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Comparison of thermal stratification,
light attenuation, and chlorophyll-a dynamics between the
ends of Lake Tanganyika
V T. Langenberg,1*
L. M. Mwape,2 K. Tshibangu,3
J.-M. Tumba,4 A.A. Koelmans,1
R. Roijackers,1 K. Salonen,5
J. Sarvala,6 and H. Mölsä7
1Aquatic
Ecology and Water Quality Management Group, Agricultural University
Wageningen, The Netherlands
2Fishery Department, Mpulungu,
Zambia
3CNR Uvira (D.R. Congo),
Bujumbura. Burundi
4FAO-Research for the Management
of the Fisheries in Lake Tanganyika. Bujumbura, Burundi 5Section
of Hydrobiology; Department of Biological and Environmental
Sciences, University Jyväskyla. Jyväskyla, Finland
6Department of Biology University
of Turku, Turks, Finland
7lnstitute of Applied Biotechnology
University of Kuopia, Kuopio, Finland
*Corresponding author: Aquatic Ecology and Water Quality Management
Group, Agricultural University, P.O. Box 8080, Wageningen.
The Netherlands; Tel: +31-317-484996; Fax: +31-317484411;
E-mail: victor.langenberg@aqec.wkao.wau.nl
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Abstract
Thermal structure, light attenuation, and
chlorophyll-a dynamics within the upper 100 us at the northern and
southern ends of Lake Tanganyika were measured from August 1995-July
1996. Pronounced variability of thermal structure in time, depth
and region were observed. During the dry windy season (June-September),
the water column in the south was largely isothermal down to 100
m, while in the north stratification was detectable. Subsequently,
in October, cessation of winds coincided with a reestablishment
and strengthening of thermal stratification in the south, and a
distinct rise of a weakened thermocline in the north was accompanied
by an increase in epilimnetic concentrations of dissolved solids.
The depth of photic zone was highly variable with the
average position of 1% of incident photosynthetically active radiance
found between 20 and 70m, sometimes showing vertical shifts of 25
m a week. Weekly observations showed that concentrations of chlorophyll-a
were rather similar at both ends of the lake. At both stations,
periods of unstable or absent thermal stratification were accompanied
by increased concentrations of chlorophyll-a, although in the north
chlorophyll-a also peaked during periods of deep thermal stratification
and low rates of mixing. Concentrations of chlorophyll-a did not
show significant relationship with the available amount of underwater
available photosynthetically active radiance from (Im
from 11-173 µEm-2s-1).
We suggest that in Lake Tanganyika chlorophyll-a maxima
can occur whenever growth rates exceed spatial mixing rates, that
is, if mixing is less than a critical turbulence. Our observations
underscore the consequences of hydrodynamic processes on biological
productivity either by securing internal nutrients or by controlling
the timing and magnitude of phytoplankton biomass production.
Keywords: Tropical Lakes, PAR, mixing, deep chlorophyll,
thermal stability
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