Short term variations of bacterial communities associated with a Mediterranean Posidonia oceanica seagrass bed

Diel changes in abundance and production of planktonic and benthic bacteria were examined at the water-sediment interface of a Mediterranean seagrass bed (Posidonia oceanica (L.) Delile) in the Bay of Calvi (Corsica). Several additional parameters (temperature, solar radiation, dissolved oxygen, DOG , Chlorophyll a) were also recorded in order to establish possible relation s with bacterial parameters. Diel changes in abundance of total and saproph ytic bacteria were relatively modest. In contrast, obvious diel cycles of b acterial production occurred, maximal values being recorded during the nigh t. No direct coupling was observed between Chlorophyll a and bacterial para meters, suggesting that phytoplanktonic exudates are not critical to bacter ial growth. Night time stimulation of bacterial production seems to corresp ond to an enlargement of cell volumes rather than to an increase in cell nu mbers. It could be interpreted as a consequence of nutrient release during the dark phase of seagrass metabolic activity or to predation release durin g the night.