Plankton of the Florida Current IV. Factors Influencing the Vertical Distribution of Some Common Copepods

The diurnal migration of sixteen species of copepods was studied at eleven 24-hour stations in the Florida Current off Miami. The extent and direction of their diurnal migration was shown to be related to their day level. In the daytime a deep-living species was more vertically dispersed, and was found at a wider range of temperature and light conditions than a shallow-living species. Night levels were controlled largely by temperature, but the light effect was sufficient to cause all species to be more abundant at the surface on moonlit than on dark nights. In Bermuda, and at the 10-15 mile station, where all species occur at relatively shallow depths, the strength of this moonlight effect is greater for deeper-living than for shallow-living species. At the 40-mile station, where all species characteristically live deeper, the moonlight effect is greater for the shallow-living than for the deep-living species. The signficance of temperature in the control of night levels is discussed in connection with geographical distribution. A diurnal rhythm in the effects of the temperature and light stimuli is suggested by the date.