86
no room for Synchococcus here. On the other hand, surface samples at Station 25 (0, 20 and
30 m) contained mostly Synchococcus: the chI a concentration was 0.0688 p.g/l, divinyl-a was
65.3% or 0.0449 p.g/l, ChI b was 0.0069 p.g/l, and zeaxanthin 0.061 p.g/l. The
divinyl-a:Chl b ratio here was 6.5 - far higher than the 1: 1 ratio at depth, where the
Prochlorophytes were clearly shade-adapted. 0.049 p.g divinyl-Chl a corresponds with 20,883
cells/ml Prochlorophytes, which contributed 0.0135 p.g zeaxanthin (Chisholm et al., 1988).
Therefore, zeaxanthin from the Synchococcus cells contributed 0.061 - 0.0135 = 0.048 p.gll,
i.e. (0.048 x 10 9 ) : 1.8 = 26,666 cells/ml.
On the basis of the simple exercise presented above, I conclude that at Banda Sea Station 25
(chosen because it is a representative station for the oligotrophic ocean: Gieskes et al., 1988)
Synchococcus cells were most abundant near the sUlface, Prochlorophytes near the bottom of
the euphotic zone. This is also the conclusion of workers who surveyed oligotrophic tropical
and subtropical regions elsewhere (Olson et al., 1990a,b; Veldhuis and Kraay, 1990). It is
remarkable that the cell numbers recorded by these colleagues with flow cytometry are not
dramatically different from the results derived by simply using taxon-specific pigment
concentrations. Apparently, the differences in the pigment content per cell, which is most
variable in a vertical direction (Bidigare et al., 1989), are not always so large in natural
populations that calculations such as those presented above are not feasible. The constancy in
picoplankton cell numbers in the euphotic zone in all the globe's oligotrophic ocean regions
(Atlantic and Pacific) implies that conditions determining the apparently delicate balance
between production and consumption are the same everywhere in the ocean's upper
100-150 m.
In Table II, I summarize the results of the exercise presented above. A similar treatment of
sample series in the Gulf of Guinea (West of Africa) and in the tropical Atlantic along 200N
yielded approximately the same cell numbers.
Grazing, degradation
Grazers such as copepods do not feed indiscriminately on algal cells of all types (Paffenh6fer
and Knowles, 1978). Species selectivity can readily be estimated by HPLC analysis of classspecific pigments in waste products of grazers (Kleppel and Pieper, 1984; Burkill et al.,
1987), and carotenoids have often been used in food chain studies (e.g., Partali et al., 1987).
However, a quantitative assessment of grazing on phytoplankton is still a problem because of
loss of pigments, i.e. incomplete recovery, between ingestion, digestion and egestion (Lopez
no room for Synchococcus here. On the other hand, surface samples at Station 25 (0, 20 and
30 m) contained mostly Synchococcus: the chI a concentration was 0.0688 p.g/l, divinyl-a was
65.3% or 0.0449 p.g/l, ChI b was 0.0069 p.g/l, and zeaxanthin 0.061 p.g/l. The
divinyl-a:Chl b ratio here was 6.5 - far higher than the 1: 1 ratio at depth, where the
Prochlorophytes were clearly shade-adapted. 0.049 p.g divinyl-Chl a corresponds with 20,883
cells/ml Prochlorophytes, which contributed 0.0135 p.g zeaxanthin (Chisholm et al., 1988).
Therefore, zeaxanthin from the Synchococcus cells contributed 0.061 - 0.0135 = 0.048 p.gll,
i.e. (0.048 x 10 9 ) : 1.8 = 26,666 cells/ml.
On the basis of the simple exercise presented above, I conclude that at Banda Sea Station 25
(chosen because it is a representative station for the oligotrophic ocean: Gieskes et al., 1988)
Synchococcus cells were most abundant near the sUlface, Prochlorophytes near the bottom of
the euphotic zone. This is also the conclusion of workers who surveyed oligotrophic tropical
and subtropical regions elsewhere (Olson et al., 1990a,b; Veldhuis and Kraay, 1990). It is
remarkable that the cell numbers recorded by these colleagues with flow cytometry are not
dramatically different from the results derived by simply using taxon-specific pigment
concentrations. Apparently, the differences in the pigment content per cell, which is most
variable in a vertical direction (Bidigare et al., 1989), are not always so large in natural
populations that calculations such as those presented above are not feasible. The constancy in
picoplankton cell numbers in the euphotic zone in all the globe's oligotrophic ocean regions
(Atlantic and Pacific) implies that conditions determining the apparently delicate balance
between production and consumption are the same everywhere in the ocean's upper
100-150 m.
In Table II, I summarize the results of the exercise presented above. A similar treatment of
sample series in the Gulf of Guinea (West of Africa) and in the tropical Atlantic along 200N
yielded approximately the same cell numbers.
Grazing, degradation
Grazers such as copepods do not feed indiscriminately on algal cells of all types (Paffenh6fer
and Knowles, 1978). Species selectivity can readily be estimated by HPLC analysis of classspecific pigments in waste products of grazers (Kleppel and Pieper, 1984; Burkill et al.,
1987), and carotenoids have often been used in food chain studies (e.g., Partali et al., 1987).
However, a quantitative assessment of grazing on phytoplankton is still a problem because of
loss of pigments, i.e. incomplete recovery, between ingestion, digestion and egestion (Lopez