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C. Brugnano et al. / Journal of Marine Systems 81 (2010) 312–322
Fig. 3. Vertical trends of temperature, salinity, fluorescence and dissolved oxygen. Data were grouped by depth range according to plankton data analysis. Layers are indicated by
capital letters (see text for explanation).
most stations (i.e. 1, 2, 5, 6 stations): in fact, average temperatures in areas (101.0 ± 25.4 and 16.8 ± 12.5 ind. m− 3, in neritic and pelagic
layer A were 22.59 ± 0.4 °C (N = 127) and 23.32 ± 0.26 °C (N = 111),
respectively.
Moreover, the Depth of the Chlorophyll Maximum (DCM) was
observed at about 40 m in station 10 coinciding with the intruding
MAW tongue, where the fluorescence maximum was recorded. In the
western-most open sea stations (stations 10, 11, and 12) the DCM
remains in layers A*/B, whereas on the Tyrrhenian side (stations 1 and
6), it deepens to 60–65 m (layer C), probably due to mixing and
coastal influence. Dissolved oxygen showed a higher average value in
layer B.
In the ordination plane of physical–chemical data set (Fig. 4), PCA
showed a clear separation of coastal, neritic and pelagic surface waters
(in the upper part of the graph) from subsurface and deeper ones (in
the middle and lower part of the graph, respectively). PC1 and PC2
axes accounted for the greatest percentage (89.9%) of total variance.
Salinity showed the highest coefficients in the linear combination
with PC1 (0.613) and temperatures with PC2 (0.774). Fluorescence
(chlorophyll a) showed a negative correlation (−0.521) with the
second axis. From BIO-ENV analysis resulted a high rank of correlation
(Spearman's coefficient R = 0.847, p ≤ 1%) between biota and physico-
chemical parameters. The best results were obtained relating copepod
species abundance data to temperature and salinity. However, high
correlation ranks were obtained also for temperature, salinity and
dissolved oxygen (0.76), Chl-a and temperature (0.74), and all
parameters together (0.69).
3.2. Copepod assemblages
Copepods were the dominant group of zooplankton community,
representing on average the 75% of total zooplankton.
Total mean abundances (adult and copepodite stages) decrease
from 150.04 ind. m− 3 in the 0–20 m layer to 111.86 ind. m− 3 in the
60–80 m layer; a sharp decrease is clear below the later layer till
200 m (5.00 ind. m− 3) (Fig. 5).
The same trend was shown by the mean abundances (adult and
copepodite stages) from inshore (262.0 ± 8.2 ind. m−3) to offshore
315
areas, respectively).
A total of 107 copepod species were identified in the investigated
area. The greatest number of species was found between 20–40 and
40–60 m layers (87 and 78, respectively), whereas the lowest (19) in
the 0–20 m one. Richness species (d) and species diversity (H′)
indices showed similar spatial trends along the study area, with
exception at 200–300 m depth, in which the former increased and the
latter decreased, with respect to the foregoing 150–200 m layer
(Fig. 5). The lowest Shannon–Wiener index values (range: 2.4–2.9)
occurred in the upper layer, corresponding to the maximum of
abundances, the highest ones (ranging between 3.2 and 3.3) between
40 and 80 m depth, due to neritic and intermediate pelagic copepod
species overlapping. The highest richness species was shown from
150 to 300 m layers (range: 17.9–51.6), in which occurred the lowest
copepod abundances, the lowest one occurred in the upper layers
(range: 15.8–23.1). The horizontal trend of diversity, calculated on the
integrated water column from 0 to 40 m depth, showed the highest
values of indices in the neritic area among the islands (81 species;
d = 22.0; H′ = 2.9), the lowest in coastal area (49 species; d = 11;
H′ = 2.5), whereas intermediate values (64 species; d = 17.2; H′ = 2.8),
in the pelagic one.
Four groups of samples were determined by cluster analysis at 42%
similarity level (Fig. 6), each one identifying a different area and/or
depth layer: I) surface water group in common between A and A*
samples of coastal, neritic and pelagic areas; II) more heterogeneous
subsurface water group, distributed preferentially along A*, B and C
samples, mainly, of neritic and pelagic areas; III) intermediate one
from C–D to E samples, and IV) deeper water group (N200 m
samples). Therefore, A and A* (0–40 m) were considered as surface
layers, the layers B and C (40–80 m) as subsurface, the layers D and E
(80–200 m) as intermediate and the layers F and G (200–600 m) as
deep layers in the study area.
Four species accounted for 53% of similarity (SIMPER test) in
sample group I: Clausocalanus furcatus (15.1%), Acartia negligens
(14.0%), Temora stylifera (12.7%) and Oithona plumifera (11.2%). In
group II, six species accounted for 40% of similarity: Nannocalanus
minor (7.9%), Centropages typicus (7.2%), Clausocalanus jobei (6.9%),