176
R. Casotti et aI.
2m
10m
...
~
20m
Co)
i:l
~
~
0
...
40m
70m
Red fluorescence (a.u.)
Fig. 6. Vertical profile of prochlorophyte red fluorescence
(arbitrary units, relative to fluorescent beads) from station 13.
Note the bimodal distribution at 70 m, where the MAW is present
ments where conditions are favourable for both
(Moore et al. 1995, 1998). In our case, the contemporary presence of these populations
occurred exclusively at 70 m, inside the MAW,
and, together with other parameters, contributed
to its characterization. Such coexistence of populations has been observed by other authors in the
Pacific Ocean (Campbell and Vaulot 1993), the
Red Sea (Veldhuis and Kraay 1993) or the
Atlantic Ocean (Partenskyet al. 1996), while this
is the first report for the Mediterranean Sea.
In order to investigate the factors causing
this bimodal distribution, a more systematic
study of a coastal stations started in June 1998.
Preliminary data from June 16 th to September
15 th will be presented here. So far, the two ecotypes have always been observed at depth. The
upper limit of the layer of coexisting ecotypes
varies with time (Fig. 7). Only on one date,
September 15 th (arrow in Fig. 7), could we
observe both limits of the "coexistence layer".
This sampling followed the first strong wind
event after the summer stability, and it induced
the breaking of the surface stratification and a
slight cooling of surface waters (Fig. Sa,b). The
mixing brought deep and saltier water in the
upper levels of the water column, pushing the
"coexistence layer" up in the water column. A significative correlation has been found between
salinity and the presence of the two populations
of Prochlorophytes (Spearman, r=0.65, P
while values corresponding to only one population (the dimmer) average 37.657 psu (SD=0.261,
n=55). A comparison of salinity means (nonparametric test of Mann-Whitney 1947) shows
that the two values are highly significantly different (U=120, P
Prochlorophytes appears to be a characteristic of
deep waters, with salinity acting as a threshold.
Although we do not know whether the co-presence is due to passive mixing of waters with different salinities or to active growth of one or the
other type of cells. Further analyses will try to
address the question whether physics or also
biology causes the observed distribution. As a
matter of fact, if culture observations can be
applied to these Mediterranean populations,
then light must playa main role in limiting the
deeper population (Moore et a1. 1995). Small
eukaryotes were never very abundant, rarely
R. Casotti et aI.
2m
10m
...
~
20m
Co)
i:l
~
~
0
...
40m
70m
Red fluorescence (a.u.)
Fig. 6. Vertical profile of prochlorophyte red fluorescence
(arbitrary units, relative to fluorescent beads) from station 13.
Note the bimodal distribution at 70 m, where the MAW is present
ments where conditions are favourable for both
(Moore et al. 1995, 1998). In our case, the contemporary presence of these populations
occurred exclusively at 70 m, inside the MAW,
and, together with other parameters, contributed
to its characterization. Such coexistence of populations has been observed by other authors in the
Pacific Ocean (Campbell and Vaulot 1993), the
Red Sea (Veldhuis and Kraay 1993) or the
Atlantic Ocean (Partenskyet al. 1996), while this
is the first report for the Mediterranean Sea.
In order to investigate the factors causing
this bimodal distribution, a more systematic
study of a coastal stations started in June 1998.
Preliminary data from June 16 th to September
15 th will be presented here. So far, the two ecotypes have always been observed at depth. The
upper limit of the layer of coexisting ecotypes
varies with time (Fig. 7). Only on one date,
September 15 th (arrow in Fig. 7), could we
observe both limits of the "coexistence layer".
This sampling followed the first strong wind
event after the summer stability, and it induced
the breaking of the surface stratification and a
slight cooling of surface waters (Fig. Sa,b). The
mixing brought deep and saltier water in the
upper levels of the water column, pushing the
"coexistence layer" up in the water column. A significative correlation has been found between
salinity and the presence of the two populations
of Prochlorophytes (Spearman, r=0.65, P
while values corresponding to only one population (the dimmer) average 37.657 psu (SD=0.261,
n=55). A comparison of salinity means (nonparametric test of Mann-Whitney 1947) shows
that the two values are highly significantly different (U=120, P
Prochlorophytes appears to be a characteristic of
deep waters, with salinity acting as a threshold.
Although we do not know whether the co-presence is due to passive mixing of waters with different salinities or to active growth of one or the
other type of cells. Further analyses will try to
address the question whether physics or also
biology causes the observed distribution. As a
matter of fact, if culture observations can be
applied to these Mediterranean populations,
then light must playa main role in limiting the
deeper population (Moore et a1. 1995). Small
eukaryotes were never very abundant, rarely