A Rocha Portugal Observatory Report 2009-2010

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.