Planktonic nitrogen fixation in Lake Malawi/Nyasa

Nitrogen (N₂) fixation has been identified as possibly an important source of “new” nitrogen (N) to the epilimnion of Lake Malawi but studies in 1999–2000 and 2002 (September–December) estimated that the contribution of N₂-fixation by heterocystous Anabaena filaments to the N budget of the lake’s epilimnion is only 3–4% of total N input. N₂-fixation rates in Lake Malawi, as estimated by the acetylene reduction assay were higher during the stratified season (September–March) than during the rest of the year. Planktonic N₂-fixation in Lake Malawi can be monitored by measuring heterocyst biovolume concentrations because a significant correlation (r ² = 0.945, P < 0.0001) was observed between the two parameters. Heterocyst density cannot be used to estimate N₂-fixation because heterocyst sizes in the lake change continuously as at least two Anabaena species with different heterocyst dimensions are present. During September–October 2002, a species similar to A. maxima, contained larger heterocysts (16.34 ± 2.46 and 13.25 ± 1.89 μm in cross and apical section dimensions, respectively). In November–December of 2002, A. discoidea dominated and had smaller sized heterocysts (8.92 ± 1.13 and 7.25 ± 0.95 μm in cross and apical section dimensions). Since planktonic N₂-fixation is higher near shore than offshore, its contribution to the N budget in the littoral zone where high densities of grazing benthic fish occur may be more critical to maintaining the high rates of benthic algal productivity observed.