Diversity and Vertical Migration of Euphausiids (Straits of Messina)
133
surface and the first 30 m of water, while the
same underflow (30 m to the bottom) northward.
However, the Tyrrhenian and Ionian Seas also
exhibit oscillatory differences caused by the tidal
currents, in fact, when there is low tide in the
Tyrrhenian Sea at its southern border with the
Straits, the Ionian Sea is in the phase of high tide
at its northern border. This means that Ionian
waters flow north, through the Straits, into
Tyrrhenian waters forming the so-called "montante" current. While, when there is high tide in
the Tyrrhenian and low tide in Ionian Sea, at
their respective bounds with the Straits, the
"scendente" current flows South. During the
"scendente" current phase, the lighter
Tyrrhenian waters flow on the top of the heavier
Ionian ones until they fill completely the entire
central part of the Straits, while in the "montante" current phase the Ionian waters sink into
lighter Tyrrhenian ones, stratifing from 200 m
down to the bottom (Magazzu and Andreoli
1971). These movements of waters occur with
considerable velocity and turbulence leading to
various phenomena that involve the water masses both horizontally ("cuts" and "sea steps") and
vertically ("eddies" and "oil spots").
Furthermore, the movements of the stationary and tidal currents may cause, sometimes. the
meeting of water masses which, on account of
their respective physico-chemical characteristics, can mix with each other leading to the formation of "mixed" waters (Guglielmo et al
1995b). The study of these complex hydrodynamic processes are undoubtedly problematical,
but the resolution of these could sometimes be
ascribed to the use of biological indicators such
as many euphausiid species (i.e. Stylocheiron
suhmi as an indicator of Oriental waters or
Thysanoessa gregaria as an indicator of Atlantic
ones, Casanova 1968, 1974) to know the features
of a certain type of water.
Sampling Procedure
Samples used for this research were carried out
during the oceanographic cruises POP -
EOCUMM '95, from 15 to 30 July 1995 by N/O
Italica. Samples were collected at 8 stations in
Ionian and in Tyrrhenian Seas, across the Straits
of Messina (Fig. 1a), by the BIONESS (Sameoto et
al. 1980) electronic multinet (1 m 2 of mouth
width and 12 nets of 500 Jlm mesh size). Each station has been sampled at regular intervals of six
hours (6.00 h, 12.00 h, 18.00 h, 24.00 h).
Simultaneously temperature, salinity, fluorescence and depth were measured by eTn and fluorometer sensors. BIONESS was hauled down, at
reduced speed, to programmed maximum depth
and then trawled up obliquely at a speed of 1.5-2
ms- I . Nets were opened and closed by operator
command at 50-100 m in profiles of 1000 m and
200-500 m between 1000 m down to maximum
depth (2030 m) recorded profiles. Each net has
filtered from 38 to 4225 m 3 of sea water.
Sample Analysis
On board samples were preserved in 5% buffered
formaldehyde and sea water solution. In the laboratory euphausiids were sorted and identified
to species level and divided into adults and juveniles according to Casanova (1968, 1974) and
Mauchline and Fisher (1969). Abundances were
combined values with respect to the total of sampled strata calculated from total specimens
counted and total filtered volume at each station
(or at each layer for vertical distribution patterns) as individuals per 10- 3 m- 3 • Similarity (S)
and Shannon and Weaver diversity (H,) indices
were calculated for specimens of both areas,
moreover the cluster analysis by Euclidean distances method (with single linkage) for species
and principal component analysis for stations
were made. Finally, the weighted mean depth
(WMD) of each species was determined according to Andersen and Sardou (1992).
Results
BIONESS Temperature-Salinity-Fluorescence
Profiles
As shown in Fig. 2a, in Ionian Sea, temperature
and salinity values of surface water masses were
about 23-24°C and 37.7 psu respectively. while,
under the pycnocline (about 15-20 m) down to
the bottom, they were colder and saltier (average
13.5°e and 38.5 psu). At stations J3 and J4 there
was not a clear pycnocline due to the presence of
mixed waters in surface layers.
