The sandy shore planktonic and benthic dinitrogen (N2) fixation (N2-fixation) rates in Lake Malawi were measured using the acetylene reduction technique during the years 1999-2000 and between Sep-Dec 2002. Spatial heterogeneity of the process in the lake was also addressed by using inshoreoffshore transects at Senga Bay and Mpwepwe in the southeast arm (SEA). While a similar study on littoral rocky surfaces in the lake showed very high N2-fixation rates (877.6 µg N m-2 h-1, Higgins 1999; Higgins et al. 2001), sandy shore planktonic and benthic N2-fixation rates were much lower (50.1 and 70.2 µg N m-2 h-1 respectively). While planktonic N2-fixation rates tended to be higher nearshore and decreased with distance offshore, sandy shore benthic N2-fixation rates increased with water depth. N2-fixation rates were generally higher during the stratified season especially between November and December than during the rest of the year. The temporal and spatial patterns of planktonic N2-fixation were also true for Anabaena filaments, heterocysts and heterocyst biovolume concentrations. Anabaena filaments, heterocysts and heterocyst biovolume concentrations increased from low values in September to high peaks in December and were higher nearshore than offshore. In the benthic community, both dark and light N2-fixation rates were substantial with dark N2-fixation accounting for between 37-283% of daytime total N2-fixation at Senga Bay and the SEA.
N2-fixation in Lake Malawi was previously suggested to be the primary source of N sustaining the generally high phytoplankton and periphyton net photosynthesis rates. The data presented in this study, however, show that N2-fixation in Lake Malawi accounts for <4% of the lake’s total epilimnion N budget. The planktonic N2-fixation contributed 13 mmol N yr-1 while N2-fixation on sandy sediments and rocky surfaces contributed 0.74 and 0.45 mmol N yr-1 respectively. This is much lower than the N contribution from riverine (18-54%) and atmospheric deposition (29-38%) to the lake’s epilimnion. Within the littoral zone where primary production supports the highest density of consumers (e.g. fish), planktonic and benthic N2-fixation could be an important source of N for primary production. Fish density in the littoral zone can be >10 fishes per square metre.
The study also examined factors controlling sandy shore planktonic and benthic algal biomass.
The dominant diatom taxa (Navicula spp. and Nitzschia spp.) were quantified and the
total algal biomass was estimated from chlorophyll concentration. The planktonic and benthic algal
biomasses seem to be controlled by different factors. In the benthic community, sandy sediment
stability to resuspension seems to be the most important factor, followed by light availability
while the planktonic community may be controlled by nutrient and light availability. Temperature
may be important during the stratified season when the nearshore is slightly warmer than the offshore.