1556
I. Siokou-Frangou et al.: Mediterranean plankton
mixing largely exceeds the critical depth but blooms might
be favoured by brief periods of quiescence. Colonial species
belonging to the genera Pseudo-nitzschia and Chaetoceros
were most dominant in spring in deep convection areas of the
North Balearic Sea (Zingone and Sarno, unpublished data).
In the Otranto Straits, high concentrations of healthy Chaeto-
ceros were found as deep as 500 m (Vilicic et al., 1989).
Prymnesiophytes and prokaryotes instead of diatoms dom-
inated in March in the Cyprus eddy, which is not a site of
deep convection, with moderately high chl a concentrations
(59 mg m
−
2
at the core and 45.5 mg m
−
2
at the boundary)
(Zohary et al., 1998). Apparently, also in highly dynamic ar-
eas, very high biomass only accumulates when diatoms are
the species involved.
Colonial, bloom forming diatoms belonging to the genera
Chaetoceros, Thalassiosira, Proboscia, Rhizosolenia, Lep-
tocylindrus are generally the main contributors also to high
chl a patches in fronts and gyres (Fiala et al., 1994; Arin
et al., 2002; Ignatiades et al., 2002; Zervoudaki et al., 2006,
2007). These structures, which are seen both in the WMS and
EMS (see the above section), have been defined the “oases”
of the Mediterranean desert (Claustre et al., 1994). The bi-
ological phenomena that they drive strictly depend on water
mass dynamics and hence are spatially heterogeneous, and
show a very high temporal dynamic, as well as a marked
inter-annual variability (Mercado et al., 2005).
Diatom-dominated chl a peaks are often found in sub-
surface waters (Arin et al., 2002), as in the exceptional
case of a monospecific bloom of a Thalassiosira (probably
Th. partheneia) forming gelatinous colonies (∼10
7
cells l
−
1
and 23 µg chl a l
−
1
), which was detected at 54 m depth in
the Almeria-Oran front area (Gould and Wiesenburg, 1990).
The formation and dynamics of these deep accumulations are
strictly linked to the frontal circulation (e.g., Raimbault et al.,
1993) and therefore are quite different from those character-
izing the development of a DCM in the stratification period in
oligotrophic waters. A significant contribution of diatoms to
the latter seasonal DCMs has been reported from many areas
of the MS, e.g., the Catalan Sea (Margalef, 1969), Southern
Adriatic Sea (Boldrin et al., 2002) and Cretan Sea (Gotsis-
Skretas et al., 1999). Frequently, the species involved are
those that are also typical of the high production events de-
scribed above, supporting the hypothesis that the DCMs are
sites of active growth, rather than of passive accumulation.
Overall, the intermittent and most probably undersampled
pulses of diatom growth in deep waters might contribute in
explaining the mismatch between the relatively few reports
of diatoms in phytoplankton samples and the high amount of
biogenic silica found in surface sediments and sediment traps
(Kemp et al., 2000).
In the DCM, diatoms are found in association with pi-
coplankton and at times they dominate the subsurface pop-
ulations (e.g., Decembrini et al., 2009; Boldrin et al., 2002).
Their relative importance may vary greatly over the time and
across sites (Estrada and Salat, 1989; Estrada et al., 1993).
Fig. 11. Vertical profiles of diatoms (dotted), dinoflagellates (light gray) and coccolitophores (dark hatched
gray) over an east-west longitudinal transect of the Mediterranean Sea in June 1999. Modified with permission
from Ignatiades et al. (2009).
84
Fig. 11. Vertical profiles of diatoms (dotted), dinoflagellates (light
gray) and coccolitophores (dark hatched gray) over an east-west
longitudinal transect of the Mediterranean Sea in June 1999. Modi-
fied with permission from Ignatiades et al. (2009).
In one of the few cases of across-basin studies at the species
level (Ignatiades et al., 2009), diatoms seemed to be less
abundant in the Levantine basin DCM as compared to the sta-
tions in the western basin (Fig. 11). Interestingly, in the sum-
mer DCM at the DYFAMED station, diatoms are associated
with the highest chl a concentrations and sit under a layer
occupied by prochlorophytes and nanoflagellates, whereas
Synechococcus dominates in the oligotrophic surface waters
(Marty and Chiaverini, 2002). This vertical zonation, similar
to that reported in the Atlantic waters (Claustre and Marty,
1995), points at a tightly structured system, within which the
distinct phytoplankton components may have different eco-
logical roles.
Colonial Chaetoceros species are a rather constant fea-
ture of diatom-dominated DCMs, but the accompanying
assemblages seem to vary from area to area.
For ex-
ample,
Pseudo-nitzschia,
Rhizosolenia and
Thalassiosira
were reported in the Catalan-Balearic DCM (Latasa et al.,
1992), while Leptocylindrus danicus, Pseudo-nitzschia
delicatissima, Thalassionema nitzschioides were found in
the Southern Adriatic Sea (Boldrin et al., 2002). To the east,
Bacteriastrum,
Hemiaulus and
Thalassionema were found
south of Crete in July (Berland et al., 1987), whereas P. del-
icatissima,
Dactyliosolen fragilissimus, and
Thalassionema
Biogeosciences, 7, 1543–1586, 2010
www.biogeosciences.net/7/1543/2010/