Distribution and Ecology of Mesozooplankton (Northern and Central Adriatic Sea)
183
coast (Fig. la,b). At the sites with a clear thermocline, two vertical samplings were carried out,
the first from the bottom up to the surface, the
other one from the thermocline up to the surface. On board, the samples were preserved in a
4% buffered formaldehyde and sea water solution.
Analysis of the Samples
In the laboratory, a qualitative-quantitative
analysis of the mesozooplankton was performed
on subsamples from 1/10 to 1/20, depending on
the total sample richness, while the whole samples were observed in their entirety for the identification of rare species. The specimens were
sorted and divided per taxon. The copepods
(adults) and cladocerans were identified at the
species level, while the remaining groups were
classified at higher taxonomic levels. Biomass
values (as dry weight) were estimated from 500
m1 of the whole sample, according to Lovegrove's
method (1966). Abundance was expressed as
indlm 3 and biomass as mg/m 3 • These parameters
were calculated according to the bottom depth
along onshore-offshore sections; the groups of
sites were re-named as no. 1 to no. 6.
Data Processing
The data were log-transformed to avoid the
excessive dominance of more abundant species
in the analysis. The Bray Curtis dissimilarity
coefficient was used to construct the betweensites distance matrix (Bakus 1990); using the
furthest neighbour amalgamation rule, we
obtained a classification of sites shown by dendrograms.
Results
BIONESS: Temperature-Salinity-Fluorescence
Isolines
In June, temperature and salinity values (recorded by the BIONESS) revealed a strong vertical
thermohaline water stratification, which determined a frontal system separating neritic ecosystem water masses (close to 40 m depth) with a
pycnocline from 10-15 m depth (Fig. 2a,b).
During this period, two sub-systems were identified in the neritic system and defined as coastal
and offshore zones. The former included the
warmer and less salty water masses from the
coast to 20 m depth, while, in the layer over the
thermocline, it spread outward until 40 m depth.
The latter included the water masses under the
pycnocline spreading from 20 m depth to the
whole neritic system. Mean values of temperature and salinity of the water masses over and
under the thermocline were 20.-l3°C and 35-37.4
PSU, respectively.
The spatial distribution of fluorescence
exhibited a decreasing inshore-offshore trend.
ranging from 0.45-1.2 mg/m3 chla over the pycnocline. Another maximum (0.75-1.3 mg/m3 chla)
was found close to the bottom at the offshore
sites (Fig. 2c). During winter, a persistent unstable frontal system was caused by large fluctuations of meteo-marine conditions and river
flows. The neritic system was characterized by a
minor extension of the less salty water of the
coastal zone and by a shift of the halocline up to
the surface (Fig. 2d, e). The spatial distribution of
mean temperature and salinity ranged from
9.38-11.64°C and from 35.46-37.85 PSU, respectively. During this period, the fluorescence values
were lower (0.1-0.45 mg/m3 chla), with decreasing inshore-offshore and surface-bottom trends
(Fig. 2f). Similar patterns, but with slightly different ranges, were recorded during AugustSeptember and March in the central Adriatic Sea.
Zooplankton Abundance and Spatial
Distribution
Late Spring-Summer '96
The zooplankton community showed wide
quantitative oscillations, both of abundance
(501-25292 ind/m 3 ) and biomass (3.80 - 173.25
mglm3); the means ± SD were 3759 ± 3767
indlm 3 and 27.06 ± 20 mglm 3 , respectively. The
spatial distribution of abundance exhibited
strongly decreasing inshore-offshore (Fig. 3) and
surface-bottom (Fig.4a.c) trends, mainly determined by cladocerans and especially by Penilia
avirostris. The copepods, instead, showed higher
values in the layer under the pycnocline.
The seasonal succession of zooplankton was
characterized by a temporal inversion of the
183
coast (Fig. la,b). At the sites with a clear thermocline, two vertical samplings were carried out,
the first from the bottom up to the surface, the
other one from the thermocline up to the surface. On board, the samples were preserved in a
4% buffered formaldehyde and sea water solution.
Analysis of the Samples
In the laboratory, a qualitative-quantitative
analysis of the mesozooplankton was performed
on subsamples from 1/10 to 1/20, depending on
the total sample richness, while the whole samples were observed in their entirety for the identification of rare species. The specimens were
sorted and divided per taxon. The copepods
(adults) and cladocerans were identified at the
species level, while the remaining groups were
classified at higher taxonomic levels. Biomass
values (as dry weight) were estimated from 500
m1 of the whole sample, according to Lovegrove's
method (1966). Abundance was expressed as
indlm 3 and biomass as mg/m 3 • These parameters
were calculated according to the bottom depth
along onshore-offshore sections; the groups of
sites were re-named as no. 1 to no. 6.
Data Processing
The data were log-transformed to avoid the
excessive dominance of more abundant species
in the analysis. The Bray Curtis dissimilarity
coefficient was used to construct the betweensites distance matrix (Bakus 1990); using the
furthest neighbour amalgamation rule, we
obtained a classification of sites shown by dendrograms.
Results
BIONESS: Temperature-Salinity-Fluorescence
Isolines
In June, temperature and salinity values (recorded by the BIONESS) revealed a strong vertical
thermohaline water stratification, which determined a frontal system separating neritic ecosystem water masses (close to 40 m depth) with a
pycnocline from 10-15 m depth (Fig. 2a,b).
During this period, two sub-systems were identified in the neritic system and defined as coastal
and offshore zones. The former included the
warmer and less salty water masses from the
coast to 20 m depth, while, in the layer over the
thermocline, it spread outward until 40 m depth.
The latter included the water masses under the
pycnocline spreading from 20 m depth to the
whole neritic system. Mean values of temperature and salinity of the water masses over and
under the thermocline were 20.-l3°C and 35-37.4
PSU, respectively.
The spatial distribution of fluorescence
exhibited a decreasing inshore-offshore trend.
ranging from 0.45-1.2 mg/m3 chla over the pycnocline. Another maximum (0.75-1.3 mg/m3 chla)
was found close to the bottom at the offshore
sites (Fig. 2c). During winter, a persistent unstable frontal system was caused by large fluctuations of meteo-marine conditions and river
flows. The neritic system was characterized by a
minor extension of the less salty water of the
coastal zone and by a shift of the halocline up to
the surface (Fig. 2d, e). The spatial distribution of
mean temperature and salinity ranged from
9.38-11.64°C and from 35.46-37.85 PSU, respectively. During this period, the fluorescence values
were lower (0.1-0.45 mg/m3 chla), with decreasing inshore-offshore and surface-bottom trends
(Fig. 2f). Similar patterns, but with slightly different ranges, were recorded during AugustSeptember and March in the central Adriatic Sea.
Zooplankton Abundance and Spatial
Distribution
Late Spring-Summer '96
The zooplankton community showed wide
quantitative oscillations, both of abundance
(501-25292 ind/m 3 ) and biomass (3.80 - 173.25
mglm3); the means ± SD were 3759 ± 3767
indlm 3 and 27.06 ± 20 mglm 3 , respectively. The
spatial distribution of abundance exhibited
strongly decreasing inshore-offshore (Fig. 3) and
surface-bottom (Fig.4a.c) trends, mainly determined by cladocerans and especially by Penilia
avirostris. The copepods, instead, showed higher
values in the layer under the pycnocline.
The seasonal succession of zooplankton was
characterized by a temporal inversion of the