A detailed study of the effects of individual bubbles at high gas flow-rate has shown, that the dominant influence on skin friction over a solid sphere is the bubble volume in compared to bubble frequency. Nevertheless the bubble frequency is very important in case of low gas flow-rate. Referring to bubbles produced by a gas distributor, statistical and spectral analyses were performed to study the influence of bubbling on exposure time and magnitude of fluctuations. Referring to a calibrated bubble train, the existence of critical frequency is demonstrated. A bubble with larger volume and a mobile, oscillatory surface generates larger velocity gradient. In the case of gas distribution, histograms of the velocity gradient for a 2mm glass sphere creating bubble coalescence reveal the maximum exceeds 48 000 s(-1) in the front zone and 2000 s(-1) in the rear zone (theta = 180 degrees). For 5mm plastic spheres creating bubble break-up, the maximum of the velocity gradient is only 8100 s(-1) for the front part of the sphere and 2000 s(-1) in the rear zone.