2.1 Study area
The study was conducted in the northern part of Lake Nyasa, Tanzania
side. Lake Nyasa, also known as Lake Malawi in Malawi and Lago Niassa in
Mozambique, is the southernmost great lake in the East African Rift
Valley system, located between Malawi, Mozambique and Tanzania. The lake
is the third largest freshwater lake in Africa after lakes Victoria and
Tanganyika and is the second largest lake by volume after Lake
Tanganyika (Darwall et al., 2010; Hampton et al., 2018; Macuiane et al.,
2015). The lake has a maximum depth of 785 m, a volume of 8,400
Km3. a surface area of 29,000 Km2,
approximate length of 550 Km and mean width of around 48-60 Km and is
located 472 m above the sea level (Bootsma and Hecky, 1993; Darwall et
al., 2010; Gonfiantini et al., 1979; Patterson and Kachinjika, 1995).
The total catchment area of the lake is 126,500 Km2(Kumambala and Ervine, 2010) of which 97,750 Km2 is
land catchment (Menz, 1995). The mean surface temperature of the lake is
between 24 and 28 °C (Vollmer et al., 2005) and the annual rainfall
ranges between 1,000 and 2,800 mm (LNBWB, 2013). The lake experiences
two main seasons, the dry season (May-August) and wet season
(November-April), which are governed by the regional climate (Lyons et
al., 2011; Vollmer et al., 2005).
Lake Nyasa is meromictic, although it may experience mixing during the
dry season in the southern tip of the lake where the depth is relatively
shallow (Darwall et al., 2010; Vollmer et al., 2005; Weyl et al., 2010).
Due to the stratification, together with the great depth of the lake,
the nutrients availability to the plankton community are very low, and
thus, the lake is considered ‘oligotrophic’ (Irvine et al., 2001;
Mwambungu and Ngatunga, 2001). The lake has more than 1,000 different
fish species, many of which are endemic (Chafota et al., 2005;
Salzburger et al., 2014). Sampling was conducted at Wissman Bay that is
located at the northern end of the lake (sampling stations of Matema
S9°29´; E34°01´, Mwaya S9°33´; E33° 57´, Kafyofyo S9°35´; E33°57´ and
Kiwira S9°37´; E33° 57´).