In the present study, the composition of unicellular eukaryotes was studied at T0 and T96h. The data provided by Bouvy et al.[24] regarding the evolution of abundances of the main biological communities (i.e. bacteria, viruses, heterotrophic flagellates) at 3 sampling times (T0, T48h, T96h) under the same experimental conditions as ours, informed this choice. Measurement of abiotic parameters Temperature was

continuously see more measured using thermistor probes (Campbell Scientific 107). Incident UVBR (280–320 nm) was constantly monitored by a UVB radiometer (SKU 430, Skye instruments). During the experiment, temperature varied between 15.7°C and 17.2°C (and between 18.7°C and 20.2°C in ‘+3°C’ treatments), while incident UVB radiations (280–320 nm), which were measured around local zenith time, varied between 150 and 185 μWcm-2 (Table 1). At T0 and T96 h, samples were taken for abiotic analysis.

A volume of 80 ml of water was filtered on pre-combusted glass fiber filters (GF/F, Whatman) and stored at −20°C until nitrate and phosphate concentrations were measured, following standard nutrient analysis methods [32]. Table 1 Environmental conditions (temperature, salinity, chlorophyll a concentration, natural UVBR intensities) during the four days experiment Environmental conditions during the 4 days of study Period Spring (18–24 April) In situ Temperature 15.7°C to 17.2°C In situ Salinity Approx. 36 In situ Chl a Approx. 1 μg/L In situ maximum UVBR incidentsN (local zenith time) 150 to 185 μW/cm2 Bacterial and viral counting by flow cytometry At T0 and T96h, 5 ml of water was collected from each of the polyethylene bags for flow cytometry counts. high throughput screening Picocyanobacteria, heterotrophic bacteria and viruses were counted using a FACSCalibur flow cytometry (Becton Dickinson) equipped with an air-cooled laser providing 15 mW at 488 nm. For photosynthetic-cells (i.e. picocyanobacteria) neither fixative nor fluorochrome were used. Samples were

stored at <4°C until analysis, which was performed within 2 h of sampling in field laboratories. Analysis was therefore performed on fresh samples, to which a suspension of 1-μm beads (Molecular probes) was added, generally for 4 to 8 minutes in order to obtain >20,000 events. For the analysis of bacteria and viruses, 1 mL fixed (glutaraldehyde 0.5% final concentration) sub-samples were incubated with SYBR Glycogen branching enzyme Green I (Molecular Probes, Eugene, OR, USA) at a final concentration of 1/10,000 for 15 min at room temperature in the dark. The cytometry flow counts were performed as described in Brussard et al. [29]. Small eukaryotes microscopy observation For enumeration of non-pigmented and pigmented eukaryotes, water samples (100 mL) taken at T0 and T96h were fixed with glutaraldehyde (1% final concentration) and stored at 4°C for 24 h. 20 to 25 ml of each preserved water sample was stained with DAPI (final concentration, 15 μg mL−1) for 15 min, filtered onto a black Nuclepore filter (0.