Diel microbial variations at a coastal Northern Adriatic station affected by Po River outflows

Abstract

Diel sampling was performed during an early spring survey in the Northern Adriatic Sea at a coastal station off the Po River delta. Samples were taken every 6 h at spring tide maxima and minima in the sub-superficial layer, at the maximum fluorescence depth (∼3 m). Variations in microbial community structure and its processes were assessed by considering heterotrophic bacteria, picocyanobacteria, viruses, exoenzymatic activities, microphytoplankton, nanoplankton and bacterial/cyanobacterial Denaturing Gradient Gel Electrophoresis (DGGE) profiles. A considerable diatom bloom, mostly supported by Skeletonema marinoi was detected. All microbial parameters except viruses, showed a sinusoidal trend with a 12 h period; only picocyanobacteria expressed relative maxima during high tide, showing a phase in opposition to the other parameters. No substantial changes in DGGE band patterns were detected. Even though the results showed bacterial activities to be influenced by the phytoplankton bloom, all microbial parameters' diel trends (except viruses) preferentially followed the tidal fluctuation rather than the light:dark cycle.