A Rocha Portugal Observatory Report 2009-2010

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16 A Rocha Portugal Observatory Report 2009-2010 Figure 4 Number of live specimens found at the Lower Estuary sampling site (5b). The sampling site at Abicada (6a) was very specific due to the wide diversity of substratum present. The majority of the sediment was composed of very fine clay creating an impermeable layer. Some big stones were also found. In these conditions it is likely that only a very few species of mollusc can survive due to the impermeability of the substrate. Colonies of Scrobicularia plana were recorded comprising mostly of large individuals, however, a number of smaller individuals were also found. High numbers of empty shells of this species also indicate that this is the main species in the area. Tapes decussatus was recorded at this sample site, located on the river bed. In total 25 live specimens were recorded from within only 4 species (Figure 9). Figure 5 Number of live specimens found at the Abicada sampling site (6a). The number of species recorded at the Alvor sampling site (6b) was significantly higher than at any of the other sampling sites in this study. This is likely to be due to the close proximity of the sea. A total of 309 live specimens from within 15 species were recorded from samples taken at this site. Hydrobia ulvae and Hydrobia neglecta were the most abundant species, with Cerithium vulgatum appearing quite frequently (Figure 10). During the May sampling period a large number of Cerithium vulgatum and Monodonta lineata
A Rocha Portugal Observatory Report 2009-2010 17 were observed around the sample areas (roughly 30 specimens of each species within a 0.5m radius of the sample area). Figure 6 Number of live specimens found at the Alvor sampling site (6b). All the sample sites in this study were located within the subtidal area and had either no, or very limited, submerged vegetation present. Aquatic vegetation within the Alvor estuary is in general very poor in diversity and extent. Discussion and Conclusion The species found alive in this study of the Alvor Estuary were also found by Almeida et al (2008) in the Ria Formosa which is located in the eastern Algarve, Portugal (Bivalvia: C. edule, O. edulis, S. plana, P. aurea, T. decussatus, and Gastropoda: C. chinensis, C. vulgatum, H. ulvae, N. pfeifferi, N. reticulatus, G. umbilicalis, J. striatus, M. lineata). Some of the species found during this study are included on lists of sensitive, tolerant and opportunistic species used by Simboura and Zenetos (2002) in their study. Most of them, like Tellina tenuis, Tapes decussatus, Spisula subtruncata, Cerithium vulgatum, Cyclope neritea, Paphia rhomboides are on the list of sensitive species; being described as “sensitive to disturbance in general”. Some species like Hydrobia acuta and Bittium reticulatum are also on the list of sensitive species but they are described as being tolerant species. A very large number of Bittium reticulatum shells were found in this survey, however no living specimens were found. Although Bittium reticulatum is an estuarine species it is mostly found on seagrass vegetation (Almeida et al, 2008; Fernandez et al, 1988), therefore, it is highly possible that most of the shells found in the samples were moved in by the tide. The Upper Estuary site (5a), where the abundance of Bittium reticulatum was almost always (with the exception of the March sample) greater than that of any other species, may be an exception due to the proximity of the sample site to a large vegetated area. Other than in this area, the aquatic vegetation within the Alvor Estuary is poor and this may explain the lack of several species of gastropods which are commonly associated with seaweed and other aquatic plants (Sousa, 2007).
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A Rocha Portugal Observatory Report 2009-2010

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