Transect Global R p

1 0.069 0.71

2 �0.081 0.93

3 �0.027 0.61

4 �0.027 0.69

5 �0.07 0.73

6 0.059 0.15

7 �0.208 0.77

8 0.064 0.06

9 �0.114 0.98

10 �0.250 0.99

11 �0.064 0.89

12 �0.122 0.92


majority of the visually identified assemblages on thetwo sampling occasions. These assemblages have muchlower diversity of the accompanying species. As anexample from one of the transects, the low shoreassemblage dominated by Chondrus crispus presenteda total of 26 accompanying species in the four replicates,while the high shore assemblage dominated by Pelvetiacanaliculata had only a total of five accompanyingspecies in the four replicates. Overall, the lack ofsignificant difference between the two sampling occa-sions allows using all the data to test different null

Table 2

One-way ANOSIM. Global R-values and associated probability for

the comparison between the same height levels sampled in the two

occasions. The null hypothesis was not rejected for the lowest (!1 m)

and the highest (O3 m) height levels. *non-significant

Height (m) Global R p

!1 0.005 0.32*

1e2 0.079 0.005

2e3 0.159 0.001

O3 �0.011 0.56*

hypothesis, without introducing significant variabilityrelated to the temporal gap between the collections ofthe two data sets.

The ordination diagrams of the individual transectsshow that height above chart datum may play differentroles in different transects. The relationship betweenassemblages and height on the shore was not the samefor all transects, as shown in the ordination diagramsdepicted in Fig. 2. The worst relationship was encoun-tered for transects illustrated in Fig. 2a and b, whereasfor those illustrated in Fig. 2cef, the relationship wasmuch clearer. In Fig. 2e and f, the variability among thesamples taken at the same height on the shore was alsoimportant, mainly at mid and higher levels. Table 3presents the Spearman rank correlation calculatedbetween the BrayeCurtis similarity matrix from in-dividual transects and a seriation model matrix accord-ing to the position of the sites along the height on theshore. The values obtained corroborate the MDS plotsand clearly indicate that the distribution of theassemblages does not show the same relationship tothe height factor across transects. Both results suggest

<1m 1 - 2m 2 - 3m 3 - 4m 4 - 5m

Stress: 0,09

Stress: 0,12

Stress: 0,07

Stress: 0,07Stress: 0,07

Stress: 0,11

Fig. 2. Two-dimensional nMDS ordinations for six of the 12 transect studied. Each point refers to a single replicate and each symbol to a different

height on the shore. Data from the first sampling occasion. (a) Moledo; (b) Apulia; (c) Montedor 1; (d) Vila Praia Ancora 1; (e) Vila Praia Ancora 2;

(f) Amorosa.


an interaction between the two main factors and as suchthe test of the null hypothesis of no differences amongtransects was conducted separately for each height level.Also, the test of the null hypothesis of no differencesamong height levels was conducted separately for eachtransect.

When testing the height factor, significant differencescould be found among all height levels, the majorexceptions being obtained between the highest levels, asshown in Table 4. The R-statistic values given in Table 4,globally for each transect (global R) and separately foreach height at each transect, vary widely from transectto transect, again suggesting that height above chartdatum plays different roles in different transects.Similarly, when testing for the difference among trans-ects, separately for each height level, the values of theR-statistic were significant at all heights on the shore asshown in Table 5, with relatively similar values for eachlevel. Table 5 only presents the global R-values for theANOSIM test among transects, separately for eachheight level, given the large number of pairwisecomparisons among transects. Although both factorsappear significant, when including all transects in thesame analysis, the global R-values for the height factor(vertical axis) and for the transect factor (horizontalaxis) were, respectively, 0.38 and 0.17, indicating that

Table 3

Spearman rank correlations for six transects obtained between the

BrayeCurtis similarity matrix and a model seriation matrix in which

sites are displayed according to their height on the shore

Site r

Moledo 0.297

Apulia 0.191

Montedor 1 0.767

Vila Praia de Ancora 1 0.496

Vila Praia de Ancora 2 0.456

Amorosa 0.718

variability between the assemblages was larger acrossthe vertical gradient.

