Effects of physical dynamics on the water column structure of Lake Bosomtwe/Bosumtwi, Ghana (West Africa)

Abstract

Thermal structure and mixing of deep tropical lakes are shaped by local meteorology and regional climatic trends, influencing the lake biogeochemical properties (VOLLMER et al. 2002, VERBURG et al. 2003). Frequently, productive tropical freshwaters develop anoxia in deep-waters that enhance dissolution of gases and nutrients in the hypolimnion, producing a sharp vertical density gradient with a fine thermal gradient. Lake Bosomtwe/Bosumtwi (hereafter referred to as Bosomtwe) is one such tropical lake permanently hypoxic below 30 m and strongly influenced by regional meteorology. The lake is located within a meteorite impact crater in the Sahel region of Ghana, West Africa (KoEBERL et al. 1997, 2007). The crater rim surrounding the lake reaches a maximum height of 460 m a.s.l., creating conditions within the crater of reduced wind stress and isolated hydrology from the surrounding Pra River Basin. The basement impact breccia from the initial meteoritic impact and overlying sediment layers are found to inhibit groundwater exchange (TuRNER et al. l996a). As a result, the hydrological balance of this closed basin is driven by longterm fluctuations in precipitation relative to rates of evaporation and reflected in the changing lake level {TuRNER et al. 1996a). Regional precipitation events in the Sahel are the result of the northward displacement of the Intertropical Convergence Zone {ITCZ), a low-pressure belt formed at the confluence of the northeastem and southwestem trade winds (JANICOT 1992). lnterannual climate variability is most closely linked to the latitudinal extent o f the ITCZ and the intensity of the convection within the ITCZ (MAMOUDOU et al. 1995, NICHOLSON 1980). Seasonally, the Sahel region is driest when northeast trade winds, known as the Harmattan, transport dust from the Sahara Desert during the months of December and January (BEADLE 1981 ). The northward migration of the ITCZ ushers in the rainy season and may travel beyond the crater walls, creating a brief dry period in July. The effects of seasonal climate variability on the thermal stratification dynamics of Lake Bosomtwe are poorly documented; therefore the objectives of our research were to characterize both the meteorology within the crater walls and the resulting physical mixing dynamics of the lake, which ultimately dictate the water chemistry and the ecology within the basin.