Langmuir circulation
In physical oceanography, Langmuir circulation consists of a series of shallow, slow, counter-rotating vortices at the ocean's surface aligned with the wind.
Irving Langmuir discovered this phenomenon after observing windrows of seaweed in the Sargasso Sea in 1927.
Stokes drift velocity of the waves stretches and tilts the vorticity of the flow near the surface.
[6] The circulation has been observed to be between 0°–20° to the right of the wind in the northern hemisphere [7] and the helix forming bands of divergence and convergence at the surface.
At the convergence zones, there are commonly concentrations of floating seaweed, foam and debris along these bands.
[8] Langmuir circulations (LCs), which are counter-rotating cylindrical roll vortices in the upper ocean, have significant role in vertical mixing.
[9][10] The wind-generated roll vortices create regions where organisms of different buoyancy, orientation and swimming behavior can aggregate, resulting in patchiness.
[10] Theoretically, LC size increases with the wind speed unless limited by density discontinuities by pycnocline.
[13] Similarly, LC are found to have higher windward surface current in convergent zones due to jet like flow.
This faster moving convergent region in water surface can enhance the transport of organisms and materials in the direction of wind.
In 1927, Langmuir saw the organized rows of Sargassum natans while crossing the Sargasso Sea in the Atlantic Ocean.
Unlike active swimmers like animals and zooplankton, plants and phytoplankton are usually passive bodies in water and their aggregation are determined by the flow behavior.
[16] In addition, Sargassum get carried from surface to benthos in downwelling zone of LC and can lose buoyancy after sinking at depth for enough time.
[17] Some of the plants that are usually observed floating in water could get submerged during high wind conditions due to downwelling current of LC.
Besides, LC could also lead to patchiness of positively buoyant dinoflagellates (including toxic red tide organisms) during blooms.
[18] Moreover, the negatively buoyant phytoplankters which would sink slowly in still water has been observed to get retained in euphotic zone which may be due to suspension created by vertical convection cells.
[19][20] Furthermore, a broader study on the Langmuir supercells in which the circulation can reach the seafloor observed the aggregation of macroalgae Colpomenia sp.
in the sea floor of shallow waters (~5 m) in Great Bahama Bank due to local wind speed of around 8 to 13 m/s.
[21] Such LC could be responsible for transport of carbon biomass from shallow water to deep sea.
This effect was evident as the concentration of the algae were found to reduce dramatically after the occurrence of LC as observed from ocean color satellite imagery (NASA) during the period of the study.
Physalia tend to drift across the windrows which also increased food or zooplankter availability in divergent zones.
[22] Moreover, studies in Lake Mendota have shown good correlation between Daphnia pulex concentration and the appearance of foam lines.
Similarly, significant differences were observed in catches of Daphnia hyaline when sampling in and out of foamlines in South Wales lake, with greater number appearing in divergent zone.
[15] Actually, the zooplankton could become trapped in upwelling zones to a point where animals are stimulated to swim downwards.
[11] There has been further improvement in such models like the modification of Stommel's model by Titman & Kilham in order to consider the difference in maximum downwelling and upwelling velocities[25] and by Evans & Taylor that discussed the instability of Stommel's regions due to varying swimming speed with depth which produced spiral trajectories affecting accumulation region.
Schools of White Bass Roccus chrysops were observed feeding upon Daphnia along the foam track.
[26] In contrast, lesser Flamingoes Phoeniconaias minor were observed feeding on bubble lines containing concentrated blue-green algae.
[27] Similarly, medusae were found to aggregate in linear pattern (average spacing of 129 m) parallel with wind in the Bering Sea which could be due to large LCs.
High concentration of surfactants (surface-active substances) produced by phytoplanktons can result higher Marangoni stress in converging regions in LC.
Numerical simulation suggest that such Marangoni stress due to surfactant can increase the size of vortical structures, vertical velocity and remixing of water and biological/chemical components in the local region compared to that without surfactant.
