I. Siokou-Frangou et al.: Mediterranean plankton
1561
Fig. 14. (A) Log–log linear regression between bacterial biomass (µg C l
−
1
) and bacterial production (µg C l
−
1
h
−
1
) and
(B) between
bacterial production and primary production (µg C l
−
1
d
−
1
) for the WMS and EMS waters; (C) Relationship between log heterotrophic
nanoflagellates abundance (HNF, cells ml
−
1
) and log bacterial prey (cells ml
−
1
) from the model of Gasol (1994). All HNF abundances
fall below the Maximum Attainable Abundance line (MAA) while 70 and 75% HNF fall above the Mean Realized Abundance for marine
environment (MRA) in the EMS and WMS, respectively, generally suggesting bottom-up control prevailing on HNF (cf. open sea studies on
Table 4).
that bacteria used the amino acids added in the samples to
meet energy requirements for cell maintenance rather than
biomass production.
Surprisingly,
little
information
exists
on
bacte-
rial respiration (BR) and bacterial growth efficiency
(BGE=BP/[BP+BR]) and furthermore, the studies are
limited to the WMS. However, the studies underline the
importance of BR to total plankton community respiration.
The mean portion of BR to community respiration was
65% in the NW MS (Lem´ee et al., 2002), and an average
value of 52% (range 41 to 85%) was recorded closer to
the coast (Navarro et al., 2004). It is noteworthy that BR
as a percent of total community respiration increased with
the percentage of High-DNA bacteria. Bacterial respiration
rates ranged from ∼0 to 3.64 µmol O
2
l
−
1
d
−
1
(Lem´ee et al.,
2002; Navarro et al., 2004).
Generally BGE tends to be low in oligotrophic systems,
perhaps because most of the DOC pool is recalcitrant and
inorganic nutrients are scarce (del Giorgio et al., 1997). In
the MS an accumulation of DOC in the surface waters has
been suggested to result from nutrient limitation of bacterial
activity (Thingstad and Rassoulzadegan, 1995; Gasol et al.,
1998). Indeed, in the Almeria-Oran geostrophic front and
adjacent Mediterranean waters BGE was estimated to be
7% (Semp´er´e et al., 2003) and lower values (2.6 ± 0.1%)
were obtained in the NW-Mediterranean through a coastal-
offshore gradient (Moran et al., 2002). Conversely, in a study
over a year in the NW MS, Lem´ee et al. (2002) report that
BGE ranged widely, from 0.1 to 43%. These authors empha-
size that they could not identify any regulatory mechanisms
of BGE and respiration over this period.
Preliminary heterotrophic microbial diversity studies from
Mediterranean samples revealed a considerable diversity of
unknown prokaryotes (e.g., Pukall et al., 1999). Commu-
nity fingerprinting by 16S rDNA restriction analysis applied
to WMS offshore waters showed that the free-living pelagic
bacterial community was very different from that living on
aggregates (Acinas et al., 1997, 1999) and similar results
were obtained in the EMS (Moesender et al., 2001). A study
of the bacterial assemblages carried out offshore Barcelona
using the DGGE (denaturing gradient gel electrophoresis)
technique showed that the diversity index followed seasonal
dynamics, but bacterial assemblages were relatively similar
over 10’s of kilometres suggesting that coastal areas might be
characterized by rather homogeneous communities (Schauer
et al., 2000). Distinct communities, stable over the time-
scale of a month were found in different depth strata be-
tween 0 and 1000 m by Ghiglione et al. (2005, 2008) in the
NW Mediterranean. In terms of temporal stability, a rather
stable taxonomic composition of bacterioplankton was re-
ported over time for Blanes Bay (Schauer et al., 2003).
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Biogeosciences, 7, 1543–1586, 2010
1562
I. Siokou-Frangou et al.: Mediterranean plankton
4.3
Heterotrophic nanoflagellates
In open MS, some heterotrophic nanoflagellates (HNF) are
usually dominated by small cells (≥80% less than 5 µm) with
total abundances between 10
5
and 10
6
cells l
−
1
(Zohary and
Robarts, 1992; Christaki et al., 1999, 2001, Tables 4 and 5).
Nanoflagellate bacterivory is important, accounting from 45
to 87% of daily bacterial production in an East-West Mediter-
ranean transect (Christaki et al., 2001). Spatially variable
bacterivory rates were reported for the NW Mediterranean,
ranging from <10 to 100% of bacterial production with bac-
terial consumption positively correlated with the presence of
High-DNA bacteria (Vaqu´e et al., 2001). In the Aegean Sea,
bacterivory by HNF and mixotrophic nanoflagellates roughly
balanced bacterial production (Christaki et al., 1999).