Exposure to wave action may be a possible explana-tion for the fact that the distribution of assemblagesalong the height factor was not similar across alltransects in the study area. Fig. 3 shows the distributionof the total number of species per height level, fortransects with different degrees of exposure to the waveaction, while Fig. 4 presents the vertical distribution of afew species in the same transects (see also Appendix A).The total number of species varies from transect totransect and seems to be higher in less exposed transects.Also the number of species at the lower levels on theshore seems to be higher on sheltered transects than onexposed ones (Fig. 3). In exposed shores, the low andmidlittoral zones are dominated by animal organisms such asMytilus sp. and incrustant algae such as Lithophyllumincrustans (Fig. 4), while the moderately exposed and thesheltered shores are dominated by macroalgae at the lowlittoral levels and by animals at mid levels. However, themacroalgae species composition and their abundance aredifferent in moderately exposed shores and in shelteredones, as show in Fig. 4 (see also Appendix A). Also, thereare differences in the vertical extent of the area colonizedin shores with different degrees of exposure to waveaction. In exposed transects, most probably due tosplash, organisms colonize higher levels on the shore (4e5 m) than in sheltered ones (Fig. 4). In spite of thesevariations, each species usually appears more abundantwithin a particular height interval on the shore in alltransects where it occurs, as shown in the ordinationdiagrams represented in Fig. 5.

Finally, the visually identified assemblages weresignificantly different within transects. The global R-values for six transects chosen at random were alwaysabove 0.96, as shown in Table 6. Of all the pairwisepossible comparisons between assemblages, only twopresented the R-value below 0.5 and the associatedprobability above 0.05. The far majority of the visually

Table 4

One-way ANOSIM for the test of the null hypothesis of no differences between assemblages located on the various heights on the shore at each

transect. The values of the R statistic are shown with the respective probability values. Notice that the associated significance depends on the number

of possible permutations (as an example, for transect 1, RZ 1 between (!1 m)e(O3 m) but r! 0.01, while RZ 0.39 between (!1 m)e(1e2 m) but

r! 0.001). *r! 0.05; **r! 0.01; ***r! 0.001; n.s., non-significant

Transects

1 2 3 4 5 6 7 8 9 10 11 12

Height (m)

(!1)e(1e2) 0.39*** 0.92*** 0.36*** 0.35*** 0.59*** 0.57*** 0.95*** 0.80*** 0.37** 0.44** 0.71*** 0.98**

(!1)e(2e3) 0.41*** 1.00*** 0.88*** 0.99*** 0.99*** 0.73*** 0.93*** 0.42*** 0.91*** 1.00* 0.68*** 0.03

(!1)e(O3) 1.00** 0.97*** 0.98*** 0.75*** 0.47*** 0.81*** 0.26***

(1e2)e(2e3) 0.35** 0.79*** 0.16* 0.57*** 0.09n.s. 0.72** 0.58* 0.01n.s. 0.22* 0.31* 0.38*** 0.13

(1e2)e(O3) 0.54*** 0.66*** 0.84*** 1.00*** 0.23* 1.00** 0.38***

(2e3)e(O3) 0.14n.s. 0.41*** 0.99*** 0.858** 0.23*** 0.19n.s. 0.01n.s.

Global R 0.40*** 0.90*** 0.46*** 0.68*** 0.56*** 0.61*** 0.88*** 0.27*** 0.47*** 0.45*** 0.62*** 0.10**


identified assemblages are thus significantly differentfrom each other, including when the comparison wasmade between the same potential assemblages located indistinct transects. Only the assemblages dominated byanimal organisms (Mytilus sp., Patella sp. and Chtha-malus sp.) were consistently, significantly similar be-tween themselves. The majority of the assemblagesdominated by algae were significantly different betweenthemselves, even the ones dominated by the samespecies. This difference was due to the variation inspecies composition and cover percentage of the ac-companying species.

