• Population counts

By 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.

Example of cytograms 

Based 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.

To 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 relation 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

The 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.

Chlorophyll 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.

The phytoplankton use the mineral carbon dissolved in the waters to produce organic matter via a process of photosynthesis. To measure the amounts of carbon incorporated in this way, natural phytoplankton populations are incubated in transparent plastic jars and replaced at similar depths to those at which they were collected. Radioactive mineral carbon (¹⁴C) is then added to the contents of the jars (Figure). After a few hours, the phytoplankton are recovered on a filter and a radioactivity count is carried out in a scintillation counter. The details of these calculations are given in the accompanying figure.

We 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).

The  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.

The chlorophyll concentrations were measured in 16 atoll lagoons and in the neighbouring ocean waters (Tableau).

The 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.

The 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.

The 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).

The 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 the table.

On 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).

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 lagoons.

Based 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). The 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).

The phytoplankton production has been frequently measured in situ in the Tikehau and Takapoto atoll lagoons

A few examples of the vertical profiles obtained are given in figures 1 and 2

Upon 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.

A 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 2). These 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.

On 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.

The 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).

The 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 atolls.

The 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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Morel A (1991) Optics of marine particles and marine optics. In : Particles analysis in oceanography. Proceedings of the NATO Advanced Study Institute of Individual Cell and Particle analysis in Oceanography. Aquatifreda di Maratea, Italy, 21-30 October 1990, Springer ; Berlin, pp 141-188

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