Biogeosciences, 7, 1543–1586, 2010
www.biogeosciences.net/7/1543/2010/
doi:10.5194/bg-7-1543-2010
© Author(s) 2010. CC Attribution 3.0 License.
Biogeosciences
Plankton in the open Mediterranean Sea: a review
I. Siokou-Frangou
1
, U. Christaki
2,3,4
, M. G. Mazzocchi
5
, M. Montresor
5
, M. Ribera d’Alcal´a
5
, D. Vaqu´e
6
, and
A. Zingone
5
1
Hellenic Centre for Marine Research, 46.7 km Athens-Sounion ave P. O. Box 712, 19013 Anavyssos, Greece
2
Universit´e Lille, Nord de France, France
3
Universit´e du Littoral Cˆote d’Opale, Laboratoire d’Oc´eanologie et de G´eosciences, 32 avenue Foch, 62930 Wimereux,
France
4
CNRS, UMR 8187, 62930 Wimereux, France
5
Stazione Zoologica Anton Dohrn, Villa Comunale, 80121 Napoli, Italy
6
Instituto de Ciencias del Mar ICM, CSIC, Passeig Mar´ıtim de la Barceloneta, 37–49, 08003 Barcelona, Spain
Received: 2 October 2009 – Published in Biogeosciences Discuss.: 27 November 2009
Revised: 7 April 2010 – Accepted: 13 April 2010 – Published: 18 May 2010
Abstract. We present an overview of the plankton studies
conducted during the last 25 years in the epipelagic offshore
waters of the Mediterranean Sea. This quasi-enclosed sea is
characterized by a rich and complex physical dynamics with
distinctive traits, especially in regard to the thermohaline cir-
culation. Recent investigations have basically confirmed the
long-recognised oligotrophic nature of this sea, which in-
creases along both the west-east and the north-south direc-
tions. Nutrient availability is low, especially for phospho-
rous (N:P up to 60), though this limitation may be buffered
by inputs from highly populated coasts and from the atmo-
sphere. Phytoplankton biomass, as chl a, generally displays
low values (less than 0.2 µg chl a l
−
1
) over large areas, with
a modest late winter increase. A large bloom (up to 3 µg l
−
1
)
is observed throughout the late winter and spring exclusively
in the NW area. Relatively high biomass values are recorded
in fronts and cyclonic gyres. A deep chlorophyll maximum
is a permanent feature for the whole basin, except during
the late winter mixing. It is found at increasingly greater
depths ranging from 30 m in the Alboran Sea to 120 m in
the easternmost Levantine basin.
Primary production re-
veals a west-east decreasing trend and ranges between 59 and
150 g C m
−
2
y
−
1
(in situ measurements). Overall, the basin
is largely dominated by small autotrophs, microheterotrophs
and egg-carrying copepod species.
The microorganisms
(phytoplankton, viruses, bacteria, flagellates and ciliates) and
zooplankton components reveal a considerable diversity and
Correspondence to: I. Siokou-Frangou
(isiokou@ath.hcmr.gr)
variability over spatial and temporal scales, although the lat-
ter is poorly studied. Examples are the wide diversity of
dinoflagellates and coccolithophores, the multifarious role
of diatoms or picoeukaryotes, and the distinct seasonal or
spatial patterns of the species-rich copepod genera or fam-
ilies which dominate the basin. Major dissimilarities be-
tween western and eastern basins have been highlighted in
species composition of phytoplankton and mesozooplankton,
but also in the heterotrophic microbial components and in
their relationships. Superimposed to these longitudinal dif-
ferences, a pronounced biological heterogeneity is also ob-
served in areas hosting deep convection, fronts, cyclonic and
anti-cyclonic gyres or eddies. In such areas, the intermittent
nutrient enrichment promotes a switching between a small-
sized microbial community and diatom-dominated popula-
tions. A classical food web readily substitutes the micro-
bial food web in these cases. These switches, likely occur-
ring within a continuum of trophic pathways, may greatly
increase the flux towards higher trophic levels, in spite of the
apparent heterotrophy. Basically, the microbial system seems
to be both bottom-up and top-down controlled. A “multivo-
rous web” is shown by the great variety of feeding modes and
preferences and by the significant and simultaneous grazing
impact on phytoplankton and ciliates by mesozooplankton.
Published by Copernicus Publications on behalf of the European Geosciences Union.
1544
I. Siokou-Frangou et al.: Mediterranean plankton
1
Introduction
La Mediterr`ania, o almenys la seva zona pel`agica, seria
comparable a una Amaz`onia marina. (Margalef, 1995)
(The Mediterranean, or at least its pelagic zone, would be
like a marine version of the Amazon forest.)
