Scientia Marina, Vol 64, No 2 (2000)

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


https://doi.org/10.3989/scimar.2000.64n2157

Michel Denis
Laboratoire d’Océanographie et de Biogéochimie, Université de la Méditerranée, France

Valérie Martin
Laboratoire d’Océanographie et de Biogéochimie, Université de la Méditerranée, France

Valérie Andersen
Observatoire Océanologique, Station Zoologique, France

Abstract


A general study of biogeochemical processes (DYNAPROC cruise, May 1995) was conducted at a station in the open northwestern Mediterranean Sea where horizontal advection was weak. During this experiment, short-term variations of the vertical distribution of cyanobacteria were investigated and a possible link with diel vertical migration was considered. Consistently, the experimental 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 during daytime and, at night, they were heavily grazed when the upper layers were occupied by organisms migrating from the aphotic zone. The corresponding 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 nanoflagellates and microzooplankton, night-time grazing could significantly involve migrant macrozooplankton organisms and sustain a periodic export of organic matter. This study stresses the importance of the potential export of organic matter as a consequence of the diel vertical migration, export that could not be accounted for by measurements at longer time scale.

Keywords


cyanobacteria; Mediterranean Sea; flow cytometry; diel vertical migration

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