Vertical Distribution Of Plankton Research Articles

Diurnal Migration of Plankton Crustacea

E VER since Forel and Weismann studied the daily vertical movements of the various plankton crustaceans in lac Leman and the Boden See in I874, various observations and experiments have been carried out to account for such migrations. The results show that diurnal migration is a very complex phenomenon, that there are great variations in the movements of different species and that the types of migrations shown by each species are quite similar from day to day in each lake. In a few cases, it has been noted that not only is the type of movement of a single species not the same in all lakes, but the character of the migration of a species sometimes changes in the same lake. Experiments indicate that the causes of the migration are not necessarily the same in all cases and it is therefore impossible to apply the facts learned from one species to others. Furthermore the simultaneous vertical movements of different species may sometimes result from quite different causes. Before Groom and Loeb (I890) explained plankton migrations on the theory of phototropism, various interpretations had been given. Forel (I874) supposed that planktonts sank during the day because the wind blew toward the land, and would drive the plankton ashore if it remained near the surface, while in the night planktonts came nearer the surface because the wind blew from the shore. Weismann (I877) was of opinion that the crustaceans performed daily vertical movements because their eyes were adapted to a low intensity of light, and were thus suited for getting food at night. A similar explanation was accepted by Fuchs (i882). Chun (I887), however, opposed this view and came to the conclusion that the diurnal migration is due not so much to the effect of light as to the influence of temperature. The later observations showed the importance of the interrelations of light and gravity as factors affecting the diurnal vertical movements. It was shown that temperature may secondarily affect vertical distribution of plankton. In this paper the writer discusses the observations and experiments relating to vertical migrations of plankton organisms and attempts to give an account of the present status of scientific knowledge relating to such phenomena.

A Net for Catching Plankton near the Bottom

In the course of the researches on the vertical distribution of plankton it became evident that a net was needed for sampling the water-layers very close to the sea bottom. While the net had to be efficient in capturing the plankton animals present in those layers, it was necessary that the catches should not be vitiated by the presence of bottom-living organisms which had been stirred up from the sea floor by the net itself.

A Releasing Apparatus for Horizontally Towed Plankton Nets

While experimenting on the vertical distribution of plankton in comparatively shallow waters I found it necessary to devise a releasing apparatus for use with horizontally towed plankton nets. Under existing working conditions there were four circumstances for which the instrument had to be adapted: