The formation of large marine snow macroflocs and stringers was monitored at a fixed station in the southern North Sea. During an 18-month investigation, large marine snow floes occurred in the water column on nine occasions for periods lasting between 5 and 16 d. Their formation coincided with both high phytoplankton biomass and low windspeeds. Retention of marine snow in the surface layer was related to slow sinking, neutral buoyancy, and rising of the large floes. Applying a specially developed instrument showed buoyancy to be caused by gas bubbles incorporated in the floes. Marine snow flotation coincided with periods of oxygen supersaturation in the water column, suggesting that oxygen production could be responsible for gas bubble formation within the floes. Disappearance of the large marine snow floes from the surface layer coincided with drastic increases in windspeed, indicating that enhanced wind-induced turbulent mixing in the surface layer could have resulted in the breakup of these floes. Formation and disappearance of marine snow had a marked influence on the light regime in the water.