Biological characterization of study sites
Methods used in the field in Tunisia
The arthropod population of Kneiss Islands (Bessila island) and of the beach dune system of Zouaraa was investigated to estimate biodiversity. To this aim samples were taken to obtain information both from a quantitative and qualitative point of view.
- At Kneiss Island a standard trapping system using pitfall traps along transects was used to capture animals spontaneously active at the sand surface. Two (transect A, B) out of four transects were placed on the coast of the island looking towards the open sea. Other two transects (transect C, D) were situated inland, on the side of the island looking towards the continent. These were reached by water only by the high waters of spring tides. In the first two transects, from sea landwards, traps were placed every two meters up to 20 m using a continuous Fiberglas strip and every 20 m up to 120 m using cross traps. The inland transects each consisted of four tetra-directional cross traps at a distance of 20 m from one another. Traps were active for 72 consecutive hours and were controlled periodically on account of the alternating high-low tides (tidal excursions in this locality reach 2.5m). At Zouaraa standard samples using pitfall traps were taken along two transects from sea towards land. Traps were placed every five meters up to 40 m and from this point every two metres until the shoreline using a continuous Fiberglas strip proceeding from the base of the dune towards sea. By contrast tetra-directional cross traps were placed at 20m, 50m, 90m, 140m from the base of the dune towards land. Traps were active for 72 consecutive hours.
- To sample arthropods associated to the halophyte vegetation cover hand nets were used. A sample of fauna was taken from the main assemblages of plants in correspondence to each pitfall trap.
- To evaluate the macrofauna/plant biomass ratio in different zones of the dune mobile cages of unitary units (1 m 3) were used. Samples were made in correspondence to the cross traps. The plants present in 1 m 3 of vegetation were taken and fresh weights calculated. All species found in 1 m3 were caught and preserved in alcohol for successive laboratory determination.
- During the entire period of study in both localities parameters were registered (temperature, relative humidity, atmospheric pressure, rainfall, wind speed and direction) with a CWMS (Compact Weather Monitoring Station) and with a BABUC-ABC (data logger for sun radiation, sand temperature and humidity).
- At Kneiss Island with a GPS the positions of the four transects were determined (transect A: N 34° 22' 10", E 10° 19' 9", transect B: N 34° 22' 3", E 10° 19' 11" transect C: N 34° 22' 4", E 10° 18' 45", transect D: N 34° 22' 17", E 10° 18' 55") together with that of the climatic stations (N 34° 22' 8", E 10° 19' 8") and the place where the orientation tests were conducted (N 34° 22' 6", E 10° 19' 10"). At Zouaraa the positions of the two transects were: transect A: N 37° 0' 46", E 8° 53' 25", transect B: N 37° 0' 43", E 8° 53' 18"; that of the climatic stations: N 37° 0' 41", E 8° 53' 26" and where the orientation tests were conducted: N 37° 0' 44", E 8° 53' 23".
- During the last days of field work other transects with pitfall traps were set to capture fresh individuals of key species to be brought back to the laboratory of Florence.
- At Zouaraa a beach profile was plotted with data obtained in correspondence to transect B. Sand samples were taken to evaluate grain size, salinities and organic components.
On Kneiss island bio-essays were carried out only for amphipods of the genus Orchestia: O. stephenseni, O. montagui and O. gammarella. For an eco-audit at Zouaraa six key species were chosen. The choice was based on a faunal gradient from sea towards land. Two sympatric amphipod species, Talitrus saltator and Talorchestia brito were chosen together with the isopod Tylos europaeus . Of the beetles present on the beach the tenebrionid Phaleria acuminata and two carabids Eurynebria complanata and Scarites laevigatus were selected.
- Orientation tests with and without landscape vision were conducted in three different hours of the day (09:00, 12:00, 15:00) for each key species. Only for the amphipods and isopods six groups of ten individuals were released in the arena. At the end of each release the position of each individual was registered. Amphipods were stored singularly in Eppendorfs and then sexed and age-classed in the laboratory of Florence. Beetles were instead tested individually. Experiments were conducted every other day of the week for three times so as to test different climatic conditions.
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03.10.2024