2.1 Study area
The study was conducted in the northern part of Lake Nyasa, Tanzania side (Figure 1). 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; Macuiane et al. , 2015; Hampton et al. , 2018). 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 (Gonfiantini et al. , 1979; Bootsma and Hecky, 1993; Patterson and Kachinjika, 1995; Darwall et al. , 2010). 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 (Vollmer et al. , 2005; Lyonset al. , 2011).
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 (Vollmer et al. , 2005; Darwall et al. , 2010; Weylet 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’ (Irvineet al. , 2001; Mwambungu and Ngatunga, 2001). The lake has more than 1,000 different fish species, many of which are endemic (Chafotaet 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´).