Short-term variations of the vertical distribution of cyanobacteria in theopen Mediterranean Sea

A general study of biogeochemical processes (DYNAPROC cruise, May 1995) was conducted at a station in the open northwestern Mediterranean Sea where ho rizontal advection was weak. During this experiment, short-term variations of the vertical distribution of cyanobacteria were investigated and a possi ble link with diel vertical migration was considered. Consistently, the exp erimental work was conducted at a time scale of a few hours. Cyanobacteria were the most abundant population in the pico- and nanoplankton size-class. Their vertical distribution was monitored over 36 h with a frequency of 4 h. With such a time resolution, they experienced a single grazing event dur ing daytime and, at night, they were heavily grazed when the upper layers w ere occupied by organisms migrating from the aphotic zone. The correspondin g integrated (0-90 m) losses of cyanobacteria during the night amounted to 534 mg C m(-2). Though daytime grazing is most likely due to nanoflagellate s and microzooplankton, night-time grazing could significantly involve migr ant macrozooplankton organisms and sustain a periodic export of organic mat ter. This study stresses the importance of the potential export of organic matter as a consequence of the diel vertical migration, export that could n ot be accounted for by measurements at longer time scale.