Abundance, frequency of dividing cells and growth rates of Synechococcus sp. (cyanobacteria) in the stratified Northwest Mediterranean Sea

The abundance, frequency of dividing cells and growth rates of the planktonic 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 magnitude greater than that at the surface, with values ranging from 1.7 to 13 x 10⁶ cells l^-1 and from 4 to 175 x 10⁶ cells l^-1 at the surface and in DCM waters, respectively. Gross growth rates, however, were 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 the surface layer were supported by a higher frequency of dividing cells (surface: 0.09-0.24; DCM: 0.01-0.12). The negative correlation between 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 evidence that grazing alone may be able to account for these losses, further, direct determinations are clearly needed to elucidate the regulation of the abundance of Synechococcus in the NW Mediterranean.