Community structure of macrobenthos in two tropical sandy ... - UFF

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Fernandes & Soares-Gomes Community structure of macrobenthos in two tropical sandy beaches McLachlan 1990; Jaramillo & McLachlan 1993). Dissipative beaches are hydrodynamically more stable than reflective ones, harboring a more diverse fauna. The composition of these two types of beaches is quite different, with polychaetes being predominant in the dissipative beaches, and crustaceans being abundant in the reflective beaches (Dexter 1984; McLachlan et al. 1993). Although not so evident as in hard-bottoms, the fauna may exhibit a discrete distribution along the beach profile in response to abiotic and biotic variables, forming a typical zonation pattern worldwide (Dahl 1952; Salvat 1964). Zonation is highly dynamic and not sharply defined (Defeo & McLachlan 2005). The influence of beach morphodynamics and sediments on benthic communities and populations was studied, among others, by McLachlan (1990, 1996), Jaramillo & McLachlan (1993), McLachlan et al. (1993), Borzone et al. (1996), Bentes et al. (1997), Defeo et al. (1997, 2001), Brazeiro (2001), Veloso & Cardoso (2001), Rodil & Lastra (2004). Only a few studies were carried out on the subtidal portion of the beach (Hill & Hunter 1976; Leber 1982; Knott et al. 1983; Borzone et al. 1996), because this required an intensive and special sampling effort (Borzone et al. 1996). The ‘‘swash exclusion hypothesis’’ (SEH) gained wide acceptability in explaining the control of species, abundance and diversity of the sandy beach macrobenthos by swash climate, determined by wave height and beach slope. This hypothesis predicts a consistent increase in species richness, abundance and biomass from reflective to dissipative conditions (Defeo et al. 2001). It is hypothesized that only true intertidal species will be affected by the differences in beach morphodynamic types (Contreras et al. 2003). Gómez & Defeo (1999) formulated new additions to the original SEH (postulated at the community level), to account for predictions at the population level (reproductive sizes, fecundity, growth and mortality rates). They argued that in harsh reflective beaches, organisms would need to divert more energy to maintenance processes, rather than for reproductive growth (Defeo & Martínez 2003). The aim of this work was to verify if the above cited paradigms are true for two beaches at tropical latitudes and to describe the local vertical zonation of their fauna. Material and Methods Study area Both beaches studied (Rio de Janeiro, Brazil) are relatively free from anthropogenic impacts, except for human recreation, which is especially intense on holidays and during summer vacations. Praia do Pontal (22°57¢58¢¢ S) is a 700m long and 35m wide intertidal beach, located in BRASIL Arraial do Cabo Municipality, with a median to fine sand granulometry. Costa Azul beach (22°31¢37¢¢) is a waveexposed, 900m long and 50m wide intertidal beach, located in Rio das Ostras Municipality, with a predominance of coarse sand (Fig. 1). Sampling design Sampling was carried out in the austral winter of 2002 and the austral summer of 2003, during low-spring tides. At each beach, two sectors (S1, S2), of 40 m distance, were sampled along five transects (T1–T5) perpendicular to the water line, from below the swash line and the supralittoral vegetation. Each transect was divided into 10 strata, equally spaced, consistent with the following beach width: Pontal beach width 35 m ¼ 3.5 m, the distance between the levels; Costa Azul Beach width 50 m ¼ 5 m, the distance between the levels. A sampling unit was taken in each stratum with a 0.04 m 2 quadrat sampler, buried to a depth of 25 cm, and the macrobenthos were sorted using a 0.5mm-mesh sieve. To verify if some species occurred beyond the limits of the intertidal zone, a triplicate sample was collected in the breaking zone contiguous to each transect, with a mean depth of 2 m, using a hand corer of 0.00785 m 2 , buried to a depth of 25 cm, and the macrobenthos were sorted using a 0.5mm-mesh sieve. As the sampling device used to sample the surf zone was different from the one used to sample the intertidal zone, a comparison between both zones was not made. However, the surf zones of the two beaches were compared so as to test the differences. Habitat featuring Rio de Janeiro 45° 44° 43°W 42° 41° Fig. 1. Geographic location of the beaches studied. Costa Azul Pontal 22°S The sediment samples for grain-size analyses were collected at each stratum by inserting a 3.5cm diameter 21° 23° Marine Ecology 27 (2006) 160–169 ª 2006 The Authors. Journal compilation ª 2006 Blackwell Publishing Ltd 161
