Nsr. Agawin et S. Agusti, ABUNDANCE, FREQUENCY OF DIVIDING CELLS AND GROWTH-RATES OF SYNECHOCOCCUS SP. (CYANOBACTERIA) IN THE STRATIFIED NORTHWEST MEDITERRANEAN SEA, Journal of plankton research, 19(11), 1997, pp. 1599-1615
The abundance, frequency of dividing cells and growth rates of the pla
nktonic cyanobacteria Synechococcus sp. during the summer of 1995 and
1996 were estimated in the Northwest Mediterranean Sea to test whether
depth-dependent growth rates of this species explain its dominance in
the deep chlorophyll maximum (DCM) layer formed during summer thermal
stratification in the NW Mediterranean, compared to the surface layer
. Abundance at the DCM layer (50-70 m) was up to two orders of magnitu
de greater than that at the surface, with values ranging from 1.7 to 1
3 x 10(6) cells l(-1) and from 4 to 175 x 10(6) cells l(-1) at the sur
face and in DCM waters, respectively. Gross growth rates, however, wer
e much higher at the surface than in the DCM layer (surface: 0.76-1.07
day(-1); DCM: 0.30-0.47 day(-1)). The higher gross growth rates at th
e surface layer were supported by a higher frequency of dividing cells
(surface: 0.09-0.24; DCM: 0.01-0.12). The negative correlation betwee
n the abundance or standing stock and growth rates of these planktonic
picocyanobacteria points to losses, and not growth rate, as the main
control on the abundance of Synechococcus. Although we provide some ev
idence that grazing alone may be able to account for these losses, fur
ther, direct determinations are clearly needed to elucidate the regula
tion of the abundance of Synechococcus in the NW Mediterranean.