Vertical distribution and migration of pelagic Copepoda in Lake Tanganyika

Abstract

The diel vertical migration and distribution of planktonic copepods were investigated at three localities in Lake Tanganyika. During the day, the surface zone was usually totally devoid of crustacean zooplankton. Even naupliar stages of Copepoda, were often absent in the surface zone in daytime, although they were numerous at night. There were clear differences in vertical distribution between species and localities, as well as between the seasons. Cyclopoida were generally found nearer the surface, as compared to the calanoid Tropodiaptomus simplex (especially in the southern arm of the lake). The maximum abundance of Cyclopoida was found both day and night at depths between 20 and 50 m. Occasionally Cyclopoids were found below 50 m and then, only in the southern end of the lake, in the Mpulungu area. Calanoids, in comparison, utilised a broader depth range. Maximum numbers were found below 100 m, but they were also very common down to 140 m. The maximum depth of their distribution was 220 m, but only in the southern part of the lake. The lower limit of vertical distribution and migration of crustaceans was evidently related to the low oxygen concentration in deep water. Thus the seasonal differences in the vertical distribution of Copepoda seem closely to follow the general mixing patterns of epilimnion. The different mixing patterns in the northern and southern arms of Lake Tanganyika also suggest differences in the biological components of the ecosystem. The northern arm of the lake is characterized by stratification and the predominance of Cyclopoida with short-range vertical migration, while the southern end is characterized by more mixing and the predominance of Calanoida, with extended vertical migration.