The Vertical Distribution of Marine Macroplankton. An Observation on Diurnal Changes

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Abstract
INTRODUCTION.1. A series of hauls with the ring-trawl were made at five depths in water 50 metres deep, so that samples were obtained approximately in daylight, at dusk, at midnight, at dawn, and again in daylight.A depth-recorder was used with the net giving a graphic record of its path through the water.2. Examination of the catches demonstrated the diurnal changes in vertical distribution of the species caught by the ring-trawl.3. Four types of behaviour were shown:—(a) A definite migration to the surface in the dark hours of certain forms living in the deeper layers in the daytime.(b) An expanding of the distribution of other organisms, that in the daytime had a zone of maximum abundance in the deeper layers, so that they were evenly scattered from the surface downwards.(c) Certain animals showed no or very little change in their vertical distribution during the dark hours.(d) A movement up to the midwater region by those forms that during the daytime adopt either a truly benthic existence or are congregated in the water layers immediately adjacent to the sea floor.(Lists of species grouped under their respective types of behaviour are to be found on page 780.)4. Many such observations are required before conclusions can be drawn as to the normal diurnal behaviour of plankton animals: this is a definite record of what occurred in this locality in mid-July at a period of full moon in fair weather.

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