A Rocha Portugal Observatory Report 2009-2010

12 A Rocha Portugal Observatory Report 2009-2010
The sampling was carried out in January, March and May 2010. This was in order to examine the possible
variation in species and numbers of individuals over time during the period available for fieldwork.
Sampling took place during the low tide of spring tides (i.e. around a full or new moon), when the sea level
was at its lowest and the time for collecting samples was the longest.
Sampling method
Sediment cores were removed, using a square-bladed spade, from areas of extractable sediment close to
the main stream of water flowing at low tide. In order to take the same quantity of sample every time a
15×15cm quadrat was used to demarcate sample areas. The depth of the sample was 20cm. At each
sampling point 3 core samples were taken. Each core was sieved in situ using a mesh screen of 1 mm
aperture size to remove the fine sedimentary material and the remaining material taken for analysis.
Initially each sample was sorted for relatively large living specimens, which would then be released
immediately, if a positive identification could be made in the field. A number of physicochemical properties
were measured at the same time that samples were taken in the field. These were air and water
temperature, water salinity, pH and dissolved oxygen.
Air and water temperature was measured by aquarium thermometer. To measure water salinity a glass
hydrometer (made by Boots) was used. The concentration of dissolved oxygen was examined using a
LaMotte kit (code 5860) and pH was also measured in situ using a pocket-sized pH meter made by HANNA
instruments. The same instruments and methodology was used as that in the 2003 assessment of water
quality in the Alvor Estuary made by Boyes and Simonson (2004).
Laboratory analysis
The core samples were sorted as soon as possible after collection. Firstly the core sample would be spread
evenly within a photographic tray and all remaining living molluscs extracted, identified and, as soon as
possible, released. If necessary for identification purposes a specimen would be preserved in 70% alcohol.
After dealing with the living specimens, the remaining shells were identified. All shells larger than 1cm in
length were extracted from the sample, identified and counted. Subsampling was applied to specimens
smaller than 1cm in length.
Subsampling method
After dealing with specimens larger than 1cm in length, the core sample was divided into 12 equally sized
subsamples. Of these, 4 subsamples were randomly selected and sorted for good condition, i.e. intact
shells. The number of each species identified in this manner was extrapolated to gain a final figure for the
core sample. Intact empty shells from within the core samples were also identified and a complete list of
species, with an indication of abundance, can be found in Appendix 1.
Results
Water sampling
The results of water sampling within the Alvor Estuary are shown below. Figures 2, 3 and 4 display the
recorded physicochemical measurements in January, March and May 2010. Water sampling was unable to
be undertaken in January at the Abicada site due to adverse site conditions which would have made
sampling unsafe. In May the pH meter failed to work at the Abicada (6a) and Alvor (6b) sample sites.