I. Siokou-Frangou et al.: Mediterranean plankton
1557
frauenfeldii were found north of Crete in June (Gotsis-
Skretas et al., 1999). Finally, in the Southern Tyrrhenian Sea,
the DCM was dominated exclusively by Leptocylindrus dan-
icus in June 2007 (Percopo and Zingone, unpublished data).
These differences in species composition are remarkable, but
more observations are needed to assess their actual consis-
tency and ecological significance.
While the above-mentioned colonial species often appear
in relatively high concentrations, other large-sized diatoms
are found at much lower concentrations in the offshore MS
waters. In fact, these large diatoms have been reported as
responsible for a substantial but underestimated fraction of
primary production in oligotropic waters characterized by
a strong seasonal thermocline and nutricline outside the MS
(Goldman, 1993). However they have a patchy or sparse dis-
tribution and are generally not sampled properly. Among
them are some of the large Rhizosolenia species, which may
form migrating mats in other oceans (Villareal et al., 1996)
and, along with species of the genus Hemiaulus, may host
the diazotrophic cyanobacteria Richelia intracellularis, thus
playing a role in nitrogen fluxes in the pelagic ecosystems
(Villareal, 1994; Villareal et al., 1996). Diatom-diazotroph
associations have been rarely been investigated in the MS,
but off the Israeli coast their role in nitrogen fixation seems
to be very limited, possibly because of P-limitation in those
waters (Zeev et al., 2008).
3.2.4
Other microplankton species
The diversity of microplanktonic dinoflagellates is very high
in the MS (Marino, 1990; G´omez, 2006), although their
importance in terms of abundance is rather low and their
ecological role is still to be assessed. Indeed, quantitative
information is very fragmentary for this group, which has
often been aggregated with the nanoplanktonic dinoflagel-
lates. With the exception of the high productivity events
mentioned above, dinoflagellates are generally more abun-
dant than diatoms in the size fraction higher than 20 µm
(Marty et al., 2009). The species most commonly reported
are those of the genera Gymnodinium, Gyrodinium, Neo-
ceratium (formerly
Ceratium),
Protoperidinium,
Oxytoxum,
which are generally associated with warm and stratified wa-
ters (Estrada, 1991). Very rarely the percentage contribu-
tion of microplanktonic dinoflagellates is high. One such
case is Oxytoxum spp. reaching 12% of total cell counts in
the Alboran Sea (Lohrenz et al., 1988). Indeed, as for di-
atoms, the larger-sized species are not sampled properly most
of the time, but their role can be significant despite their
low abundances.
Some of them, e.g., in the genera Or-
nithocercus,
Histioneis and
Citharistes, can host endosym-
biotic cyanobacteria that allow them to survive even un-
der conditions of severe nitrogen limitation (Gordon et al.,
1994). Species of the widespread genus Neoceratium may
be mixotrophic (Smalley and Coats, 2002). They occupy
selected depths (Tunin-Ley et al., 2007), and their distribu-
tion could change as a consequence of warming in the MS
(Tunin-Ley et al., 2009). Finally, Protoperidinium spp. and
several athecate dinoflagellates in the genera Gymnodinium,
Gyrodinium and Lessardia are truly phagotrophic and may
constitute a main part of the microzooplankton (Sherr and
Sherr, 2007), but their importance in the offshore MS has
rarely been assessed (see Margalef, 1985).
Among other algal groups, the silicoflagellates Dictyocha
and Distephanus are also a constant although scarce compo-
nent of offshore MS microplankton, their abundance reach-
ing the highest values in surface waters in winter (Totti et al.,
2000) or in deeper waters in spring-summer (Lohrenz et al.,
1988; Estrada et al., 1993). In addition, a few other flag-
ellates that can form large colonies are also part of the off-
shore microplankton at least in some phases of their life cy-
cle. One of these is the key species Phaeocystis cf. globosa
(often reported in the MS under the name of the congeneric,
cold-water species P. pouchetii), which can form spherical
colonies reaching a few millimeter diameter. The species
has occasionally been recorded as abundant in the Catalan
Sea (e.g., Estrada, 1991) where its importance is apparently
increasing over the recent years (Margalef, 1995). Another
interesting species is the prasinophyte Halosphaera viridis,
which is found up to depths of 1000 m in autumn-winter
(Wiebe et al., 1974; Kimor and Wood, 1975) but then rises
to shallow water in spring. Such extensive migrations could
account for considerable upward recycling of carbon and nu-
trients (Jenkinson, 1986). Unfortunately, like in the case of
large dinoflagellates and diatoms, there are not many data on
the distribution of these interesting microplanktonic taxa in
offshore waters, due to the limited usage of net samplers in
recent phytoplankton studies.
4
Heterotrophic microbes and viruses
In the MS the hypothesis of phosphate limitation on pri-
mary production, first demonstrated by Berland et al. (1980),
and the remarkably pronounced gradient of P depletion from
west-to-east (Krom et al., 1991; Thingstad and Rassoulzade-
gan, 1995, 1999), have inspired numerous studies dealing
with microbial processes. Recent technological and concep-
tual breakthroughs are beginning to allow us to address bio-
logical complexity in terms of diversity and open new per-
spectives in integrating microbial loop processes into pre-
dictive models of ecosystem functioning. Here we describe
the different components and processes within the microbial
food web focusing on heterotrophic microbes, including the
viral shunt, in the Mediterranean open sea waters. Based
upon data published over the last 25 years, we attempt to es-
tablish some large-scale patterns of abundance and activities
for viruses, bacteria and protists along the Mediterranean
west-east gradient.
www.biogeosciences.net/7/1543/2010/
Biogeosciences, 7, 1543–1586, 2010