Limnological annual cycle inferred from physical-chemical fluctuations at three stations of Lake Tanganyika

Abstract

Ten variables were measured at least twice per month at three locations of Lake Tanganyika (East Africa) over one year . Upwelling was observed in the south of the lake during the dry, windy season from May to September. Stratification was variable in strength but always present in the north. The lake showed a marked tilting of the epilimnion during the dry season (0-20 m in the South, 60-70 m in the North). This period was followed by oscillations of water masses towards an equilibrium when the strong winds from the south east ceased. Conductivity and pH fluctuations indicated dampened oscillations, particularly at the ends of the lake. Movements of the epilimnion toward an equilibrium position generated and/or re-inforced internal waves. These waves were inferred from fluctuations of chemical and physical characteristics of the lake. The concentrations of inorganic P and N commonly fluctuated by a factor of 3 or more in the epilimnion. The period of long-period internal waves was estimated to be ca. 28-33 days. Turbidity changes suggested pulse production caused by internal waves linked to non-random patchiness in nutrients and organisms. Turbulence resulting from highly dynamic physical events also induce random-patchiness in water composition. The lake water generally showed oligotrophic characteristics near the surface but had high concentrations of nutrients in deep water. The results showed that the trophic state of Lake Tanganyika, like that of the oceans, seems to depend largely on regeneration processes. The annual limnological cycle in Lake Tanganyika appears closely linked to the climatic conditions.

An abrupt change in the African monsoon at the end of the Younger Dryas

“…Primary production in many of the larger lakes is dependent upon nutrient regeneration in the epilimnion, and subsequent return by vertical mixing of the regenerated nutrients into the photic zone. In the absence of marked seasonal contrasts in surface temperature, vertical mixing is mainly controlled by the wind; any change in wind direction or intensity can thus impact the rate of primary production and, depending upon nutrient availability, the composition of the phytoplankton assemblage [Beadle, 1981;Hecky and Kling, 1987;Hecky et al, 1991;Patterson and Kachinjika, 1995;Plisnier et al, 1999]. Variations in either of these may in turn lead to changes in the flux and preservation of autochthonous biogenic material at the lake floor.…”

Interpreting stable isotopes in food webs: Recognizing the role of time averaging at different trophic levels

“…Lake Tanganyika is permanently stratified, but strong winds during the dry season cause seiche activity with a 28-to 36-d period, leading to episoidic vertical metalimnion entrainment (Plisnier et al 1999). Wind speeds vary diurnally, but daily mean speeds were significantly higher in the week preceding sampling (1.48 Ϯ 0.09 m s Ϫ1 ) than in the week following sampling (0.81 Ϯ 0.08 m s Ϫ1 ), with four consecutive days where speeds were higher than the long-term seasonal mean (Johannes et al 1999).…”

Clone library analysis reveals an unusual composition and strong habitat partitioning of pelagic bacterial communities in Lake Tanganyika

“…The similarity of the BCC in the epilimnion in the south of the lake with that of the hypolimnion was also noted in a DGGE survey of BCC that had a higher spatial resolution than the present study (De Wever et al 2005). This similarity was probably related to the upwelling of deep water in the south of the lake, which is especially pronounced during the dry season when this study was carried out (Plisnier et al 1999). Upwelling of deep water may result in the import of deep-water bacteria to surface waters and may influence BCC in the epilimnion.…”