Dynamics of phytoplankton groups in three contrasting situations of the open NW Mediterranean Sea revealed by pigment, microscopy, and flow cytometry analyses

•Good correspondence between pigments and flow cytometry, fair with microscopy.•Significant contribution of haptophytes and cryptophytes in the <3 μm fraction.•Record high concentrations of Synechococcus; cryptophytes accumulated at surface.•Phytoplankton groups had different ecological preferences related to depth and nutrients.•No clear correspondence between cell size and nutrient environmental conditions.