Community structure of macrobenthos in two tropical sandy beaches Fernandes & Soares-Gomes cylindrical corer to a depth of 15 cm. Samples were dried at 70 °C in an oven and sieved through graded screens so as to determine mean particle size and sorting parameters (Folk & Ward 1957); the sediments were classified according to the Wentworth (1922) scale. Wave height was estimated by measuring the height of the waves with graduated poles against the horizon. Beach slope at each site was determined by Emery’s (1961) profiling technique, from the average wave height (Hb), wave period (T) and sand fall velocity of the particles (values estimated using the mean grain size from the swash zone and conversion tables given by Gibbs et al. 1971). Dimensionless Dean’s parameter (X) (Short & Wright 1983) was calculated for each beach as a measure of its morphodynamic state: X ¼ Hb/Ws · T. The wave period was the time interval between breakers, and the swash period was estimated measuring the maximum swash amplitude, calculated as the distance between the highest and the lowest turning points of the up-swash and back-swash, respectively, during a 10- min period. Data analysis The hypothesis that total macrobenthos density and species richness were different in the two sectors within each beach was tested by the one-way ANOVA (T1 · T2 · T3 · T4 · T5). The a posteriori Tukey test was adopted to determine if there were significant differences among the transects. The one-way ANOVA was also adopted to test the differences between the sectors (S1 · S2). A Student’s t-test was used to compare the total abundance of sublittoral macrofauna between the beaches. All data were first tested for normality (Kolmogorov– Smirnov test) and homoscedasticity (Bartelett test) prior to all parametric analyses. The data set of each beach were arranged in an ecological matrix and analyzed by the Unweighted Pair Group Method, Average Clustering and Non-metric Multidimensional Scaling Ordination Methods, adopting the Bray–Curtis similarity index. To test the hypothesis that the strata were different along the vertical distribution (levels, zonation), a one-way analysis of similarity (ANOSIM) was carried out using the Bray–Curtis similarity matrix. The samples were pooled for each level/stratum as the sum of the species abundance of the five transects for each stratum and sector, because the ANOVA results showed that there were no differences between the transects. The relationship between the abiotic and biotic variables was determined by the Spearman rank correlation. Results Well-sorted fine sands predominated at Pontal Beach while moderately sorted coarse sand was the main type of sediment found at Costa Azul. According to the mean Dean’s X index, Pontal is a dissipative beach (X ¼ 6.120), and Costa Azul is a reflective beach (X ¼ 0.568). Table 1 shows the other abiotic features of the two sectors at each beach, separated by summer and winter surveys. Eleven species belonging to crustaceans, molluscs, polychaetes and insects were collected, four of which were common to both beaches. At each beach, a set of seven species was collected and some species were found only in one of the studied periods. In Pontal Beach, organisms were collected in all the 10 levels analysed, while in Costa Azul, organisms were missing from levels 9 and 10. The polychaete Scolelepis squamata, the crustacean Excirolana armata and the insect Phaleria testacea were found only at Pontal Beach; the latter species were found only in the winter survey. The polychaetes Pisionides indica and Hemipodus olivieri, and the mollusc Olivancillaria vesica vesica occurred only at the Costa Azul Beach. Hemipodus olivieri was found only in winter and O. vesica vesica only in summer sampling. The dominant species at Pontal, on both surveys, was S. squamata while Excirolana braziliensis dominated at Costa Azul Beach. At Pontal E. armata co-dominated in the winter survey and at Costa Azul, Donax hanleyanus co-dominated in winter, while Emerita brasiliensis Table 1. Physical characteristics of the beaches studied in winter (W) and summer (S). physical characteristics/ beaches Pontal, sector 1 Pontal, sector 2 Costa Azul, sector 1 Costa Azul, sector 2 wave height [m] W 1.0 1.0 1.0 1.0 S 1.0 1.0 0.8 0.8 wave period [s] W 6.0 6.0 16.93 11.93 S 5.8 5.8 10.2 10.2 swash period [s] W 16.2 15.3 7.4 6.2 S 14.1 14.5 7.3 7.2 intertidal slope [1/m] W 1:21.4 1:20 1:10.4 1:12.1 S 1:22.5 1:20.8 1:12.1 1:15 length [m] W 35 35 50 50 S 35 25 50 50 Dean parameter X W 6.01 6.01 0.48 0.51 S 6.22 6.22 0.78 0.49 162 Marine Ecology 27 (2006) 160–169 ª 2006 The Authors. Journal compilation ª 2006 Blackwell Publishing Ltd
Page 1: Marine Ecology. ISSN 0173-9565 ORIG
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