The Mediterranean Sea (MS) is the largest quasi-enclosed
sea on the Earth, its surface being similar to that of the largest
semi-enclosed (e.g., the Gulf of Mexico) and open (e.g., the
Caribbean Sea) marginal seas of the extant ocean (Meybeck
et al., 2007). The MS’ size, location, morphology, and exter-
nal forcing allow for a rich and complex physical dynam-
ics that includes: i) unique thermohaline features, ii) dis-
tinctive multilayer circulation, iii) topographic gyres, and
iv) meso- and sub-mesoscale activity. Nutrients and chloro-
phyll a (chl a) pools rank the basin as oligotrophic to ultra-
oligotrophic (Krom et al., 1991; Antoine et al., 1995). Olig-
otrophy seems to be mainly due to the very low concentra-
tion of inorganic phosphorus, which is assumed to limit pri-
mary production (Berland et al., 1980; Thingstad and Ras-
soulzadegan, 1995, 1999; Thingstad et al., 2005). Additional
features of the MS are i) the decreasing west-east gradient
in chl a concentration, as shown by color remote sensing
(D’Ortenzio and Ribera d’Alcal´a, 2009; Barale et al., 2008)
as well as by in situ data (Turley et al., 2000; Christaki et al.,
2001), ii) a high diversity compared to its surface and volume
(Bianchi and Morri, 2000), and iii) a relatively high number
of bioprovinces (sensu Longhurst, 2006), with boundary def-
inition mostly based on the distribution of the benthos and
the necton (Bianchi, 2007). The MS is also a site of intense
anthropic activity dating back to at least 5000 years BP, the
impact of which on the marine environment has still to be
clearly assessed and quantified. All these peculiar and con-
trasting characteristics should likely be reflected in the struc-
ture and dynamics of plankton communities.
Numerous investigations have been conducted on the
fluxes of the main elements, as linked to the biological pump.
Studies on structure and dynamics of plankton communities
in the open MS have increased in the last decades. A first
synthetic overview of the pelagic MS ecosystems was pro-
vided by the collective efforts reported in Margalef (1985)
and Moraitou-Apostolopoulou and Kiortsis (1985), followed
a few years later by a collection of scientific papers edited
by Minas and Nival (1988). Most of those contributions
focused on bulk parameters (e.g., chl a, primary productiv-
ity, mesozooplankton biomass) and organism distributions.
In the following years, the discovery of picoplankton (e.g.,
Waterbury et al., 1979) and the consequent increased at-
tention for the role of microheterotrophs within the pelagic
food web provided new perspectives for the understanding
of oligotrophic seas such as the MS (Rassoulzadegan, 1977;
Hagstr¨om et al., 1988). Numerous research efforts starting
from the nineties were hence devoted to study carbon and nu-
trient fluxes and to provide insight into the key players of the
MS pelagic food web (e.g., Lipiatou et al., 1999; Thingstad
and Rassoulzadegan, 1999; Tselepides and Polychronaki,
2000; Monaco, 2002; Mazzocchi et al., 2003; Krom et al.,
2005). An increasing number of studies have focused on rel-
evant biological processes and/or physiological rates (e.g.,
Calbet et al., 1996; Estrada, 1996; Saiz et al., 1999; Moutin
and Raimbault, 2002), while the phosphorus limitation hy-
pothesis has inspired studies on the effects of phosphorus en-
richment on the pelagic food web (Thingstad et al., 2005).
Physical-biological coupling in general (Crise et al., 1999;
Pinardi et al., 2004), as well as in relation to mesoscale dy-
namics, has also been addressed more frequently during the
last decades (e.g., Champalbert, 1996; Alcaraz et al., 2007).
Clearly these studies have provided valuable insights on the
components of the MS plankton in different areas of the
basin. Overall, significant knowledge was provided by in-
ternational and European projects at basin (MATER) or sub-
basin scale (POEM-BC).
The present review aims at providing an updated and in-
tegrated picture of the Mediterranean plankton in the off-
shore epipelagic waters (0–200 m depth) based on studies
conducted during the last 25 years.
The key issues ad-
dressed in the review are: i) the plankton components, from
the viruses, bacteria and picoautotrophs, up to mesozoo-
plankton, with a prevalent focus on the key players, i.e.,
with a species-oriented approach; ii) their mutual interactions
within the pelagic realm, with the aim of corroborating or im-
proving existing descriptions of planktonic food web struc-
ture (Thingstad, 1998; Sommer et al., 2002) and highlighting
the principal carbon producers. A review could be helpful,
among other things, as a baseline for the assessment of global
change impact on MS ecosystems. In addition, as detailed
in the following sections, the peculiar features of the main
forcings in the basin and of their scales of variability might
trigger non-trivial responses in plankton communities, which
could be of general ecological interest beyond the Mediter-
ranean boundaries.
2
Physical and chemical framework
Physical dynamics is a crucial driver for seasonal cycle of
production in the pelagic environment (Mann and Lazier,
2006,
and references therein). Here we use the term of
“physical dynamics” in a broad sense, to include both ma-
rine and atmospheric processes. The latter are particularly
important in the MS because, besides determining the gen-
eral circulation, they contribute to the fluxes of elements en-
tering the basin. As compared to the open ocean or other
internal seas, the inputs from land play a greater role in the
MS, because the perimeter to surface ratio of the basin is par-
ticularly high and the catchment area of the inflowing waters
is one of the largest for marginal seas (Meybeck et al., 2007).
As it will be discussed later, this enhances the role of external
inputs in regulating nutrient fluxes to the photic zone.
Biogeosciences, 7, 1543–1586, 2010
www.biogeosciences.net/7/1543/2010/