Table 5

One-way ANOSIM for the test of the null hypothesis of no differences

between assemblages located on the same height on the shore, on

different transects. Values of the global R and associated probability.

***r ! 0.001

Height (m) Global R

!1 0.303***

1e2 0.324***

2e3 0.356***

O3 0.376***

38 27 42 77 69 52

} } }

Fig. 3. Total number of species along the height on the shore and for

the whole transect, in 6 of the 12 transects studied. Two transects

correspond to exposed shores (E) (Moledo (ML) and Apulia (AP)),

two transects refer to moderately exposed shores (SE) (Montedor 1

(MT) and Vila Praia de Ancora 1 (VPA1)) and two transects refer to

sheltered shores (S) (Vila Praia de Ancora 2 (VPA2) and Amorosa

(AMO)).

4. Discussion

For the northern Portuguese intertidal rocky shorewe could not find consistent patterns of distribution oforganisms along the vertical axis across sites and nohomogeneity between assemblages located within thesame level on the shore, but on different transects. Theseresults are in agreement with those reported byBenedetti-Cecchi and Cinelli (1997) for the Strait ofMagellan and by Benedetti-Cecchi (2001) for Mediter-ranean rocky shores. Important horizontal variability ata scale of a few meters in species dominating assemb-lages was also referred by Underwood (1981), fora rocky intertidal community of New South Wales.

When analysing the individual distribution of organ-isms along transects, it was clear that the majority of thespecies appeared only at some levels on the shore. Theoccurrence of any given species at a particular height onthe shore is well documented (Underwood, 1981), beingthe patterns of distribution of organisms in intertidalhabitats attributable to physiological needs in relation tophysical stress and biological interactions (Schonbeckand Norton, 1978, 1980; Kastendiek, 1982; Benedetti-Cecchi et al., 2000a,b). However, it was not possible toidentify groups of species recurrently occurring withineach tidal level across sites. In fact, the various transectspresented large differences in specific composition.

Apart from the observed distribution patterns foreach species, it must be emphasized that withina particular level on the shore, different mosaics ofspecies coexist (assemblages). The dominance in each ofthese assemblages is usually attributable to particularspecies with high cover percentage. Within the sameheight level, large variability existed in the assemblages’composition across transects. For instance, in somesites, communities dominated by Chondrus crispuscoexist with assemblages with high cover percentage ofGelidium sesquipedale, while in other sites, at the sameheight level, assemblages of Gigartina pistillata appearedtogether with communities of Bifurcaria bifurcata.

Increasing evidence points to the importance ofsmall-scale variation in the species composition ofassemblages within large zones (low, mid and highlittoral), along the vertical gradient (Menge et al., 1993).This variability was a conspicuous feature of the systemstudied here. Thus, it is important to consider small-scale variability between assemblages; otherwise thehorizontal variability between assemblages in any givensite can be underestimated.

The processes that regulate the variability in speciescomposition at small spatial scales have been the subjectof recent studies. Recent findings show that small-scalesubstratum heterogeneity is important in determining theassemblages’ structure within any particular height.Archambault and Bourget (1996), studying the pro-portion of variability explained by large and small spatial


Chondracanthus teedii

} } } } } } } } }

} } } } } } } } }

} } } } } } } } }Chondracanthus acicularis Chondrus crispus

Rhodothamniella floridula Mytilus sp. Chthamalus sp.

Patella sp. Lithophyllum incrustans Lomentaria articulata

Fig. 4. Representation of the vertical distribution of some species, in 6 of the 12 transects studied. Two transects correspond to exposed shores (E)

(Moledo (ML) and Apulia (AP)), two transects refer to moderately exposed shores (SE) (Montedor 1 (MT) and Vila Praia de Ancora 1 (VPA1)) and

two transects refer to sheltered shores (S) (Vila Praia de Ancora 2 (VPA2) and Amorosa (AMO)).


