1572
I. Siokou-Frangou et al.: Mediterranean plankton
(Agostini and Bakun, 2002). Indeed, the Alboran Sea, the
Gulf of Lion and the nearby Catalan Sea, the Adriatic Sea
and the North Aegean Sea are known as successful spawn-
ing grounds and areas of high yield of small pelagic fish,
mainly anchovy and sardine (Stergiou et al., 1997; Agos-
tini and Bakun, 2002; Palomera et al., 2007). In these ar-
eas, the larvae of small pelagic fish feed mainly on copepods
and other mesozooplankters, as mentioned in the previous
chapter, but ciliates and flagellates have also been found to
contribute to their diet (Rossi et al., 2006). New production
also occurs at the DCM (with frequent presence of diatoms)
close to the nutricline over a broad time interval, but its over-
all weight on the production of the basin is poorly quantified.
Whether the DCM hosts a significantly different planktonic
web is still an open question (e.g., Estrada et al., 1999).
A first approximation may then be that the microbial food
web is the prevailing structure in the offshore waters of the
MS, with few exceptions where larger, bloom forming phy-
toplankton might initiate the “classical” food web. However
this simplification is becoming less and less robust in the
light of new findings on the nutritional potential of marine
organisms. What were considered as heterotrophic bacteria
when estimated with normal counts, turned out to include
groups capable of diversified metabolic strategies (Moran
and Miller, 2007; Van Mooy and Devol, 2008; Zubkov and
Tarran, 2008) and the MS should not be different from the
global ocean in this respect. Part of flagellates, ciliates and
dinoflagellates are mixotrophic (Sects. 3 and 4) and their
contribution is significant in the EMS (Sect. 4). Metazoans
display also a wide range of feeding modes and food pref-
erences. Among copepods, the genera more abundant in
the MS are known to exploit a large variety of food re-
sources, including fecal pellets (e.g., Oithona, Gonz´alez and
Smetacek, 1994; Svensen and Nejstgaard, 2003) and marine
snow (e.g., oncaeids, Alldredge, 1976; Ohtsuka et al., 1993).
Appendicularians, which are capable to feeding on pico- and
small nanoplankton (Deibel and Lee, 1992), constitute a by-
pass from the lower trophic levels to fishes (Deibel and Lee,
1992), contributing to a more efficient food web like the
one described in the oligotrophic North Aegean Sea (Siokou-
Frangou et al., 2002).
The variable grazing impact on larger than 5 µm primary
producers by mesozooplankton, despite the prevalence of
ciliates in their diet, during both mixing and stratified sea-
sons (Sect. 5), indicates a flexible and possibly efficient con-
nection between autotrophs and microheterotrophs and the
higher trophic levels. All this suggests that the MS is charac-
terized by a “multivorous food web” (sensu Legendre and
Rassoulzadegan, 1995), including a continuum of trophic
pathways spanning from the herbivorous food web to the mi-
crobial loop and dynamically expanding or contracting along
the seasons, areas and transient processes. The high diver-
sity in species, feeding and reproduction modes, and conse-
quently in functional roles, might support a more efficient
energy transfer to the higher trophic levels, a common fea-
ture among oligotrophic systems (Margalef, 1986, chap. 23
and chap. 26).
Most of the studies describing phytoplankton biomass dy-
namics in the MS (Sect. 3) have stressed the bottom up con-
straints on phytoplankton growth and accumulation to justify
the generally low standing stocks of autotrophs. On the other
hand, Thingstad et al. (2005) have shown that purely het-
erotrophic processes may produce, even in the extreme olig-
otrophy of the EMS, a rapid transfer to higher levels, which
suggests an efficient top-down control, which was revealed
for both ciliates and bacteria (Sects. 4 and 5). We are then
confronted with two possible views: i) the low standing stock
of autotrophs results from low availability of dissolved nu-
trients which, as typical for oligotrophic regime, also deter-
mine a low standing stock of intermediate and top predators
(bottom-up control); ii) the low standing stock of autotrophs
would rather result from a very effective top-down control
that propagates along the food web, ultimately affecting the
top predators (see Sects. 4 and 5). The two views are not
in contrast, although the former points at geochemical con-
straints while the latter emphasizes the ecosystem dynamics
as a whole. Indeed, a bottom-up control does exist, and deter-
mines the carrying capacity of the system. This is relatively
low because of the moderate nutrient fluxes and the continu-
ous loss of the internal nutrient pool into the Atlantic Ocean.
On the other hand, the view of a top-down control is sup-
ported by the structure of the planktonic food web discussed
above as well as by the evidence that fisheries in the MS are
richer than expected on the basis of measured chl a and nutri-
ent concentrations (Fiorentini et al., 1997). It is the latter as-
pect that is at the origin of the so called “Mediterranean para-
dox” (Sournia, 1973; Estrada, 1996), which however is less
paradoxical in the light of the effective food web discussed
above, and is coherent with general ecological theories (e.g.,
Margalef, 1986). We can thus hypothesize that the intricate
and very flexible food web (e.g., Paffenh¨ofer et al., 2007)
contributes in minimizing carbon loss to deeper layers (POC
export is very low in the MS, e.g., Wassmann et al., 2000;
Boldrin et al., 2002) and predators can optimally profit from
carbon produced and transformed within the system, thus be-
ing the ultimate controllers of plankton abundance in the MS.
The paradox becomes even less paradoxical if one takes into
account the significant role of external organic matter inputs,
which corroborates the view of the MS as a coastal ocean.
7
Perspectives
Despite the numerous investigations of the last decades, the
emerging picture of plankton dynamics in the MS is far from
satisfying, both at the spatial and at the temporal scales.
Apart from the satellite images, some areas, especially in the
southern part of both basins, are still insufficiently known.
The temporal variability at short, seasonal and interannual
scale also calls for more intensive sampling: in addition to
Biogeosciences, 7, 1543–1586, 2010
www.biogeosciences.net/7/1543/2010/