Sediment and nutrient deposition in Lake Volta in Ghana due to Harmattan dust

Harmattan is a dust-laden north-easterly wind that blows from the Sahara towards the Gulf of Guinea in the period November to March. It is the dominant wind in the north of Ghana while at the coast in the south it only occurs sporadically and here westerly or south-westerly winds dominate. Some of the dust is trapped in the vegetation, in lakes and other inland waters, and a little on the bare land. In this study, we determine the amount of sediment and nutrients that are deposited by this wind in big water bodies, exemplified by the dust deposition in the Harmattan period in the man-made Lake Volta. These depositions are compared with similar inputs by the rivers. On average, approximately 146,000 t of mineral matter and 42,000 t of organic matter are deposited per year in the Lake during the Harmattan periods. This is the equivalent of approximately 1% of the suspended sediment input by the rivers. The total amounts of Ca, P and Mg deposited during the Harmattan period are3000 t, 350 t and 810 t, respectively. About 40% of the Ca deposited is readily or plant available, for P and Mg it is about 50% and 20%, respectively. If the amount of readily available nutrients coming from the Harmattan dust is held against the influx of readily available (dissolved) Ca, Mg and P from the rivers to Lake Volta, it appears that the Harmattan dust accounts for only 0.7‰ Mg, 4.8‰ Ca, and 2.1% P. It must therefore be concluded that the contribution of nutrients to Lake Volta by the Harmattan dust is very limited.

► Method for determination of dust deposition in big water bodies.
► A comparison of sediment deposition in big water bodies by wind and rivers.
► A determination of nutrient deposition in big water bodies.