Regularly spaced rows of medusae in the Bering Sea : Role of Langmuir circulation

In summer at night in the Bering Sea hydromedusae and scyphomedusae formed dense aggregations at the surface in convergences apparently produced by Langmuir circulation cells. During strong winds distance between parallel rows regularly exceeded 100 m; densities of medusae within rows reached 1,000 m-3. Physical and behavioral factors can increase spatial aggregation and thereby alter the rate of encounter of medusae with prey, predators, and competitors. The importance of Langmuir circulation in rough seas has been underestimated. Wind-driven Langmuir circulation is widely regarded as one of the more important mechanisms that mix the epilimnion (e.g. Langmuir 1938; Faller 197 1; Leibovich 1983; Weller et al. 1985). Adjacent circulation cells, aligned with the wind, roll in opposite directions, producing linear, parallel convergences and divergences both at I This research was supported 82-05723 and DPP 82-06036. by NSF DPP the surface and at depth. Organisms with persistent directional behavior (Stavn 197 1) and passively buoyant or slowly sinking particles (Stommel 1949) will be retained in these cells. Since Langmuir circulation cells always exist when winds exceed 3 m s-l (Pollard 1977), one might expect to find many examples of linear surface aggregations of organisms in the literature, yet very few have been reported. Langmuir (1938) and Woodcock (1944, 1950) first described linear rows of Physalia and Sargassum. Subsequently Bar-y (1953) and Bainbridge ( 195 7) discussed accumulations of phytoplankton on Langmuir convergences. McNaught and Hasler (196 1) observed airlocked Daphnia in convergent foamlines, whereas normal Daphnia seems to be concentrated between foamlines in areas of upwelling and divergence (George and Edwards 1973). Although these few examples have generated much speculation and many reviews in the biological literature (e.g.