Lake Malawi/Nyasa Experience and Lessons Learned Brief
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Lake Malawi/Nyasa Experience and Lessons Learned Brief Harvey A. Bootsma*, Great Lakes WATER Institute, University of Wisconsin-Milwaukee, USA, [email protected] Sven Erik Jorgensen, Royal Danish University of Pharmaceutical Sciences, Copenhagen, Denmark * Corresponding author 1. Introduction Lake Malawi/Nyasa (Figure 1) is the ninth largest, and third /$.(0$/$:,1<$6$%$6,1 deepest, freshwater lake on Earth (Bootsma and Hecky 2003). In 'UDLQDJH%DVLQ%RXQGDU\ addition to its great size, it is distinguished by being home to a ,QWHUQDWLRQDO%RXQGDU\ greater diversity of fi sh species than any other lake, the majority 5LYHU /DNH being endemic (Fryer and Iles 1972; Ribbink et al. 1983). As a 6HOHFWHG&LW\ result of these two qualities—its great size and biodiversity—the NP lake is recognized as part of the global heritage. At a time when 6RQJZH5 7 $1=$1,$ both the quantity and quality of freshwater are becoming issues .\HOD of concern in many parts of the world, the value of a lake that &KLWLSD contains nearly 7% of the Earth’s available surface freshwater is .DURQJD becoming increasingly obvious. 1 5XKXKX5 15XNXUX5 Within the Lake Malawi/Nyasa catchment, the lake’s fi sheries / are seen as its primary asset. The fi sheries are an important D source of protein for riparian populations within Malawi, N 5XPSKL H 7$1=$1,$ Tanzania and Mozambique, and make a signifi cant contribution =$0%,$ 5 UX 0 X to the regional economy, especially within Malawi. Other N X 5 D 1NKDWD DVLWX5 benefi ts of the lake include water for irrigation, transportation 6 . %D\ O and hydroelectric generation. D 0]LPED &+,6808/8,6 0DODZL Z Formal management activities of one form or another have L /,.20$,6 0DODZL 0$/$:, been carried out for Lake Malawi/Nyasa since the 1930s, 1 0HWDQJXOD although local management practices were probably \ implemented for centuries before then (Munthali 1994). D 'ZDQJZD5 V 02=$0%,48( 1NKRWDNRWD Virtually all management activities, and much of the D .DVXQJX research, have focused on the lake’s fi sheries. Despite these %XD5 1WFKLVL management activities, total fi sh catches have declined, and the catch of some fi sh species has been dramatically reduced, 'RZD 6HQJD WR/XJHQGD5LYHU 6DOLPD especially in the southern part of the lake (Tweddle and 0FKLQML %D\ /LORQJZH 0RQNH\ 5 H %D\ Magasa 1989; Turner et al. 1995; Banda et al. 1996; Bland and Z J /LQWKLSH5 Q R Donda 1995). Thus, there remains a need to refi ne and better LO / 'HG]D implement effective fi sheries management strategies. 0DQJRFKLFKL /&KLXWD /0DORPEH While fi sh species composition has been altered as a result 1WFKHX 02=$0%,48( of fi shing pressure, Lake Malawi/Nyasa has not experienced 0DFKLQJD 6KLUH5 a decline in species numbers, in contrast to some other African /DNH =RPED &KLOZD lakes (e.g., Lake Victoria). Nevertheless, continued increasing 0ZDQ]D fi shing pressures, along with changes in plankton community &KLUDG]XOX WR=DPEH]L5LYHU %RXQGDULHVDQGORFDWLRQVDUHDSSUR[LPDWH structure and water quality, may lead to a decreased DQGVKRXOGQRWEHFRQVLGHUHGDXWKRULWDWLYH ,/(&6( biodiversity, and even losses of species, if preventive action is Figure 1. The Lake Malawi/Nyasa Basin. not taken. Sediment core data and historic phytoplankton data (Hecky et al. 1999) suggest that nutrient inputs to the lake may be increasing, and possibly causing changes in phytoplankton species composition. The current issues of primary concern for Lake Malawi/ Brachystegia) and agriculture. The southern third is woodland Nyasa, therefore, are fi sheries management, biodiversity on the Mozambique coast, but almost completely cultivated conservation, and water quality. This brief provides an overview land within the Malawi portion of the watershed, with the of the historic and current management activities related to exception of the steep hillsides on the western side of the rift these issues, and, based on lessons learned from previous valley, which are forest covered. research and management activities, recommendations regarding management strategies for promoting sustainable The largest part of the lake’s watershed is within Malawi, fi sheries management and conservation of biodiversity and followed by Tanzania and Mozambique (Figure 1). The water quality. watershed population density is greatest in the southern Malawian portion, although it is also relatively high at the It should be noted that the lake is called “Malawi” in Malawi, northern end, in the Songwe and Kiwira River catchments. “Niassa” in Mozambique and “Nyasa” in Tanzania. For The watershed is more densely populated than that of Lake simplicity, this brief refers to the lake as Lake Malawi/Nyasa, Tanganyika, but less than that of Lake Victoria. with no preference implicit in the order or exclusion of names; however, exceptions made for terms such as formal project or Despite its large