Extreme variations in temperature and salinity have been observed at depth, during the summer months, off North Rustico, P.E.I., and Cheticamp, N.S. These were correlated with wind force and direction, controlling factors of internal adjustment of water layers according to Ekman's theory of wind-driven currents.Organisms which cannot tolerate these sudden changes in temperatures and salinities will not form important groups in populations along the margins of the Magdalen Shallows.