The Effects of Temperature, Illumination and Pressure on the Vertical Distribution of Zooplankton

Thirteen 24-hour plankton stations were worked in the Florida Current and counts made for four species of siphonophores. Depth, temperature and illumination data accompanied all net hauls. An equation is developed relating the depth of each species at any given time to external conditions of temperature, illumination and pressure. This involves null value of each at which there is no stimulus to vertical movement, and factors giving relative response to each of the three environmental influences. The nulls are not solved here; the factors show a diurnal rhythm and also a progressive change from predominance of light control in the upper part of the population to predominance of temperature control in the lower part.