size, the lake does not have a high outfl ow commission names. water volume (Table 1). Of the approximately 68 km3 of water that enters the lake annually, only about 16% fl ows out the 2. Background Shire River, with the remainder being evaporated directly from the lake surface. Thus, the lake has a very long fl ushing time 2.1 Biophysical Features (Table 1). This distillation effect results in the lake water being more concentrated with regard to conservative ions than are Lake Malawi ranks among the world’s largest lakes, being the its infl owing rivers. The long fl ushing time also has important third deepest and ninth largest by surface area. Its geography water quality ramifi cations. Nutrients or other chemicals refl ects the fact that it is situated in a rift valley, the lake entering the lake essentially become trapped in the lake, and being long, relatively narrow, and deep. Its basin consists can only be removed by burial in the sediments, loss to the of a series of half-grabens (blocks of earth that have tilted atmosphere (if the chemical has a gaseous phase), or the very and dropped during rifting). Some parts of the lakeshore slow process of water outfl ow through the Shire River. are bordered by steep mountains, while the mountains that defi ne the edges of the rift valley are separated from the The dominance of precipitation and evaporation in the lake’s lake by extensive lakeshore plains in other parts. As a result, hydrologic cycle means that it also is very susceptible to nearshore topography varies between gently sloping beaches changes in climate. A small increase in the precipitation: and steep, rocky coastline. The lake is underlain by more than evaporation ratio can result in fl ooding, as occurred in 1979- 4 km of sediment in its deepest regions, refl ecting its great 80; in contrast, a small decrease in the ratio can result in the age, estimated at several million years (Johnson and Ng’ang’a basin becoming closed with no outfl ow, as occurred between 1990; Owen et al. 1990). 1915 and 1937 (Kidd 1983). In recent years, the lake level has again been declining, and the lake nearly became closed at the Although some of the plateau regions around the lake end of 1997. contain thick colluvial soils, the drainage basin is dominated by metamorphic and igneous gneiss, schist and granite The lake is permanently stratifi ed into three layers, separated (Carter et al. 1973). The northern two-thirds of the watershed by differences in water density (controlled primarily by are predominantly a mixture of woodlands (evergreen, temperature). The top, warm layer (epilimnion) varies in depth between 40-100 m, being deepest during the cool, Table 1. Physical Characteristics of Lake Malawi/Nyasa. windy season (May to September). Most algal growth occurs in this layer, supporting the lake’s food web. The middle layer Surface area (km2) 29,500 (metalimnion) is several degrees cooler than the surface layer, Maximum depth (m) 700 extending from the bottom of the epilimnion to about 220 m. Mean depth (m) 264 Within this layer there are strong vertical gradients of dissolved Volume (km3) 7,775 nutrients and oxygen. The deepest layer (hypolimnion) extends from about 220 m to the lake bottom. This layer is the coolest Altitude (m asl) 474 (most dense), and contains high concentrations of dissolved 2 Drainage area (km ) 100,500 nitrogen, phosphorus and silica. It is completely anoxic (no River infl ow (km3/yr) 29 dissolved oxygen); thus, virtually no fi sh are found below River outfl ow (km3/yr) 12 220 m. Rainfall (km3/yr) 39 Although the lake has never been known to completely Evaporation (km3/yr) 57 mix vertically, there is a slow exchange of water between Residence time (years) 114 the three layers. The exchange rate varies with season and Source: Modifi ed from Bootsma and Hecky (2003). location. Generally, there is a greater upward fl ux of nutrient- 260 Lake Malawi/Nyasa rich waters from the hypolimnion and metalimnion to the spp.) have decreased, currently making up less than 20% of surface during the cool, windy season when the southeast the total catch (Irvine et al. 2002). trade winds blow (known locally as the mwera). In addition to vertical entrainment and mixing, upwelling can occur 2.2 Political Features in parts of the lake during this season. Due to the lake’s morphometry (shape), upwelling tends to be strongest in the Lake Malawi/Nyasa is shared by three countries—Malawi, lake’s southeast arm. Thus, plankton production is usually Tanzania and Mozambique (Figure 1). The majority of the lake greatest in the southeast arm during and shortly after the and its catchment lie within Malawi. The position of the Malawi- windy season (Bootsma 1993a; Patterson and Kachinjika Tanzania border within the lake is contested; Malawi accepts 1995). Phytoplankton species composition also varies with the border running along the eastern shore of the lake, while season and location.