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Phytoplankton production and growth rate in Lake Tanganyika: evidence of a decline in primary productivity in recent decades

Authors


Jean-Pierre Descy, Laboratory of Freshwater Ecology, Department of Biology, FUNDP – University of Namur, B-5000 Namur, Belgium. E-mail: jpdescy@fundp.ac.be

Summary

1. This study focused on phytoplankton production in Lake Tanganyika. We provide new estimates of daily and annual primary production, as well as growth rates of phytoplankton, and we compare them with values published in former studies.

2. Chlorophyll-a (chl-a) in the mixed layer ranged from 5 to 120 mg chl-a m−2 and varied significantly between rainy and dry seasons. Particulate organic carbon concentrations were significantly higher in the south basin (with 196 and 166 mg C m−3 in the dry and the rainy season, respectively) than in the north basin (112 and 109 mg C m−3, respectively).

3. Carbon : phosphorus (C : P) ratios varied according to season. Phosphorus limitation seemed to occur more frequently than nitrogen limitation, especially during the rainy season. Severe P deficiencies were rare.

4. Measured particulate daily primary production ranged from 110 to 1410 mg C m−2 day−1; seasonal contrasts were well marked in the north basin, but less in the south basin, where primary production peaks occurred also in the rainy season. Estimates of annual primary production, based on daily primary production calculated from chl-a and water transparency, gave values lower than those reported in previous studies. Picophytoplankton accounted on average for 56% of total particulate production in the south basin during the wet season of 2003.

5. Phytoplankton growth rates, calculated from primary production, ranged from 0.055 to 0.282 day−1; these are lower than previously published values for Lake Tanganyika.

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