Journal
JOURNAL OF GREAT LAKES RESEARCH
Volume 29, Issue -, Pages 76-88Publisher
ELSEVIER SCI LTD
DOI: 10.1016/S0380-1330(03)70540-1
Keywords
N-fixation; phytoplankton productivity; light limitation; nutrients; Lake Victoria
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Between 1994 and 1998 biological nitrogen fixation was measured using the acetylene reduction method at inshore and offshore sites on northern Lake Victoria. Rates of biological N-fixation were high and often exceeded 0.5 mug N/L/h. Average rates of volumetric N-fixation at optimal irradiance were 8 times higher at inshore locations than in the offshore. Rates of annual areal N-fixation, modeled from the N-fixation light-response and light attenuation in the lake were moderate to high (1.8 to 23.1 g N/m(2)/y) depending on location within the lake, and were, on average, only twice as high inshore as offshore. Variations in the light extinction coefficient explained a small but significant proportion of the variation in the optimal N-fixation in Lake Victoria. Algal biomass and N-fixation were lower in the more deeply mixing (> 20 m) offshore waters, because of the persistently low mean light intensities (108 +/- 41 muE/m(2)/s) in the water column most of the year. N-fixation increased with increased light availability, and maximal rates occurred when the lake was shallowly mixing anti thermally stratified. At both inshore and offshore stations, minima of algal biomass and N-fixation were consistent with low light availability in July when the lake was most deeply mixing. The ratio of the mean water column irradiance (I-24) to the irradiance at which N-fixation approaches saturation (I-k) was often < 1, and provides evidence that N-fixation was light-limited, chronically so in the offshore region. Light limitation of algal growth lessens the algal demand for N, and it constrains algal biomass development anti N-fixation more in the offshore compared to the inshore surface waters of Lake Victoria. Biological N-fixation is the largest input of fixed N to Lake Victoria, greatly exceeding estimates of atmospheric deposition and river inputs of N.
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