1566
I. Siokou-Frangou et al.: Mediterranean plankton
et al., 2005; Isari et al., 2006). In the South Aegean Sea,
Evadne spinifera contributed 6% to mesozooplankton abun-
dance in September (Siokou-Frangou et al., 2004), while in
the Straits of Sicily and the EMS, all cladocerans accounted
for just 0.3% of total zooplankton during autumn (Mazzocchi
et al., 1997).
Ostracods, which are not numerous in the mesozoo-
plankton communities at temperate latitudes (Angel, 1993),
present a remarkably consistent distribution in different
Mediterranean regions (Scotto di Carlo et al., 1984; Mazzoc-
chi et al., 1997; Isari et al., 2006). Their contribution to to-
tal zooplankton numbers increases gradually with depth and
varies from ∼2% in the upper 50 m to ∼11% in the 200–
300 m layer. The highest ostracod abundance is recorded in
the winter period in neritic waters, probably in relation to
temperature conditions and the scarcity of potential preda-
tors (Brautovic et al., 2006).
Gelatinous zooplankton represents an important group of
various organisms that play different and significant roles in
the pelagic communities as efficient filter-feeders or vora-
cious predators. However, they are generally underestimated
because standard sampling devices used for mesozooplank-
ton damage or destroy their fragile bodies and are therefore
inappropriate for their quantitative estimation. The pelagic
filter-feeding tunicates, and especially the salps, are known
to occur periodically in dense swarms, sometimes with out-
breaks lasting for days or weeks (Bone, 1998; M´enard et al.,
1994). It seems, however, that salps form smaller swarms in
the MS than in other oceans, which could be related to the
oligotrophic nature of this sea (Andersen, 1998). Doliolids
and salps together accounted for 4% of the total zooplankton
abundance in the Catalan Sea in June (Saiz et al., 1999) and
only 0.04–1.3% in the EMS in October–November (Mazzoc-
chi et al., 1997). However, doliolids made up to 9% of to-
tal zooplankton in the North Aegean Sea in September (Isari
et al., 2006). Appendicularians represent a more constant
component in open-water zooplankton, but, given their high
population growth rate under favourable conditions (Gorsky
and Palazzoli, 1989), their abundance seems to depend on
the selected sampling area and time. They accounted for 8%
of the total spring zooplankton abundance in the open Cata-
lan Sea (Saiz et al., 1999) and between 1 and 8% in the Io-
nian Sea (Mazzocchi et al., 2003). The range of their relative
abundance was very wide among several regions of the EMS
in the autumn of 1991, from 1% in the West Levantine Sea up
to 23% at a station in the Rhodos Gyre area (Mazzocchi et al.,
1997). The contribution of appendicularians to zooplankton
abundance was very high (38%) in the Ligurian Sea in win-
ter, when the group was dominated by Fritillaria (Licandro
and Icardi, 2009).
Among the carnivorous gelatinous zooplankton, chaetog-
naths are more abundant than siphonophores (Mazzocchi
et al., 1997; Isari et al., 2006). However, the latter group
can easily be underestimated because of the damage caused
by nets during sampling (Luˇci´c et al., 2005). In both WMS
and EMS during autumn, the most abundant chaetognaths
are Sagitta enflata, S. decipiens, S. bipunctata, S. minima
and S. serratodentata (Dallot et al., 1988; Kehayias, 2003).
During the same season, the siphonophores Muggiaea at-
lantica,
Abylopsis tetragona,
Lensia subtilis,
Eudoxoides spi-
ralis and the medusae
Rhopalonema velatum,
Liriope tetra-
phylla,
Aglaura hemistoma are common in the WMS (Dallot
et al., 1988).
5.3
Influence of hydrology
Mesozooplankton abundance and biomass display patterns
at sub-basin scale that broadly follow hydrological features,
similarly to the distribution of primary producers discussed
in the previous sections. In the Alboran Sea, the highly en-
ergetic dynamics contributes to higher plankton biomass and
diversity on the Mediterranean than on the Atlantic side of
the Strait of Gibraltar (Vives et al., 1975). In the same area,
the sustained productivity caused by processes linked to the
Atlantic Water inflow results in high zooplankton dry mass
(18 mg m
−
3
) and copepod abundance (up to 5000 ind. m
−
3
)
in the upper 200 m of the Almeria-Oran frontal area (Seguin
et al., 1994; Thibault et al., 1994). This enrichment is also re-
flected in the demographic structure of the chaetognaths and
siphonophores, which are found to have a high proportion of
juveniles and eudoxids, respectively (Dallot et al., 1988). A
great spatial variability of biomass values (5.5 to 25 mg m
−
3
)
was observed in this region among sites positioned within
different water masses and hydrological features and at a dis-
tance of 15–20 miles (Fig. 19). Increased mesozooplankton
standing stock values are associated with the fronts in the
Balearic, Catalan, and Ligurian Seas (Razouls and Thiriot,
1973; Sabat´es et al., 1989; Alcaraz et al., 1994; Pinca and
Dallot, 1995; Mc Gehee et al., 2004; Licandro and Icardi,
2009). The hydrographic features of the frontal system in
the Catalan Sea also affect the metabolic activities (e.g., res-
piration, excretion) of zooplankton in the area, as well as
their variability in different seasons (Alcaraz et al., 2007).
The zooplankton abundance in the Straits of Sicily seems
to be enhanced by intermittent upwelling (Mazzocchi et al.,
1997). The strong thermohaline front between the inflowing
modified Black Sea water and the Aegean Sea water harbors
the highest mesozooplankton standing stock recorded in the
epipelagos (0–100 m) of the very oligotrophic EMS (up to
3875 ind. m
−
3
and 26.73 mg m
−
3
dry mass, Siokou-Frangou
et al., 2009). The permanent or semi-permanent cyclonic
gyres of the EMS (e.g., the Rhodos Gyre and the cyclonic
gyre South-West of Crete Island) revealed higher meso-
zooplankton abundance than the neighboring anticyclonic
gyres (Mazzocchi et al., 1997; Christou et al., 1998; Siokou-
Frangou, 2004).
Mesoscale circulation and hydrodynamic features affect
not only standing stock but also composition and structure
of mesozooplankton communities. In the Alboran Sea, the
copepods Centropages typicus and Clausocalanus furcatus
Biogeosciences, 7, 1543–1586, 2010
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