Mesozooplankton in a Mediterranean Coastal Sound (Stagnone di Marsala, Western Sicily)
167
and bottom organisms are re-suspended.
Secondly, the presence of these demersal groups
in our samples could be due to specific daily
migratory behaviour (Alldredge and King 1980).
In all the 24-h surveys, mysids (mainly Diamysis
bahirensis, Siriella armata, S. clausii and
Mysidopsis gibbosa) showed one or two clear
abundance peaks during the dark period. as
found in other shallow-water environments
(Genovese 1963). This phenomenon could be
related to the vertical migration performed by
these organisms (Mauchline 1980).
Isolated abundance peaks of decapod larvae
were observed during the dark hours in all the
24-h cycles, confirming reports by Ferrari et al.
(1985a) for a lagoon of the Po River Delta. The
diel abundance pattern of these organisms
appears to be due to endogenous circatidal
rhythms in their behaviour (Cronin and Forward
1979).
The copepod community was characterised
by low diversity and the marked dominance of
one or two species (Acartia margaleJi and Isias
clavi pes), as observed by numerous authors for
brackish-water environments of temperate climates (Alcaraz 1983; Colombo et al. 1983-84;
Ferrari et al. 1985b; Belmonte and Bianchi 1992;
Zagami and Guglielmo 1995). Other coastal-neritic species (mainly Oithona nana, A. clausi, A.
josephinae, and Labidocera brunescens) entered
the basin but showed very low abundance and
frequency of occurrence, suggesting that they
were unable to stay and reproduce in the area.
This confirms that a certain degree of adaptability to the frequent temporal variations in the
hydrological parameters is required to establish
a population in this environment; this is the case
for A. margaleJi, an «rn strategist species (Alcaraz
1983; Ferrari and Carried 1985; Zagami and
Guglielmo 1995) which is able to enter the inner
basin and spend its entire life cycle there
(Campolmi 1998). The temporal variations in
physical-chemical parameters have a greater
importance than their range of values in determining the spatial distribution of species; the
species that are more "r" strategist occupy the
less stable areas (Alcaraz 1983).
The seasonal succession of species is different from that reported for coastal waters of the
Mediterranean Sea (Scotto di Carlo 1985). In the
Stagnone mouth area, two species that usually
characterise the Mediterranean neritic community, A. clausi and Paracalanus parvus. show very
low abundances throughout the year. This is
probably due to the location of the area between
the coastal waters and the inner basin. The structure of the mesozooplankton assemblage of this
area is intermediate between the two adjacent
environments (Campolmi 1998). following a gradient of abundance and species diversity which
overlaps with that of the hydrological features, as
also reported in the Sacca di Scardovari (Ferrari
and Colombo 1988). This transition environment, although influenced by the nearby sea
water, shows high instability and significant diel
and seasonal fluctuations in physical-chemical
parameters.
The finding of rare hyperbenthic calanoid
species (Pseudocyclops spp., Stephos spp.,
Metacalanus acutioperculum and Ridgewayia
marki minorcaensis) is noteworthy, and their
greatest occurrence at night confirms the
hypothesis of demersal behaviour. As reported
for other shallow-water environments. these
species probably migrate at various times
between the bottom and the water column
(Damkaer 1970; Alldredge and King 1985;
Ohtsuka 1985; Jacoby and Greenwood 1988).
The variability of the zooplankton structure
throughout the 24-h period seems to be affected
by hydrodynamic factors (tidal and wind-driven
currents). The diel tidal range. although low in
comparison with other Mediterranean shallowwater environments (Ferrari et al. 1985a, b),
seems to affect the currents; they show a dear
periodicity of about six hours, especially when
the winds are weak or absent. Incoming {N IE}
and outgoing (S/W) currents alternate through
the sea mouth during the 24-h period, which
contribute to the water-mass turnover and
strengthen the effects of wind action. These
phenomena contribute to the transportation of
coastal-neritic species into the basin, but are not
sufficient to explain the large diel fluctuations in
zooplankton abundance. Independently of the
tidal phase and the presence of outgoing currents (which transport the zooplankton organisms out of the basin), high peaks in abundance
were observed at night in all the time series.
These were due mainly to an increase in coastalneritic species and to a lesser extent autochthonous forms. Thus it can be hypothesised that
these increases in abundance are due to a specific daily migratory behaviour, which seems to
be related principally to the light and dark periods. These species may stay near the bottom
167
and bottom organisms are re-suspended.
