Scientific results of the Cambridge Expedition to the East African Lakes 1930-1.-No. 17. The vertical distribution and movements of the plankton in Lakes Rudolf, Naivasha, Edward, and Bunyoni.

Abstract

Since little quantitative work has been done on the vertical distribution of the plankton in tropical lakes, it is considered important to put on record the data from Lakes Rudolf, Naivasha, Edward, and Bunyoni. The plankton was collected by vertical hauls with Nansen type nets of both fine and coarse mesh. The material was counted by special methods devised for the purpose, and the results were plotted in the form of histograms representing the actual numbers of individuals. Lake Rudolf is characterized by a very high temperature, alkalinity, and pH. The phytoplankton was scarce and poor in species, particularly of diatoms. The ciliate Thuriwla folliculata was the most abundant form of the small zooplankton, Lake Rudolf is characterized by a very high temperature, alkalinity, and pH. The phytoplankton was scarce and poor in species, particularly of diatoms. The ciliate Thuriwla folliculata was the most abundant form of the small zooplankton, and its association with symbiotic vorticellids is analysed. The macroplankton was very rich in Copepoda and poor in Cladocera. A 24-hour series of observations showed the diurnal migration of Diaptomus to be in the opposite direction from the usual, so that the animals were near the surface in the day and in the deeper water at night. This reversal may be due to a low sensitivity to light. The diurnal migrations of Cyclops and the Cladocera, though less marked, were in the normal direction Lake Naivmhu has a lower temperature, alkalinity, and pH, and is comparatively shallow. The phytoplankton was abundant both in numbers and species, and the diatom Melosira was dominant. The day and night distribution of the species of the phytoplankton and small zooplankton is compared, and shows an apparent migration of non-motile as well as active forms. Water currents were probably responsible for these changes in distribution. The mecroplankton contained four species at the station where the observations made, but only two were abundant. The Cladocera were nearly as numerous as the Copepoda, and their diurnal migrations were similar as shown by a 24-hour series of hauls. The numbers caught by day were much smaller than those by night, probably because the animals were concentrated at or near the bottom during the light. The nauplii and young Cyclops showed little movement. Lake Edward is the deepest of the lakes, with a thermocline between 40 and 60 metres. The temperature, alkalinity, and pH were intermediate between Lakes Rudolf and Naivasha. The phytoplankton was abundant, and consisted mainly of diatoms and blue-green Algae. Fine net hauls were taken in the deep water, and showed that there was very little life below the thermocline. The macroplankton was rich in species and individuals of both Copepoda and Cladocera. Cycclops hyalinus was the dominant form in numbers, but the greatest volume of the plankton was contributed by the larvae of Chaoborua. Lake Bunyoui is a small lake situated at a high altitude and characterized by relatively fresh water. The graphs representing the temperature and pH values at different depths show that there was no turnover of the water. The phytoplankton caught was extremely scarce, but, since the zooplankton was abundant, there must have been more plant material present, which probably escaped through the meshes of the net. The macroplankton was rich in species, and its distribution at a day and a night station waa examined. The Copepoda were more abundant than the Cladocera, and Chaoborus larvae were again present. Prawns also occurred in appreciable numbers. The main sources of error in this type of quantitative work are discussed: the most important are the lack of uniformity in the horizontal distribution, the inability to fish the bottom 2 metres, and the differences in the sizes of the day and night catches. The importance of these factors depends partly on the depth of the lake and partly on the differences in behaviour of the various species. The flora and fauna of the plankton in each lake is compared. All the lakes differ from those of temperate regions in the uniformity of the conditions and the absence of a true thermocline. The limited range of migration is perhaps due to the turbidity of the water. The East African lakes are aberrant from the Naumann Thienemann classification: they are essentiafiy eutrophic, but are modified by alkalinity. Much work on the measurement of controlling factors in the field and experimental work on the behaviour of different organisms must be performed and correlated before a complete and general explanation can be given of the distribution and movements of plankton.