A Rocha Portugal Observatory Report 2009-2010

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.