Secondly, the presence of these demersal groups
in our samples could be due to specific daily
migratory behaviour (Alldredge and King 1980).
In all the 24-h surveys, mysids (mainly Diamysis
bahirensis, Siriella armata, S. clausii and
Mysidopsis gibbosa) showed one or two clear
abundance peaks during the dark period. as
found in other shallow-water environments
(Genovese 1963). This phenomenon could be
related to the vertical migration performed by
these organisms (Mauchline 1980).
Isolated abundance peaks of decapod larvae
were observed during the dark hours in all the
24-h cycles, confirming reports by Ferrari et al.
(1985a) for a lagoon of the Po River Delta. The
diel abundance pattern of these organisms
appears to be due to endogenous circatidal
rhythms in their behaviour (Cronin and Forward
1979).
The copepod community was characterised
by low diversity and the marked dominance of
one or two species (Acartia margaleJi and Isias
clavi pes), as observed by numerous authors for
brackish-water environments of temperate climates (Alcaraz 1983; Colombo et al. 1983-84;
Ferrari et al. 1985b; Belmonte and Bianchi 1992;
Zagami and Guglielmo 1995). Other coastal-neritic species (mainly Oithona nana, A. clausi, A.
josephinae, and Labidocera brunescens) entered
the basin but showed very low abundance and
frequency of occurrence, suggesting that they
were unable to stay and reproduce in the area.
This confirms that a certain degree of adaptability to the frequent temporal variations in the
hydrological parameters is required to establish
a population in this environment; this is the case
for A. margaleJi, an «rn strategist species (Alcaraz
1983; Ferrari and Carried 1985; Zagami and
Guglielmo 1995) which is able to enter the inner
basin and spend its entire life cycle there
(Campolmi 1998). The temporal variations in
physical-chemical parameters have a greater
importance than their range of values in determining the spatial distribution of species; the
species that are more "r" strategist occupy the
less stable areas (Alcaraz 1983).
The seasonal succession of species is different from that reported for coastal waters of the
Mediterranean Sea (Scotto di Carlo 1985). In the
Stagnone mouth area, two species that usually
characterise the Mediterranean neritic community, A. clausi and Paracalanus parvus. show very
low abundances throughout the year. This is
probably due to the location of the area between
the coastal waters and the inner basin. The structure of the mesozooplankton assemblage of this
area is intermediate between the two adjacent
environments (Campolmi 1998). following a gradient of abundance and species diversity which
overlaps with that of the hydrological features, as
also reported in the Sacca di Scardovari (Ferrari
and Colombo 1988). This transition environment, although influenced by the nearby sea
water, shows high instability and significant diel
and seasonal fluctuations in physical-chemical
parameters.
The finding of rare hyperbenthic calanoid
species (Pseudocyclops spp., Stephos spp.,
Metacalanus acutioperculum and Ridgewayia
marki minorcaensis) is noteworthy, and their
greatest occurrence at night confirms the
hypothesis of demersal behaviour. As reported
for other shallow-water environments. these
species probably migrate at various times
between the bottom and the water column
(Damkaer 1970; Alldredge and King 1985;
Ohtsuka 1985; Jacoby and Greenwood 1988).
The variability of the zooplankton structure
throughout the 24-h period seems to be affected
by hydrodynamic factors (tidal and wind-driven
currents). The diel tidal range. although low in
comparison with other Mediterranean shallowwater environments (Ferrari et al. 1985a, b),
seems to affect the currents; they show a dear
periodicity of about six hours, especially when
the winds are weak or absent. Incoming {N IE}
and outgoing (S/W) currents alternate through
the sea mouth during the 24-h period, which
contribute to the water-mass turnover and
strengthen the effects of wind action. These
phenomena contribute to the transportation of
coastal-neritic species into the basin, but are not
sufficient to explain the large diel fluctuations in
zooplankton abundance. Independently of the
tidal phase and the presence of outgoing currents (which transport the zooplankton organisms out of the basin), high peaks in abundance
were observed at night in all the time series.
These were due mainly to an increase in coastalneritic species and to a lesser extent autochthonous forms. Thus it can be hypothesised that
these increases in abundance are due to a specific daily migratory behaviour, which seems to
be related principally to the light and dark periods. These species may stay near the bottom