Estimating Synechococcus spp. growth rates and grazingpressure by heterotrophic nanoplankton in the English Channel and the Celtic Sea

Marine chroococcoid phycoerythrin-containing Synechococcus spp. recently have been implicated as a substantial component of the photosynthetic picoplankton in the ocean. Although the importance of Synechococcus as food sources for heterotrophic nanoplankton are now recognized, the information about its cycling of biomass and diel patterns is limited and the methodology used varies according to different authors. A selective metabolic inhibitor method was used to allow simultanous estimation of both growth rates and grazing disappearance rates of Synechococcus. Results obtained in the English Channel show growth rates ranging from 0.25 to 0.72 d^-1 with an average value of 0.51 d^-1 and grazing disappearance rates ranged from 0.21 to 0.64 d^-1 (mean=0.44 d^-1). Offshore in the Celtic Sea of the Northeast Atlantic Ocean, both rates were lower than in the channel. The similarity between average growth and grazing rates suggests a rapid recycling of Synechococcus biomass.