Phytoplankton in Lake Tanganyika - vertical and horizontal distribution ofin vivo fluorescence

Determinations of chlorophyll a and in vivo fluorescence of photosynthetic pigments were used to study vertical and horizontal distribution of phytopl ankton in Lake Tanganyika (East Africa). Blue excited fluorescence (IVFb) w as an approximate predictor of chlorophyll a at different depths and locati ons. Green excited fluorescence (IVFg), which reflects phycoerythrin in cya nobacteria, explained chlorophyll a variation equally well, and in combinat ion with IVFb the degree of explanation was improved to 87% (n = 90). Parti cularly during the shallow stratification in March-May, the maxima of chlor ophyll a, IVFb and IVFg were located within the thermocline. Such distribut ion may have resulted from the high penetration of UV light, often accentua ted by very shallow daytime thermal stratification, leading to inhibition o f phytoplankton near the surface. Because the decrease of chlorophyll a spe cific IVFb was less striking towards the surface, the decrease of IVFb was not caused by light inhibition only. In October-November, epilimnetic IVFb and chlorophyll a values seemed to be consistently higher than in April-May and often showed remarkable patchiness. The sometimes very dense phytoplan kton blooms (Anabaena sp., Cyanobacteria) observed in the central and south ern parts of the lake, suggest that local upwelling or mixing events may be important for the development of phytoplankton in Lake Tanganyika.