A general description is given of Lake Hayes, a freshwater lake in the South Island of New Zealand, with a brief analysis of its physical conditions. Preliminary samplings from a variety of lakes showed that Lake Hayes possesses a comparatively rich zooplankton, noteworthy for the fact that its bulk is made up of few species, chiefly entomostracan Crustacea. The greatest number of organisims occur in the upper 6 fathoms but there is a vertical distribution through the complete range. Seasonal fluctuations are marked, the phytoplankton showing greatest productivity in spring, while summer and autumn peaks are dominated respectively by cladocera and copepods. The Entomostraca manifest a diurnal vertical migration. There is a concentration of the population in the surface waters at dawn and dusk. During the day, tile migrants desert the upper waters, while at night the numbers assume average proportions but no peak periods occur. It appears that the Entomostraca of Lake Hayes exhibit n facultative phototropism with a negative reaction to direct sunlight but a positive reaction to weak light.
