A Rocha Portugal Observatory Report 2009-2010

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18 A Rocha Portugal Observatory Report 2009-2010 The most numerous species identified in this study was Hydrobia ulvae, known to be one of the most common inhabitants of European estuaries (McLusky, 1989). This species is significantly smaller in size than, for example, Cerastoderma edule and therefore we might expect a greater number of individuals per mᶾ. In this case, unlike B. reticulatum, many living specimens were recorded. H. ulvae is a species normally associated with higher and more fluctuating salinity levels (McLusky, 1989). This study also indicated that the highest number of individuals of this species was found in locations with the highest salinity: the Western Marsh and the Lower Estuary sample sites. The most important physicochemical properties affecting the occurrence and distribution of molluscs in estuaries are salinity (Ysebaert et al, 2003; Holland et al, 1987; Hampel et al, 2009), type of sediment (Sousa et al, 2006; Ysebaert et al, 2003) and the level of dissolved oxygen (Sousa et al, 2007). Salinity readings varied at each of the sample sites over the course of the study, however the salinity was consistently higher at the Alvor and Western Marsh sampling sites (Figures 2, 3 and 4). The Alvor sampling site (6b) is relatively close to the river mouth, and far from the main stream of the Alvor River, which explains the higher salinity readings. In the Western Marsh the relatively high salinity measurements can largely be attributed to water evaporation. Seawater enters the Western Marsh at high tide and, as it receives no freshwater input from the river or from rain in the summer months, the salinity is likely to increase due to evaporation during the summer period. The highest diversity of species was recorded in the Lower Estuary (13 species) and Alvor (15 species) sample sites. This may be due to the proximity of the sites to the river mouth as the higher salinity levels and generally greater stability of hydrological factors, (largely salinity) is conducive to higher levels of biodiversity. The mouth of the estuary is also richer in organic matter compared to the Upper Estuary, resulting in a greater quantity of potential food particularly for deposit-feeding species (Sousa et al, 2006; Ysebaert, et al, 2003). Loripes lacteus, which was found in high numbers in the Western Marsh, has been characterised as a tolerant species and is also known to be an indicator of instability and/or pollution. The high abundance of this species suggests that it may be a key species within the Western Marsh, providing an important food resource to other trophic levels at this site, for example wading birds. Scrobicularia plana is another species which could be considered to be a key species, and was recorded in high abundance at the Abicada sampling site. However, it was less abundant within samples than L. lacteus, but this may be related to size as most specimens observed were between 4 and 5 cm in length. Galván (2010) in her research of Cambrian estuaries found that Scrobicularia plana was the most abundant mollusc. In addition, in research undertaken by Sousa et al (2006), S. plana together with Cardium edule were the most abundant species in areas of shallow water. Interestingly, in their study the sediment recorded at the sampling station was coarse, while at Abicada it was found to be a very fine sediment together with relatively big stones. Although the data collected are not sufficient to estimate the ecological quality of the Alvor Estuary, the presence of sensitive species suggests that the ecosystem is at least partly in good health. Following on from this project, studies could be undertaken using the new biotic index (BENTIX) to establish ecological quality within the estuary.
A Rocha Portugal Observatory Report 2009-2010 19 Further monitoring of molluscs within the Alvor Estuary could be undertaken, with additional sampling points within the estuary and further upstream in the Odiáxere River. Another interesting issue would be to investigate the status of Loripes lacteus in the Western Marsh. References: Adam P. (1990). Saltmarsh ecology. Cambridge University Press. Al-Dabbas, M.A.M., Hubbard, H. and McManus, J. (1984). ‘The shell of Mytilus as an indicator of zonal variations of water quality within an estuary’. Estuarine, Coastal and Shelf Science 18: 265-270. Almeida, C., Coelho, R., Silva, M., Bentes, L., Monteiro, P., Ribeiro, J., Erzini, K. and Gonçalves, J.M.S. (2008). ‘Use of different intertidal habitats by faunal communities in a temperate coastal lagoon’. Estuarine, Coastal and Shelf Science 80: 357–364. Barnes R.S.K. (1984). Estuarine Biology, Edward Arnold. Boyes, C. and Simonson, W. (2004). A preliminary assessment of water quality In the Alvor Estuary. A Rocha Portugal Observatory Report for 2003. Associação A Rocha, Portimão, Portugal. Carpenter, B. (2005). Towards better characterisation of the macro-invertebrate fauna of the Alvor Estuary, A Rocha Portugal Observatory Report for 2004. Associação A Rocha, Portimão, Portugal. Constanza, R., Kemp, M. and Boyton, W. (1997). ‘Scale and biodiversity in coastal and estuarine ecosystems’. In C. Perrings, K.G. Mäler, C. Folke and C. S. Holling (eds.) Biodiversity loss: economic and ecological issues. Cambridge University Press. Fernàndez, E., Anadòn, R. and Fernàndez, C. (1988). ‘Life histories and growth of the gastropods Bittium reticulatum and Barleeia unifasciata inhabiting the seaweed Gelidium latifolium’. J. Moll. Stud. 54: 119-129 Galván C., Juanes J.A., Puente A. (2010). Ecological classification of European transitional waters in the North-East Atlantic eco-region, Estuarine, Coastal and Shelf Science 87 p. 442-450 Hampel, H., Elliott, M. and Cattrijsse, A. (2009). ‘Macrofaunal communities in the habitats of intertidal marshes along the salinity gradient of the Schelde Estuary’. Estuarine, Coastal and Shelf Science 84: 45–53. Holland, A. F., Shaughnessy, A. T. and Hiegel, M. H. (1987). ‘Long-term variation in mesohaline Chesapeake Bay macrobenthos: Spatial and temporal patterns’. Estuaries 10: 227-245. McLusky, D.S. (1989). The estuarine ecosystem. Second Edition. Blackie, Glasgow. Nakamura Y., Kerciku F. (2000). ‘Effects of filter-feeding bivalves on the distribution of water quality and nutrient cycling in a eutrophic coastal lagoon’. Journal of Marine Systems 26 (2): 209-221.
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A Rocha Portugal Observatory Report 2009-2010

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