assess the biomass of phytoplankton, the organisms of which it
consists can be identified and counted. When these organisms are
smaller than 3 µm, it is necessary to use an epifluorescence
microscope, or better still, a flow cytometer. In both cases, the
plankton cells can be identified thanks to their autofluorescence.
performing flow cytometry, it is possible to analyse both the size and the
type of fluorescence (green, orange or red) produced by natural pigments or by
colouring agents added to the sample under investigation.
On the basis of these optical criteria, the components of the
phytoplankton can be assigned to various groups. Synechococcus are
characterised, for example, by their green, orange and red fluorescence. Prochlorococcus
and picoeucaryotes give off red fluorescence. These three groups of cells can
then be distiguished on the basis of their size.
on the flow cytometry data, it is possible to describe the structure of
picoplankton communities and to assess their respective contributions to the
chlorophyll concentration, the carbon biomass and the primary production.
Details of the model used to evaluate these contributions are given in Figure 1
and Figure 2.
calculate the volumes of the picoplankton cells, we used the "forward
light scattering" (FSC) index, which depends on the size of the cells as
well as their shape and their refraction index. The cells under investigation
were taken to be spherical and to all have the same refraction index. The
between the FSC index and the size of the spheres (Morel 1991) can
be used to calculate the diameter of a sphere, given the superscript
x. One can calculate x by applying this relation to cells with a known
microscopically measured diameter. If superscript x is assumed to be the same
in the case of all 3 taxa, a single measurement will suffice for this purpose
contribution of procaryotic cells to the phytoplankton production can be
assessed by determining the proportion of the production for which the <1 µm
fraction is responsible. The contributions of the 2 groups of cyanobacteria to
the <1 µm production can be taken to be the same as their contributions to
the chlorophyll levels.
is the main pigment involved in the process of photosynthesis occurring in the
plant kingdom. Its sea water concentrations are therefore a good index to the
phytoplankton biomass. The method used to determine these concentrations is
perfectly straightforward: phytoplankton
are collected with fine filtres through which none of the cells can
pass, the chlorophyll is extracted by means of a solvent (such as acetone or
methanol), and the fluorescence of the extract is measured.
have now been collecting data on the chlorophyll levels in the Tikehau lagoon
for 9 years (Figure).
A fairly stable mean annual value of 0.2
mg m-3 has been obtained (except for 1993, when the chlorophyll
levels were abnormally high due to the presence of cells measuring <1 µm).
The chlorophyll concentrations
were recorded every month on the atoll of Takapoto from 1991 to 1994. No
clear-cut seasonal changes were observed (Figure). Similar
findings were obtained on Tikehau, where the chlorophyll biomass was monitored
from 1982 to 1985 (Figure).
plankton ecosystem can therefore be said to be well balanced.
It is probably controlled by the grazing activities of the zooplankton
inhabiting the same biotope.
chlorophyll concentrations were measured in 16 atoll lagoons and in the
neighbouring ocean waters (Tableau).
phytoplankton biomass was found to range between 0.2 and 0.3 µg chlorophyll
per litre, apart from the Tekokota atoll lagoon, where the values recorded
were as low as those obtained in the surrounding sea, and in the Reka-Reka
lagoon, where they were greater than 0.4 µg l-1. These 2 lagoons
have rather unusual characteristics, however: the former is widely open to the
oceanic waters, and the latter is very shallow (it is less than 1 m deep), and
the organisms it contains are a mixture of phytoplankton and phytobenthos.
contribution of picoplankton (<3 µm) to the total phytoplankton levels
were calculated on the basis of the amounts of chlorophyll collected on
filtres with pore sizes of 3 µm,
1 µm and 0.2 µm. On the 2 most intensively studied atolls, 80 % of all
the phytoplankton cells were found to measure less than 3 µm, and 60 %,
less than 1 µm.
(mean ± standard error) of the various size classes to the total
19.2 ± 0.4
23.0 ± 0.5
57.6 ± 0.4
80.8 ± 0.5
22.4 ± 0.9
17.4 ± 0.5
60.1 ± 0.9
77.6 ± 0.9
phytoplankton cells were found to be equally small in the 14 other lagoons
studied, where the percentages again reached approximately 80 % in the case of the cells measuring less than 3 µm, and
around 50 to 60 % in the case of
those measuring less than 1µm (Figure).
abundance of the picoplankton
biomass of the phytoplankton, the contribution of the picoplankton and the
abundance of the 3 taxa in the
11 atoll lagoons studied and in the surrounding oceanic waters are given in
Takapoto, during the rare periods when no trade winds are blowing, the number
of picoplankton cells present in the lagoon reaches a peak at depths of 15 m
and below (Figure).
