Studies on the Interrelationships of Zooplankton and Phytoplankton

Abstract

An apparatus in which it is possible to observe the horizontal migrations of zooplankton in gradients of phytoplankton is described, and an account given of its use in experiments involving various organisms. A preliminary search for toxic organisms is also described.The experiments comprise a demonstration of a migration by various animals into concentrations ofthe diatoms Skeletonema, Thalassiosira, Biddulphia, Nitzschia and various mixed cultures and the flagellates Chlamydomonas, Peridinium, Dicrateria, K, and Oxyrrhis. Cultures of Rhodomonas and Gymnodinium II are found to produce the opposite effect, and no reaction towards Lauderia, Coscinodiscus, Eucampia, Syracosphaera and Exuviaella could be shown. Results with bacteria cultures andvarious inorganic gradients are also discussed.A second apparatus in which it is possible to observe the vertical migrations of zooplankton in the presence and absence of phytoplankton is also described, and an account given of experiments in it involving various organisms.The experiments comprise a demonstration that greater numbers of animals swim up in the presence of mixed phytoplankton cultures, Coscinodiscus, Skeletonema, Ditylum, Chlamydomonas, Gymnodinium I and II and Oxyrrhis than in unenriched water. Flagellate One has no effect and Chlorella possibly depresses the number swimming up.Under-water observations on the swimming movements of some of the animals used are described. The possible application of the observations towards resolving the problem of the inverse distribution of phytoplankton and zooplankton in the sea is discussed, and the hypothesis is proposed that this may be accounted for by a combination of migration and grazing. It is considered that plankton animals must migrate both horizontally and vertically into patches of phytoplankton and, when present in sufficient numbers, graze these down very quickly.