catastrophic geological event: the 1908 Messina earthquake. The alien species with this
unique invasion history is the benthic foraminifer Amphistegina lobifera Larsen, a
unicellular organism with a small-sized carbonatic test, which had entered the
Mediterranean Sea through the Suez Canal (Prazeres et al., 2020).
Materials and Methods
The research was carried out in Marsamxett Harbour (35°54’16.7”N; 14°30’27.5”E), a
natural bay on the western coast of Malta island where a well-established population of
the Indo-Pacific foraminifer Amphistegina lobifera was already known since 2006
(Yokeş et al., 2007; Guastella et al., 2019). This site was selected after multiple and
careful inspections, because it displays the required condition of sediment grain size,
shelter, depth and absence of human activities that could have altered sedimentation on
the sea-floor.
Two sediment cores were collected in May 2018 and September 2019 at 16 and 17 m
depth, respectively, using a hand-corer operated by scuba divers (Fig. 1A). As core
samples reflect the sedimentation pattern on the sea floor that has occurred in the past
decades, our cores were expected to record the colonization history of A. lobifera in the
area. After collection, both cores were vertically sectioned in two halves (Fig. 1B),
photographed, lithologically described and finally crosscut at each centimeter, thus
obtaining 41 sediment samples from the 2018 core (Fig. 1C) and 50 sediment samples
from the 2019 core (Fig. 1D).
Grain size analysis was performed using five overlapped sieves of 1 mm, 500, 250, 125,
and 63 μm mesh size. Sediment porosity was calculated from the loss of water between
wet and dry sediments, by weighing samples before and after drying sessions at 55 °C
and applying Berner (1971) equations.
The analysis of foraminiferal content was carried out on samples from one half of each
core, to assess occurrence and quantify absolute abundance of A. lobifera at each
centimeter of sediment. The other core half was utilized for radiometric analysis, in
order to identify an age model corresponding to the different depth levels of the cores.
For foraminiferal analyses, sediment samples were prepared as washed residues
following the standard procedures suggested by the FOBIMO protocol (Schönfeld et al.,
2012), which involves the following steps: oven-drying at 40 °C for one day, washing
over a sieve of 63 μm mesh size and then oven-drying at 40 °C for another day. Discrete
sample aliquotes obtained with a micro-splitter were then analysed at the dissecting
microscope. Specimens of A. lobifera were identified following Hottinger et al. (1993)
and counted as number of individuals per gram of dry sediment.
The most common technique to date recent sediments (up to 100-150 years) applies the
radiometric decay of
210
Pb isotope, a natural radio-nuclide (Incarbona et al., 2016) and
137
Cs, an artificial radio-nuclide introduced in atmosphere after the nuclear tests around
the world starting from the 1950s and is used as an independent tracer for validation of
the
210
Pb chronology (Smith, 2001).
Results
Foraminiferal analyses revealed a similar pattern of A. lobifera occurrence and
abundance along both cores. The upper portions of the cores, corresponding to recent
times, exhibit high abundances of the alien foraminifer. In the central part of both
records, the species is absent, but reappears in the lower portions, that correspond to
older times. This is more evident in the 2018 core where the species is present from cm
2nd Mediterranean Symposium on the Non-Indigenous Species (Genoa, Italy, 22-23 September 2022)
51
unique invasion history is the benthic foraminifer Amphistegina lobifera Larsen, a
unicellular organism with a small-sized carbonatic test, which had entered the
Mediterranean Sea through the Suez Canal (Prazeres et al., 2020).
Materials and Methods
The research was carried out in Marsamxett Harbour (35°54’16.7”N; 14°30’27.5”E), a
natural bay on the western coast of Malta island where a well-established population of
the Indo-Pacific foraminifer Amphistegina lobifera was already known since 2006
(Yokeş et al., 2007; Guastella et al., 2019). This site was selected after multiple and
careful inspections, because it displays the required condition of sediment grain size,
shelter, depth and absence of human activities that could have altered sedimentation on
the sea-floor.
Two sediment cores were collected in May 2018 and September 2019 at 16 and 17 m
depth, respectively, using a hand-corer operated by scuba divers (Fig. 1A). As core
samples reflect the sedimentation pattern on the sea floor that has occurred in the past
decades, our cores were expected to record the colonization history of A. lobifera in the
area. After collection, both cores were vertically sectioned in two halves (Fig. 1B),
photographed, lithologically described and finally crosscut at each centimeter, thus
obtaining 41 sediment samples from the 2018 core (Fig. 1C) and 50 sediment samples
from the 2019 core (Fig. 1D).
Grain size analysis was performed using five overlapped sieves of 1 mm, 500, 250, 125,
and 63 μm mesh size. Sediment porosity was calculated from the loss of water between
wet and dry sediments, by weighing samples before and after drying sessions at 55 °C
and applying Berner (1971) equations.
The analysis of foraminiferal content was carried out on samples from one half of each
core, to assess occurrence and quantify absolute abundance of A. lobifera at each
centimeter of sediment. The other core half was utilized for radiometric analysis, in
order to identify an age model corresponding to the different depth levels of the cores.
For foraminiferal analyses, sediment samples were prepared as washed residues
following the standard procedures suggested by the FOBIMO protocol (Schönfeld et al.,
2012), which involves the following steps: oven-drying at 40 °C for one day, washing
over a sieve of 63 μm mesh size and then oven-drying at 40 °C for another day. Discrete
sample aliquotes obtained with a micro-splitter were then analysed at the dissecting
microscope. Specimens of A. lobifera were identified following Hottinger et al. (1993)
and counted as number of individuals per gram of dry sediment.
The most common technique to date recent sediments (up to 100-150 years) applies the
radiometric decay of
210
Pb isotope, a natural radio-nuclide (Incarbona et al., 2016) and
137
Cs, an artificial radio-nuclide introduced in atmosphere after the nuclear tests around
the world starting from the 1950s and is used as an independent tracer for validation of
the
210
Pb chronology (Smith, 2001).
Results
Foraminiferal analyses revealed a similar pattern of A. lobifera occurrence and
abundance along both cores. The upper portions of the cores, corresponding to recent
times, exhibit high abundances of the alien foraminifer. In the central part of both
records, the species is absent, but reappears in the lower portions, that correspond to
older times. This is more evident in the 2018 core where the species is present from cm
2nd Mediterranean Symposium on the Non-Indigenous Species (Genoa, Italy, 22-23 September 2022)
51