Spatial variability of intertidal rocky shore assemblages in the northwest coast of Portugal

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  • ao

    o

    A

    o d

    3810-193 Aveiro, Portugal

    ec Departamento de Bioloxia Animal, Bioloxia Vegetal e Ecoloxia, Campus da Zapateira, s/n. 15071 A Coruna, Spaind 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 identied assemblage. Multivariate analysis was used to test the null hypothesis of nosignicant dierences among assemblages located on dierent heights along the transect and among assemblages located at the same

    height on dierent transects. The distribution pattern of organisms along the height was not consistent across transects andsignicant 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 identiedassemblages, 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 signicant 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 specic levels along a height axis, from

    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: theSpatial variability of intertidin the northwest c

    R. Araujo a,b, I. Barbara c, I. Sa CIMAR, Centro Interdisciplinar de Investigacao Marinha e

    b Departamento de Biologia, Universidade de Aveiro, CESAM, Centr

    Estuarine, Coastal and Shelf Scienc* 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: vquintino@bio.ua.pt (V. Quintino).

    0272-7714/$ - see front matter 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.ecss.2005.03.020l rocky shore assemblagesast of Portugal

    usa-Pinto a,d, V. Quintino b,*

    mbiental, Rua dos Bragas, 289, 4050-123 Porto, Portugal

    e Estudos do Ambiente e Mar, Campus Universitario de Santiago,

    64 (2005) 658e670

    www.elsevier.com/locate/ECSSupper shore, the midshore and the low shore (Stephen-son and Stephenson, 1949; Lewis, 1964; Pere`s andPicard, 1964; Seoane-Camba, 1969). Nevertheless,some authors considered these general zonation schemes

  • nover-simplied, 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 notconrmed 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 onsignicant dierences 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. Dierent scales of variability were analysed:global horizontal and vertical variability along the studyarea, variability among and between dierent heights on

    R. Araujo et al. / Estuarine, Coastal athe 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), Pere`s and Picard (1964) andSeoane-Camba (1969). The uppermost zone of the shoreis dominated by incrustant lichens such as Verrucariamaura, Lichina pygmaea and Lichina connis and by thegastropod Melaraphe neritoides. In the colder seasons,this level is also colonized by Porphyra linearis. Sessilelter 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 identied 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 to

    659d Shelf Science 64 (2005) 658e670each sub-quadrate. Whenever necessary, sampling was

  • nstratied in dierent 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 identied in theeld were collected and identied in the laboratory. Allmacroalgae were identied to species level except for thegenus Ulva. Only sessile animal species were considered

    lichens Verrucaria maura, Lichina pygmaea and Lichinaconnis 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 of

    Cepes

    Aplia

    Amorosa

    Montedor

    Forte do Co

    Viana do Castelo

    S. Bartolomeu do Mar

    95'0"W 90'0"W 855'0"W 850'0"W 845'0"W

    4130'0"N

    4135'0"N

    4140'0"N

    4145'0"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.660 R. Araujo et al. / Estuarine, Coastal ain this study and were identied to the genus level. TheMoledo

    Vila Praia de ncora

    4150'0"N

    d Shelf Science 64 (2005) 658e670communities whose dominant species lead to the notion

  • aof 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 coecient, 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 signicantdierences between: (1) sampling occasion; (2) assemb-lages located on dierent heights along transects evertical gradient; (3) assemblages located at the sameheight on dierent transects e horizontal gradient and(4) visually identied 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 dierent 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 identied 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 identied 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 rst 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 comparisons

    R. Araujo et al. / Estuarine, Coastalbetween transects (66). To assure data independencefrom 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 identied 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 rst 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 rst 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 signicance ofa Spearman rank correlation between the two matrices(Clarke and Warwick, 2001).

    3. Results

    Concerning the variability between samples taken atthe same sites on dierent occasions, no signicantdierences were found between each transect (Table 1)or between some of the height levels (Table 2). However,signicant dierences were found between some of thevisually identied assemblages on the two samplingoccasions. In spite of presenting similar percent cover ofthe dominant species, these assemblages showed highvariability in the species composition and pe...

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