Ventilation of Lake Malawi / Nyasa

Seasonal and interannual particle export in an African rift valley lake: A 5‐yr record from Lake Malawi, southern East Africa

“…Even though the dissolved and particulate nutrient influx to the lake via river discharge is elevated at this time, we observed a minimal response of the offshore phytoplankton community, as reflected by the production of sinking particulate biogenic material (Bootsma 1993;Bootsma et al 2003;Hecky et al 2003). Approximately 25% of the annual silica input to the epilimnion of Lake Malawi is from rivers, whereas 75% is provided by vertical exchange with silicarich deep waters, although this varies regionally to some extent (Vollmer et al 2002;Bootsma et al 2003;. Additionally, a greater amount of the total silica transferred annually to the epilimnion via upwelling, compared to river-input silica, is soluble-reactive silica and thus immediately available for diatom uptake (Hamblin et al 1999;Bootsma et al 2003).…”

“…The site change for the final year of the trap time series was necessitated by complications arising from crew and vessel availability for working at the northern end of the lake in 1991-1992. At the northern site, dissolved oxygen concentrations have been shown to decrease rapidly below 20 m, with an anoxic water column present below ϳ170 m (Vollmer et al 2002). The central lake trap site is characterized by a slightly deeper anoxic hypolimnion boundary at 240 m (Vollmer et al 2002).…”

“…At the northern site, dissolved oxygen concentrations have been shown to decrease rapidly below 20 m, with an anoxic water column present below ϳ170 m (Vollmer et al 2002). The central lake trap site is characterized by a slightly deeper anoxic hypolimnion boundary at 240 m (Vollmer et al 2002). The subsurface mooring was anchored to the bottom by railroad wheels.…”

“…1) and the abundance of silt and sand in the northern lake basins (Eccles 1974;Johnson and Davis 1989;Pilskaln and Johnson 1991;Halfman and Scholz 1993;Hecky et al 2003). Upwelling, cold, deep-water formation and possibly internal wave propagation appear to be focused in the shallower southern reaches of the lake, although localized, divergent upwelling zones occur throughout the lake (Eccles 1974;Hecky et al 1996;Hamblin et al 1999;Vollmer et al 2002). Phytoplankton production, biomass, and deposition, as well as fisheries yield, are all enhanced in the southern lake region when compared to the north (Eccles 1962;Hecky and Kling 1987;Owen 1989;Owen and Crossley 1992;Bootsma 1993).…”

“…Permanent stratification and the oxic-anoxic boundary are maintained by moderately small chemical and thermal gradients in the metalimnion (105-220 m) between the 0-to 105-m epilimnion and deep (Ͼ220 m) hypolimnion (Hutchinson 1957; Wüest et al 1996;Vollmer et al 2002). Recent studies of density stratification and ventilation in Lake Malawi have shown that the temperature gradient has a stronger effect on the density structure than the salinity gradient (Wüest et al 1996;Vollmer et al 2002). Total dissolved nutrient flux into the surface mixed layer is thus highly dependent on seasonal…”