The temporal variability of the
picoplankton levels was assessed by performng epifluorescence counts on Synechococcus
picoeucaryotic cells. The numbers of Synechococcus were found to
increase from the late afternoon up to 10 p.m., whereas the dividing cells
were rated at approximately 0 during the night, but increased from 6 a.m. to 6
p.m. It is therefore in the late afternoon that the cell division processes
are completed in these cells. The decreasing numbers recorded after 10 p.m.
were due to the grazing activities of the zooplankton (Figure).
The monthly variability of the
numbers of Synechococcus and picoeucaryotes was monitored for 2 years
in the Takapoto lagoon. The maximum levels were found to occur during the
austral winter in the case of the cyanobacteria and during the austral summer
in that of the algae (Figure).
of the 3 taxa to the biomass
The excellent correlations
obtained between the total fluorescence levels measured after performing
extraction procedures and the fluorescence detected in the 3 taxa, the coefficients y (fg chl a)
per relative red
fluorescence unit were calculated (Figure). Excellent agreement was found
to exist between the chlorophyll levels measured and those calculated on the
basis of our model (Figure). We therefore calculated the
contributions of the 3 taxa to the
biomass and the phytoplankton production in all the lagoons on which we had
carried out cytometric studies (Figure). Procaryotic
cells were found to be predominant in
all the lagoons apart from those on the following 2 very untypical atolls: Taiaro, which is completely enclosed and has a
hypersaline lagoon, and Tekokota, an atoll which has a very shallow lagoon and
is widely open to the ocean waters. In most of the lagoons studied, Synechococcus
ranked first in terms of both the biomass and the production rates. Prochlorococcus
abound only in lagoons with a mean depth of more than 30 m. The
efflorescence of Prochlorococcus observed
in the Hiti lagoon in November 1995 (281 103 cells / ml-1)
and in the Haraiki lagoon in March 1996 (210 103 cells / ml-1)
completely changed the taxonomic composition of the picoplankton in these
size of the picoplankton and their C content
on the relation between the FSC and the size of the spheres, as well as exact
measurements of the diameter of Synechococcus (0.8 µm), the value of x
was calculated and found to be 3.94 in the case of the Takapoto lagoon waters
and 4.34 in that of the surface ocean waters. It is possible to calculate the
size of the cells in the ocean and in the lagoons in the same way, using the
FSC values. The size of the 3 taxa in question was found to vary depending
on the time of day, but not on the depth (Figure).
carbon content was then measured from the C/Volume conversion factors
published by Verity et al. (1992). At this lagoon, we thus obtained C contents
per cell of 53 fg C cell -1 in the case of Prochlorococcus,
180 fg C cell-1 in that of Synechococcus and 4970
fg C cell-1 in that of the picoeucaryotic cells. The
corresponding values obtained in the ocean waters were as follows: 53 fg C
cell -1 in the case of Prochlorococcus, 191 fg C cell-1
in that of Synechococcus and 2568 fg C cell-1 in
that of the picoeucaryotic cells. In the Takapoto lagoon, the contribution of Prochlorococcus
to the overall C levels was rather small; similar low levels were determined
in the case of both Synechococcus and the picoeucaryotic cells (Figure).
phytoplankton production has been frequently measured in situ in the
Tikehau and Takapoto atoll lagoons
few examples of the vertical profiles obtained are given in figures
1 and 2
averaging all the measurements recorded on these 2 atolls and integrating the
production values obtained up to depths of 25 m (the mean depth of these 2
lagoons), the mean phytoplankton
production rate was found to be 0.7 g C m-2 day-1 on
Tikehau and 0.8 g C m-2 day-1 on Takapoto.
useful index for comparing the fertility of the lagoons is the
production-to-biomass ratio (P/B). The P/B
ratios obtained on these 2 atolls are extremely high, reaching 20 mg C per mg
of chlorophyll per hour (Figure 1 et Figure
figures indicate that the phytoplankton in these lagoons have a very fast
growth rate, which is certainly due to the fact that the picoplankton increase
1.3-fold every day.
all the atolls studied, the picoplankton measuring <1 µm account for
between 30 and 80 % of the total primary production. Apart from the atoll of
Hiti, where an efflorescence of Prochlorococcus occurred in March 1996,
Synechococcus and the picoeucaryotic cells are responsible for most of
the primary production.
integrated primary production was found to reach a maximum on Kauehi, but the
mean depth of the lagoon on this atoll is greater than 30 m (table).
phytoplankton inhabiting the atoll lagoons and the neighbouring ocean waters
consist mainly of picoplankton
measuring <3 µm. Cyanobacteria account for most of the biomass of the
picoplankton measuring <1 µm.
In this size class, Prochlorococcus predominate in the ocean waters and
Synechococcus in the lagoon waters. Picoeucaryotic cells are present in
all the lagoons but constitute the main component of the biomass in only a few
phytoplankton production rate in the lagoons with a mean depth of 25 m is
approximately one gramme of carbon per m2 per day. The picoplankton
multiply about once a day on average, via a cell division process
occurring towards the end of the day.
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