Stress: 0,13

Chondrus crispus

Stress: 0,13

Chondracanthus acicularis

Stress: 0,13

Mastocarpus stellatus

Stress: 0,13

Sacchorhiza polyschides

Stress: 0,13

Fucus spiralis

Stress: 0,13

Chthamalus sp.

Stress: 0,13

Mytilus sp.

Stress: 0,13

Porphyra linearis

Stress: 0,13

Verrucaria maura

Stress: 0,13

Pelvetia canaliculata

Fig. 5. Two-dimensional nMDS ordinations for all transects sampled. Each plot represents the distribution pattern of a single species, being the circle

area proportional to the mean percent cover of species at each site.


scales, found that variability in species richness wasexplained by large scale variability while differences inabundances were attributable to small-scale spatialvariability (!20 cm). Lapointe and Bourget (1999) alsofound that the substratum heterogeneity was the mostimportant factor determining community structure.Also, Bourget et al. (1994) demonstrated that substratumheterogeneity and complexity strongly structured thesmall-scale distribution of species during the earlydevelopment phases of a marine epibenthic community.Structural complexity may reduce environmental stressexperienced by intertidal communities during low tide(Thompson et al., 1996), allowing the development ofa higher diversity of assemblages. Biological interactionsare also responsible for the development of mosaics ofcommunities at small-spatial scales (Kaehler and Wil-liams, 1997; Johnson et al., 1998; Leonard, 1999;Benedetti-Cecchi et al., 2000b; Williams et al., 2000).

In this study, the major source of variability was thecommunities’ species composition within transects. Thecolonization history, small-scale substratum heterogene-ity, biological interactions and organisms’ physiologicalneeds can, altogether, contribute to the heterogeneity inassemblage structure found within each site. Benedetti-Cecchi (2001) suggested that the relative importance ofphysical factors such as tidal height and of physicalattributes of the habitat, such as small-scale spatialvariability, could depend on the magnitude of each ofthese factors. In microtidal systems, like the Mediterra-nean Sea, where physical factors are not importantlimiting factors, variations in physical attributes of thehabitat may be important in determining communitystructure (Benedetti-Cecchi, 2001). The relative impor-tance of each of these factors remains unknown for thePortuguese coast.

In our study area, the distribution patterns oforganisms showed different relations with height onthe shore. The worst relationship was observed in veryexposed sites. In these sites, the number of macroalgaein the low littoral zone was much lower than the onefound in other transects, except for incrustant forms likeLithophyllum incrustans, while the number of sessilesuspension feeders (mainly Mytilus sp.) and invertebrate

Table 6

One-way ANOSIM. Values of global R and associated probability for

the comparison between the visually identified assemblages within six

transects randomly chosen. Values of the global R and associated

probability. ***r ! 0.001

Transect R

1 0.983***

2 0.993***

5 0.961***

8 0.979***

11 0.978***

12 0.967***

herbivores (mainly Patella sp.) was higher. The exposedsites present very harsh conditions for algae to settle,while mussels profit from these environments where theyfind great feeding opportunities due to high flow andenhanced ‘splash’ (Dahlhoff and Menge, 1996). In veryexposed systems, the degree of exposure may thus be themost important factor affecting communities, disablingthe identification of clear distribution patterns inrelation to shore height or to small-scale spatialvariability. In this study, the strongest relation betweenthe distribution patterns of organisms and height on theshore was encountered in moderately exposed andsheltered shores. Several authors mention the grade ofexposure as an important factor regulating the assemb-lages’ composition (Underwood, 1981; Hawkins, 1983;Menge et al., 1993). Menge et al. (1993) refer thatprocesses maintaining macrophyte mosaics in the lowlittoral zones differ from wave-exposed to wave-shel-tered sites. The exposure grade can also influence thespatial distribution patterns of organisms along theshore. A tendency for shifting upwards the distributionof organisms with increasing wave exposure is welldocumented (Underwood, 1981). This tendency was alsoobserved in the present study. Moreover, the grade ofexposure to waves can also explain the vertical patternsobserved in the sheltered places. In these habitats, waveaction is unable to reach high shore levels which maylimit the vertical extent of the colonized shore area.Thus, organisms occurring at mid and higher levelsoverlap their distribution due to limitations on verticalspace.

