Subsea gas releases can produce visible surface patches whose size is much larger than the stochastic lateral wandering of individual bubbles during ascent. This distinction is important when laboratory measurements of bubble-swarm dispersion are extrapolated to environmental releases. Here we formulate a scale comparison between three lateral length scales: the Taylor dispersion scale for random bubble motion within a swarm, the entrainment scale for coherent widening of a rising gas–water plume and the surface-spreading scale produced after a plume reaches the free surface and forms a fountain. Applying this framework to the Nord Stream pipeline leaks shows that the observed surface expression was controlled by plume entrainment and surface spreading, rather than by bubble-swarm Taylor dispersion. The resulting regime classification clarifies when bubble-swarm Taylor dispersion can control the observed lateral spread and when plume entrainment and surface transport dominate.
