A Rocha Portugal Observatory Report 2009-2010

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10 A Rocha Portugal Observatory Report 2009-2010 Introduction MONITORING OF MOLLUSCS IN THE ALVOR ESTUARY, ALGARVE, PORTUGAL Aleksandra Cisek and Esther Pawley Estuaries are very important ecosystems. They are classified as ‘transitional waters’ under the Water Framework Directive (Directive 2000/60/EC), where, in Article 2 they are defined as: ” bodies of surface water in the vicinity of river mouths which are partly saline in character as a result of their proximity to coastal waters but which are substantially influenced by freshwater flows”. As ecosystems existing on the boundary between sea and river, they experience big fluctuations of physicochemical factors, mainly due to the tidal cycle. Estuarine conditions are very specific and only some organisms are able to withstand such stresses as rapid and substantial changes in salinity levels, dissolved oxygen concentrations, turbidity and the presence or absence of water (McLusky, 1989; Ysebaert et al, 2003). For these reasons the biological diversity within estuaries is quite limited. It has been reported by many authors, for example, Constanza, 1997, Barnes, 1984, McLusky, 1989, that while the number of different species living in estuaries is quite low, the abundance of species is very high. Due to relatively low inter-species competition those species which have managed to adapt to the difficult conditions can occur in very high numbers. Estuarine organisms are also very often restricted to particular places largely defined by salinity gradient and sediment composition (Ysebaert et al, 2003; Sousa et al, 2006). In spite of these difficult living conditions, estuaries have been claimed to be amongst the most productive natural habitats in the world (McLusky, 1989). For this reason estuaries are very attractive areas for secondary consumers, especially for large groups of wading birds, which are higher up the food chain. Salt marsh habitats also belong to this group of highly productive ecosystems (e.g. Adam, 1990; Vinagre et al, 2008). This habitat is strongly associated with estuaries. It also attracts many species of secondary consumers (Hampel et al, 2009) and often provides a safe place for nesting waders (Adam, 1990). Molluscs are one of the largest and most abundant groups of estuarine animals (Sousa et al, 2007; Ysebaert et al, 2003). In a previous study of the benthic fauna of the Alvor estuary, Portugal it was found to be the dominant group (Carpenter 2004). Many estuarine molluscs can feed on a range of food sources and are tolerant of low salinities, providing success for this group in the colonization of estuaries (McLusky, 1989). As one of the most numerous groups of animals, the impact they have within estuarine and marsh ecosystems is not only as a food source for higher trophic levels; by way of feeding they exert a control on the biomass of phytoplankton and stimulate primary production by excreting nutrients (Nakamura & Kerciku, 2000). In many cases a few species of molluscs, together with (Arthropoda and Crustacea), are considered to be the key species within estuarine or salt marsh ecosystems because of their high abundance (e.g. Sousa et al, 2007). Many mollusc species are used as indicators of the ecological status of transitional waters (Al-Dabbas, 1984). Simboura and Zentos (2002), for example, used benthic organisms as indicators of the ecological quality using their new biotic index (BENTIX). The aim of this study was to analyse the spatial variability of molluscs within the Alvor Estuary, as they comprise a significant part of benthic macro-invertebrates.
Location of the study A Rocha Portugal Observatory Report 2009-2010 11 The Alvor Estuary is one of the most important wetlands for wildlife in the Algarve and covers an area of 1,454 ha. In September 2006 it was established as a Natura 2000 site and is one of 14 Natura 2000 sites in the Algarve. Ten years before this designation, in May 1996, it was accredited as a wetland of international importance, according to the Convention of Wetlands (Ramsar, 1971). It is one of just three wetlands in the Algarve on ‘The Ramsar List of Wetlands of International Importance’ (Ramsar (Iran). (1971) Due to its significance and proximity to the Cruzinha centre, it is the focus of many studies by A Rocha. It is important to monitor the ecological health of this area due to its high conservation value. Methodology Sampling scheme Five sampling sites were chosen as being representative of the main habitats within the Alvor Estuary. A preliminary site visit was also undertaken to determine accessibility and safety considerations of sampling points. The technique employed in a previous invertebrate study at this site (Carpenter, 2004) was examined, but not used in this study due to various impracticalities such as the sample not being removed effectively. Sampling at the same points as Carpenter (2004) was undertaken where possible. Some previous survey sites, such as within the Eastern Marsh and salinas areas of the Alvor Estuary, are no longer available for study or public access. Figure 1 Location of sampling points within the Alvor Estuary. The Western Marsh sample site (4a) was located at the most south-western part of the marsh where the sea wall turns north. The Upper Estuary site (5a) was on the muddy area, on the opposite side of the river to the fish farm. The Lower Estuary sample site (5b) was in the area where the Alvor and Odiáxere rivers join. The Abicada site (6a) was close to the upper end of the Eastern Marsh, located near to a house with a jetty. The Alvor sample site (6b) was near to the river mouth.
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