Although the number of papers reporting HNF abundance
and their grazing activity is limited (Table 5), they provide
a quite good spatial coverage of the open MS, and over-
all suggest that bacterivory is the dominant cause of bac-
terial mortality. According to the model by Gasol (1994),
the plot of the relationships between log HNF abundance
(HNF ml
−
1
) and log bacterial abundance (ml
−
1
) suggests
that HNF are resource, or bottom-up, controlled by bacteria
(Fig. 14c). A tight coupling of HNF and bacterial concentra-
tions supports the view that bacteria are top-down controlled
as we have suggested above (Fig. 14a).
Little is known about HNF diversity in the MS; four
genetic libraries for surface waters from Blanes Bay
(NW Mediterreanean) showed that some heterotrophic pi-
coeukaryotes belong to the marine stramenopiles (MAST)
(Massana et al., 2004). In the Alboran Sea, MAST were
found in the upper ocean, including the photic zone and the
upper aphotic zone, and appeared to be more abundant at
subsurface (near the DCM) than at surface (5 m, Rodr´ıguez-
Mart´ınez et al., 2009).
4.4
Ciliates
Ciliate abundance in the MS at different sites and in dif-
ferent seasons displays a remarkably high variability. For
example, in the Catalan Sea in June, the highest values of
about 850 cells l
−
1
were found at the DCM, whereas in the
Ligurian Sea in May average surface layer values (5–50 m)
were ∼3.3×10
3
cells l
−
1
with a maximum of ∼10
4
cells l
−
1
(Per´ez et al., 1997). These high values contrast with those for
the Aegean Sea, where ciliate abundance was always lower
than 5×10
2
cells l
−
1
(Pitta and Giannakourou, 2000). Pitta
et al. (2001) reported a 2-fold decrease in ciliates concen-
tration from west to east. However, a decline in concentra-
tions along the west-to-east oligotrophy gradient has not been
found to be always true for the ciliate standing stock (e.g.,
Dolan et al., 1999). It could be that the relationship between
ciliate abundance and chl a concentration is stronger in the
WMS than in the EMS indicating a better coupling with phy-
toplankton stock in the WMS (Fig. 15). However, differences
Fig. 15. Log–log linear regression between chl a concentration
(µg l
−
1
) and ciliate abundance (cell l
−
1
), taken at depths between
5 and 200 m, for Western and Eastern Mediterranean waters.
between slopes are not statistically significant (t
value
=
1.7;
p=
0.23) probably, due to the restricted data set for the EMS
(Table 4).
Since most of the primary production in the MS is due
to nano- and picophytoplankton (see phytoplankton section
of this review) one can expect that ciliates are likely im-
portant grazers (Rassoulzadegan, 1978; Rassoulzadegan and
Etienne, 1981). Ciliate grazing impact can be about 50%
of the primary production in the Catalan Sea, where ciliate
maximum abundance was found near the DCM (Dolan and
Marras´e, 1995). In the Ligurian Sea, Per´ez et al. (1997) es-
timated that ciliates could graze from 8 to 40% of primary
production. The importance of ciliates as primary produc-
tion consumers seems to be higher in the EMS (Dolan et al.,
1999; Pitta et al., 2001).
In the MS, as in all marine systems, planktonic ciliates
are dominated by the order Oligotrichida (Lynn and Small,
2000). Within that order, the aloricate naked forms are the
main group (Margalef, 1963; Travers, 1973; Rassoulzade-
gan, 1977, 1979). An important aspect of ciliate ecological
diversity is linked to their trophic type as well as their size,
since both affect their role within the food web. As a per-
centage of total ciliates, the mixotrophs can vary between
<
10% to almost 100% (Verity and Vernet, 1992; Bernard
and Rassoulzadegan, 1994). Dolan et al. (1999) found that
large mixotrophic ciliates were more abundant in the EMS
than in the WMS both in absolute and relative terms. In
a later study across the MS, Pitta et al. (2001) confirmed
that pattern reporting that mixotrophs represented 17% and
18% in abundance and biomass, respectively and they were
from 3 to 18 times more abundant in the EMS (although
with lower total ciliate abundance) than the WMS. In the
Ligurian Sea, nano- and micro-mixotrophic ciliate contribu-
tion to total oligotrich biomass and abundance ranged from
31 to 41% and from 42 to 54%, respectively and they were
mainly located at the level of the DCM (Per´ez et al., 1997). In
Biogeosciences, 7, 1543–1586, 2010
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