Picoplankton population dynamics in coastal waters of the northwestern Mediterranean Sea

Abstract

High‐frequency sampling of surface picoplanktonic populations was performed in Villefranche Bay (northwestern Mediterranean Sea) during the first week of July 1996. The evolution of abundance and cell parameters were monitored once per hour by flow cytometry for these populations: Synechococcus cyanobacteria, photosynthetic picoeukaryotes, and heterotrophic bacteria plus Prochlorococcus. Some parameters, such as the right‐angle light scatter of Synechococcus and picoeukaryotes, or the red chlorophyll fluorescence of picoeukaryotes, exhibited a very clear 24‐h diel periodicity. For other parameters, such as Synechococcus red chlorophyll fluorescence, it was necessary to perform a Fourier analysis to establish a major 24‐h period unambiguously. This analysis also revealed that some other parameters, however, such as the cell concentration, right‐angle light scatter, and red chlorophyll fluorescence of picoeukaryotes, or the red chlorophyll and orange phycoerythrin fluorescence of Synechococcus, had a period of 17–18 h, corresponding to the inertial frequency at this latitude. The cell cycle of Synechococcus, sampled twice per hour, was synchronized with the daily light cycle, allowing an estimate of their growth rate, which averaged 0.95 d−l (SD = 0.16, n = 7). Its large day‐to‐day variability was related to the duration of the interval between the maxima of S and G2 phases that ranged from 2 to 3.5 hours. Generally, the division rate was depressed on sunny days, The loss rate of Synechococcus was in general lower than the division rate and appeared to follow the evolution of the latter with a 1‐d lag, as if grazers or viruses adapted very rapidly to changes in division rates. Over the period of study, concentrations of Synechococcus and other bacteria were significantly correlated (r2 = 0.60, p < 0.01, n = 340), suggesting the possibility of a common controlling factor for these populations, e.g., phosphorus or grazing.