In the studied area, the global variability in theassemblages’ composition along the vertical axis wasmore pronounced than along the horizontal axis. Heightin relation to chart datum seems to be the mostimportant factor affecting globally the communities’structure. Moreover, horizontal variability between siteswas also significant. We suggest that in the northwestcoast of Portugal, with broad tidal amplitude and whereextremely exposed sites are scarce, height on the shore isthe major factor determining spatial variability in theassemblages’ species composition. However, horizontalvariability, namely related with grade of wave-exposureat larger spatial scales and small-scale spatial heteroge-neity, was also significant and must be considered instudies within this area.

Acknowledgments

This work was developed under the project ‘Distri-bution, use and conservation of the botanical heritageof the Northern Coast of Portugal: a sustainabledevelopment approach for a (future) ‘‘Natura 2000’’site’ (PNATBIA15204/99) funded by the Portuguese


Foundation for Science and Technology (FCT). Thefirst author was the recipient of a fellowship from thisproject. The authors acknowledge the help of CristinaCorreia during the field work.

Appendix A.

Species composition in an exposed (E e Apulia), amoderatly-exposed (M e Vila Praia de Ancora 1) and asheltered (S e Vila Praia de Ancora 2) transect, atdifferent heights on the shore. Only species with morethan 1% of mean cover percentage were considered.

Species composition of transects with different degrees of wave

exposure

!1 m 1e2 m 2e3 m 3e4 m O4 m

S M E S M E S M E S M E S M E

Ahnfeltiopsis

devoniensis

C

Bifurcaria

bifurcata

D C D

Calliblepharis

jubata

C

Callithamnion

tetricum

C

Ceramium

botryocarpum

C

Ceramium

echionotum

C

Ceramium

pallidum

C

Chondracanthus

acicularis

D C D

Chondracanthus

teedii

D C

Chondria

coerulescens

D

Chondrus crispus D D D C

Cladostephus

spongiosus

C

Codium

tomentosum

C

Corallina

elongata

C C D D D

Cryptopleura

ramosa

C C

Cystoseira

tamariscifolia

C

Desmarestia

ligulata

C

Dictyopteris

membranaceae

C C

Dictyota

dichotoma

C C C

Dumontia

contorta

C C

Enteromorpha

sp.

C C C C C C C

Gastroclonium

ovatum

C

Appendix A (continued )

!1 m 1e2 m 2e3 m 3e4 m O4 m


Gelidium

pulchellum

C

Gelidium

sesquipedale

C

Gigartina

pistillata

C

Gymnogongrus

crenulatus

C C

Ahnfeltiopsis

devoniensis

C

Gymnogongrus

griffithsiae

C

Haliptilon

squamatum

C

Halurus

equisetifolius

C

Himanthalia

elongata

C

Jania longifurca C

Laminaria

hyperborea

C

Leathesia

difformis

C

Leptosiphonia

schousboei

C

Lithophyllum

incrustans

C C C C C

Lomentaria

articulata

C

Mastocarpus

stellatus

C C C C

Mesophyllum

lichenoides

C

Mytilus sp. C C C C C

Ophidocladus

simpliciusculus

C

Osmundea

pinnatifida

C C C C C C

Palmaria palmata C

Plocamium

cartilagineum

C

Porphyra leucosticta C

Porphyra umbilicalis C C C C C C

Pterocladiella

capilaceae

C

Pterosiphonia

ardreana

C

Pterosiphonia

complanata

C C C

Pterosiphonia

pennata

C

Rhodothamniella

floridula

C C

Sacchorhiza

polischides

C C C

Scinaia furcellata C

Streblocladia

collabens

C C C C

Ulva sp. C C C C C C CUlvaria obscura C

Blidingia minima D D D D D D

Chthamalus sp. D D D D D C C C


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