MESOSCALE DYNAMICS OF PICOPHYTOPLANKTON IN THE MEDITERRANEAN SEA, WITH A
FOCUS ON PHOTOACCLIMATION RESPONSES
C. Brunet
1
*, R. Casotti1 and V. Vantrepotte
2
1
Stazione Zoologica di Napoli, Villa Comunale, 80121 Napoli, Italy - * brunet@szn.it
2
Université du Littoral, LISE, UPRES-A ELICO 8013, 62930 Wimereux, France
Abstract
Phytoplankton dynamics were investigated at the Italian side of the Strait of Sicily in July 1997 using HPLC pigment analysis of
fractionated samples (< 3
µ
m and > 3 µm) and ?ow cytometry. Phytoplankton were dominated by the picoplankton fraction (more than
80% of total chlorophyll), which was numerically dominated by cyanobacteria of the genus Prochlorococcus, with average concentrations
of 6.2 x 10
4
cell ml
-1
. Picophytoplankton composition varied with water mass characteristics and along the water column, with the highest
pigment diversity observed in the DCM. Photoacclimation responses were investigated using both, on board incubations and high-rate
sampling at one site, and showed a strong response of the picoeukaryotic component to the diel light cycle.
Keywords: phytoplankton pigments, diel variability, ?ow cytometry, photoacclimation
Rapp. Comm. int. Mer Médit., 37,2004
267
The Strait of Sicily is a highly dynamical area, where different
water masses mix, originating from the Atlantic Ocean and the
western basin, the Levantine basin, and the Adriatic Sea (1). As a
consequence, the area is hydrologically dynamic and several
mesoscale physical structures are frequently present, such as filaments
or meanders. It is therefore a site very suited to study the physical-
biological coupling at mesoscales. In July 1997 a cruise took place in
the framework of the SYMPLEX project (Synoptic Mesoscale
Plankton Experiment), aimed at investigating phytoplankton
dynamics as related to mesoscale physical structures. Several
transects were sampled across a coastal filament, originating from
horizontal advection of deep waters upwelling along the southern
coast of Sicily. Apart from discrete samples taken at 45 stations (8
depths), also a fixed station was sampled every 3 h for a total of 50 h
at 6 depths. At the same time as the fixed station, on-board incubations
on filter-fractionated samples were done, to compare physiological
responses to the diel light cycle and variations induced by changes in
the light field, as for example induced by upwelling or mixing.
Mesoscale sampling
The filament separated a western area, dominated by the modified
Atlantic Water (MAW), detected from its lower salinity in the first 50
m, from the so-called Ionian Water (IW), present in the eastern area,
which was saltier. A strong horizontal temperature gradient (front)
delimited the filament of cold water upwelling along the southern
coast of Sicily, which was also very evident from satellite imagery
(AVHRR). Phytoplankton biomass was low in the whole area (chl a
average of 0.09
µ
g l
-1
±0.08
µ
g l
-1
), characterized by a recurrent Deep
Chlorophyll Maximum (DCM), situated between 60 and 100 m depth)
with picophytoplankton dominating (up to 94% of total chl a). The
highest phytoplankton diversity was observed in the picoeukaryotic
component (< 3
µ
m in size) of the DCM, with dinophytes,
pelagophytes, prymnesiophytes and chlorophytes contributing to a
significant fraction of the total chl abiomass of this fraction.
Prokaryotic phytoplankton (Synechococcusand Prochlorococcus)
dominated mainly in the MAW (more than 60%), while in the deeper
DCM picoeukaryotes dominated (69%). In the IS, the relative
percentage of phytoplankton < 3
µ
m was slightly lower than in the
MAW, but picoeukaryotes dominated as well. Inside the filament, high
concentrations of Prochlorococcus, with a higher cell red ?uorescence
with respect to the surrounding stations, clearly marked the deep
origin of this cold water.
Diel sampling and on-board incubations
In order to estimate velocities of the vertical transport of
phytoplankton during the upwelling, cellular red fluorescence
estimated through ?ow cytometry on the picophytoplankton fraction
was analyzed, and compared with both the data from the diel cycle
sampling and the on board-incubations. The water column at the fixed
station, located east of the filament, was strongly thermally stratified
for the whole sampling period. Picophytoplankton dominated the
DCM (70% of total chl a), located between 70 and 90 m depth (1.55
to 0.45% of incident light), and Prochlorococcusand picoeukaryotes
dominated the picophytoplankton. Larger size phytoplankton
dominated, instead, the surface layer of the water column.
Photoprotective pigment markers, such as the ratios
zeaxanthin/(violaxanthin + antheraxanthin + zeaxanthin) and
Diatoxanthin/ (Diatoxanthin + Diadinoxanthin) showed high values in
surface waters in the picophytoplankton fraction (both ratios) or in the
larger phytoplankton (the second ratio), indicating the need of
photoprotection in surface waters due to the excess light intensities
present at the time of sampling. This process showed a significant diel
variation in both size classes in the first 20 m, but indicated a faster
reaction in the picophytoplankton fraction, suggesting a higher
physiological plasticity of these small-sized algae, at least with
respect to light utilization. Surprisingly, diel variations were observed
even in the DCM, even though very little light irradiances were
measured, suggesting that a response to the diel light cycle was
present even at very low light intensities.
The on-board incubations consisted of incubating natural
phytoplankton assemblages at higher (shift-up) or lower (shift-down)
light intensities than present at their sampling depth, in order to
estimate kinetic coefficients of the photoacclimation reaction. The
shift-up experiments showed a very fast synthesis of Diatoxanthin
with respect to total chl ain the picophytoplankton, suggesting faster
photoacclimating responses of these algae as compared to larger
phytoplankton (0.29 versus 0.10 h
-1
). For the shift down experiment
(from 1% to 0.1% incident light), no change in pigments was
observed, but a significant increase in divinyl-chl a, a marker of
Prochlorococcus, indicating a very strong photoacclimation response
of this species, which successfully occupies the very deep layers of
the water column. From ?ow cytometric detection of cellular red
?uorescence of chlorophyll, it has been observed that the kinetics of
low-light acclimation of Prochlororcoccuswas much faster than that
of Synechococcus, while the opposite was true for the kinetic of high-
light acclimation. This confirms the hypothesis that Synechococcusis
more adapted to high-light, nutrient poor surface waters, while
Prochlorococcusis better suited to low-light, high nutrients waters
re?ecting their relative vertical distribution. The analysis of the
picophytoplankton community proved to be a powerful tool to
describe water mass changes and to estimate variations of
physiological processes over time, which, in turn, may help to
estimate vertical velocities of mixing in the surface layer of the water
column (2).
References
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