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Fernandes & Soares-Gomes Community structure of macrobenthos in two tropical sandy beaches Fig. 4. Variation in the number of individuals and slope along the water level. Table 5. Spearman Rank Correlation between abiotic and biotic variables of the beaches studied. variables R P significance grain size versus faunal density )0.8982 0.0046 * grain size versus intertidal slope )0.7785 0.0279 * grain size versus species richness 0.2191 0.6021 n.s. Intertidal slope versus faunal density 0.9048 0.0046 * Intertidal slope versus species richness 0.5361 0.1710 n.s. X versus density 0.8675 0.007 * X versus richness 0.3061 0.418 n.s. n.s., not significant. *P < 0.05. & Dolan (1985), beaches that have slight slopes and long swash periods are more favourable for filterfeeding animals. The difference in the abundance of the sandy crab, E. brasiliensis between the beaches in this study, corroborates these ideas. The swash period at Pontal Beach was two times that at Costa Azul Beach, where the abundance of E. brasiliensis was very low, thus in agreement with the SEH, proposed by McLachlan et al. (1993). At Costa Azul Beach, where the animals were more exposed to wave action, crustaceans were the dominant taxa, whereas the polychaete S. squamata dominated at Marine Ecology 27 (2006) 160–169 ª 2006 The Authors. Journal compilation ª 2006 Blackwell Publishing Ltd 165
Community structure of macrobenthos in two tropical sandy beaches Fernandes & Soares-Gomes Pontal Beach. Along the Brazilian coast, S. squamata occurs in beaches along São Paulo and Paraná State coasts (Amaral 1979; Amaral et al. 1990; Shimizu 1991, 1995; Omena & Amaral 1997), and it is the most characteristic macrofauna species, both in abundance and frequency (Souza & Gianuca 1995; Borzone et al. 1996). Similarly, Barros et al. (2001) found that S. squamata was the most representative species on a dissipative beach of the Paraná State coast, representing 70% of the total macrofauna abundance. Cirolanidae crustaceans were abundant at both the beaches studied. At Costa Azul, E. braziliensis was the most abundant species, and at Pontal, E. armata was the second most abundant species, coexisting with the hippidean E. braziliensis. Defeo et al. (1997), comparing the different beaches along the Uruguayan coast, concluded that E. braziliensis and E. armata prefer fine sandy habitats, although E. braziliensis seems to be more eurytopic, also occurring at high densities in coarse sandy habitats. The lower abundance of E. braziliensis at Pontal Beach compared with Costa Azul might suggest a competitive interaction with E. armata. Defeo et al. (2001), showed that E. brasiliensis was more abundant at dissipative beaches, corroborating the ‘swash exclusion hypothesis’, which predicts a consistent increase in species richness, abundance and biomass from reflective to dissipative conditions (Defeo et al. 2001). Conversely, it has been hypothesized that only true intertidal species will be affected by differences in beach morphodynamic types (Contreras et al. 2003). In the present study, Pseudorchestoidea brasiliensis was found in both dissipative and reflective beaches. Gianuca (1983) has found that this crustacean was more abundant in dissipative beaches. On the other hand, it has been demonstrated that P. brasiliensis was more typical of reflective beaches (Cardoso & Veloso 1996). Gómez & Defeo (1999) suggested that P. brasiliensis was more resistant to immersion, desiccation and least prone to predation at reflective beaches. These results show that this species is able to successfully colonize beaches that are morphodynamically distinct. However, in this study, P. brasiliensis was found at low densities at both the beaches studied. Barros et al. (2001) suggested that the absence of some species in the beaches could be associated with a high level of recreational activities (i.e. tramping) and the usual practice of removal of artificial and natural detritus from those beaches. Studies have shown that some species can be eliminated or have their population densities reduced as a result of the increase of recreational activities in beach ecosystems (Moffet et al. 1998). The total abundance and dominance of species were not different between winter and summer surveys. The same has been found in beaches along the Paraná State coast in Southern Brazil, where crustaceans dominated or co-dominated together with polychaetes, regardless of the period (Barros et al. 2001). Therefore, the absence and/or the low abundance of some species can also be related to their life cycle. Monthly and annual fluctuations in population density are common among sandy beach macrofauna, where peaks in abundance are often a consequence of variations in recruitment rates (Souza & Gianuca 1995). Species richness in this study was low and similar to that recorded in the exposed Chilean beaches (Jaramillo et al. 2001), when compared with the west coast beaches of the United States, South Africa, Australia, and Oman. Otherwise, the macrofaunal density in the Chilean beaches is often higher than at other localities. Those high densities, in the majority of the cases studied, were largely a result of great densities of the sand crab Emerita analoga, that could reach 50% of the total density in the different types of beaches in Chile (Jaramillo & Lastra 2001). Similar results have also been found by Dugan et al. (2000) along the California beaches. At Pontal Beach, densities were comparable with those of the Chile and California beaches, but are attributed to the high density of the polychaete S. squamata. Dahl (1952) and Salvat (1964), among others, have proposed a vertical zonation pattern for sandy beach assemblages. For Brown & McLachlan (1990) it was possible to recognize two vertical zones: a subaerial or resurgence zone and a subaquatic or retention zone. However, there are controversies about a global pattern of zonation in sedimentary beaches. Defeo et al. (1992) were not able to recognize a unique pattern for five beaches studied in the coast of Uruguay. In addition Brazeiro & Defeo (1996) demonstrated that zonation at dissipative beaches could exhibit either a seasonal or a short-time variation. Beach species could have their distribution altered in response to environmental changes such as beach width and slope, position of the swash zone, food availability, and competition (Brazeiro & Defeo 1996; Giménez & Yannicelli 1997). At Pontal Beach, our results pointed out three groups of stations corresponding to biological zones: a lower zone, where S. squamata was a conspicuous species, an intermediate zone, characterized by the high abundance of E. brasiliensis and E. armata, and a higher zone, where E. braziliensis was the characteristic species. At Costa Azul, only two zones could be distinguished: a lower zone characterized by E. brasiliensis and D. hanleyanus, and a higher zone by E. braziliensis. When zonation patterns were tested using ANOSIM, only two zones were evident at both beaches; a zone located in the lower portion of the beach, encompassing intertidal migratory species, and a zone where those migratory species were excluded. These results are in agreement with Brown & McLachlan 166 Marine Ecology 27 (2006) 160–169 ª 2006 The Authors. Journal compilation ª 2006 Blackwell Publishing Ltd
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Community structure of macrobenthos in two tropical sandy ... - UFF

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