Vertical distribution and migration of pelagic Copepoda in Lake Tanganyika

The diel vertical migration and distribution of planktonic copepods were in vestigated at three localities in Lake Tanganyika. During the day, the surf ace zone was usually totally devoid of crustacean zooplankton. Even nauplia r stages of Copepoda, were often absent in the surface zone in daytime, alt hough they were numerous at night. There were clear differences in vertical distribution between species and localities, as well as between the season s. Cyclopoida were generally found nearer the surface, as compared to the c alanoid Tropodiaptomus simplex (especially in the southern arm of the lake) . The maximum abundance of Cyclopoida was found both day and night at depth s between 20 and 50 m. Occasionally Cyclopoids were found below 50 m and th en, only in the southern end of the lake, in the Mpulungu area. Calanoids, in comparison, utilised a broader depth range. Maximum numbers were found b elow 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 p atterns in the northern and southern arms of Lake Tanganyika also suggest d ifferences in the biological components of the ecosystem. The northern arm of the lake is characterized by stratification and the predominance of Cycl opoida with short-range vertical migration, while the southern end is chara cterized by more mixing and the predominance of Calanoida, with extended ve rtical migration.