In South Tyrrhenian Sea, different, temperature and salinity values of about 27.5 °C and
37.8 psu were found dose to the surface layers
133
surface and the first 30 m of water, while the
same underflow (30 m to the bottom) northward.
However, the Tyrrhenian and Ionian Seas also
exhibit oscillatory differences caused by the tidal
currents, in fact, when there is low tide in the
Tyrrhenian Sea at its southern border with the
Straits, the Ionian Sea is in the phase of high tide
at its northern border. This means that Ionian
waters flow north, through the Straits, into
Tyrrhenian waters forming the so-called "montante" current. While, when there is high tide in
the Tyrrhenian and low tide in Ionian Sea, at
their respective bounds with the Straits, the
"scendente" current flows South. During the
"scendente" current phase, the lighter
Tyrrhenian waters flow on the top of the heavier
Ionian ones until they fill completely the entire
central part of the Straits, while in the "montante" current phase the Ionian waters sink into
lighter Tyrrhenian ones, stratifing from 200 m
down to the bottom (Magazzu and Andreoli
1971). These movements of waters occur with
considerable velocity and turbulence leading to
various phenomena that involve the water masses both horizontally ("cuts" and "sea steps") and
vertically ("eddies" and "oil spots").
Furthermore, the movements of the stationary and tidal currents may cause, sometimes. the
meeting of water masses which, on account of
their respective physico-chemical characteristics, can mix with each other leading to the formation of "mixed" waters (Guglielmo et al
1995b). The study of these complex hydrodynamic processes are undoubtedly problematical,
but the resolution of these could sometimes be
ascribed to the use of biological indicators such
as many euphausiid species (i.e. Stylocheiron
suhmi as an indicator of Oriental waters or
Thysanoessa gregaria as an indicator of Atlantic
ones, Casanova 1968, 1974) to know the features
of a certain type of water.
Sampling Procedure
Samples used for this research were carried out
during the oceanographic cruises POP -
EOCUMM '95, from 15 to 30 July 1995 by N/O
Italica. Samples were collected at 8 stations in
Ionian and in Tyrrhenian Seas, across the Straits
of Messina (Fig. 1a), by the BIONESS (Sameoto et
al. 1980) electronic multinet (1 m 2 of mouth
width and 12 nets of 500 Jlm mesh size). Each station has been sampled at regular intervals of six
hours (6.00 h, 12.00 h, 18.00 h, 24.00 h).
Simultaneously temperature, salinity, fluorescence and depth were measured by eTn and fluorometer sensors. BIONESS was hauled down, at
reduced speed, to programmed maximum depth
and then trawled up obliquely at a speed of 1.5-2
ms- I . Nets were opened and closed by operator
command at 50-100 m in profiles of 1000 m and
200-500 m between 1000 m down to maximum
depth (2030 m) recorded profiles. Each net has
filtered from 38 to 4225 m 3 of sea water.
Sample Analysis
On board samples were preserved in 5% buffered
formaldehyde and sea water solution. In the laboratory euphausiids were sorted and identified
to species level and divided into adults and juveniles according to Casanova (1968, 1974) and
Mauchline and Fisher (1969). Abundances were
combined values with respect to the total of sampled strata calculated from total specimens
counted and total filtered volume at each station
(or at each layer for vertical distribution patterns) as individuals per 10- 3 m- 3 • Similarity (S)
and Shannon and Weaver diversity (H,) indices
were calculated for specimens of both areas,
moreover the cluster analysis by Euclidean distances method (with single linkage) for species
and principal component analysis for stations
were made. Finally, the weighted mean depth
(WMD) of each species was determined according to Andersen and Sardou (1992).
Results
BIONESS Temperature-Salinity-Fluorescence
Profiles
As shown in Fig. 2a, in Ionian Sea, temperature
and salinity values of surface water masses were
about 23-24°C and 37.7 psu respectively. while,
under the pycnocline (about 15-20 m) down to
the bottom, they were colder and saltier (average
13.5°e and 38.5 psu). At stations J3 and J4 there
was not a clear pycnocline due to the presence of
mixed waters in surface layers.
In South Tyrrhenian Sea, different, temperature and salinity values of about 27.5 °C and
37.8 psu were found dose to the surface layers