Sandy shore benthic N2-fixation in Lake Malawi, Africa

Abstract

Epipelic N2-fixation in Lake Malawi (9°30"S to l4°25"S) has been shown to be a very important local source of nitrogen (N) for epipelic algal growth, which supports highly diverse and populated endemic cichlid species (HIGGINS et al. 200 l). The rocky habitat, however, covers only about 30% of the lake's littoral area, while the remainder is almost entirely covered by sand (R.IBBINK et al. 1983). Despite sand covering a larger expanse of the littoral benthic zone with an almost entirely different biota from that of the rocky zone, N2-fixation in the lake's sand community has never been studied before. This is important particularly because algal growth in the lake has generally been perceived as N limited, which may become critical with the continued threat of eutrophication by increasing human populations and the associated land-use changes in the lake's catchment area (GUILDFORD et al. 2003). HECKY et al. (2003) showed that phosphorus (P) loading to the lake is increasing more rapidly than N loading, so that the epilimnion may become more N limited, thereby causing harmful N2-fixing cyanobacterial species to proliferate in the lake. We report results of a study conducted on 2 sandy beaches in the more productive southern part of Lake Malawi. Study objectives were to (l) estimate Nz-fixation rates on sandy sediments by using the acetylene reduction technique, and (2) evaluate the N input from epipsammic N2-fixation to the lake's epilimnion.