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Created in 1952 to supply water to Harare, Lake Chivero became hypertrophic 15 years later. Nutrient removal and wastewater treatment
reduced the trophic state in the 1970s. Urban growth in the 1980s resulted in increased discharge of partially treated wastewater into the lake.
Aquatic weeds, mostly free-floating species
such as Water Hyacinth (
Eichhornia crassipes
),
Water Lettuce (
Pistia stratiotes
), RedWater Fern
(
Azolla filiculoides
), and Kariba Weed (
Salvinia
molesta
), are dominant in the Zambezi basin
(Hirji
et al.
2002).
Although water hyacinth growth is a problem
throughout the basin, areas that are particularly
problematic are the Kafue Flats, Lower Shire, Lake
Kariba, and Lake Chivero. In Lake Kariba water
hyacinth and hippo grass (
Vossia cuspidate
) are
found in the estuaries and along the shoreline.
Lake Chivero
Lake Chivero was created in 1952 with the
damming of the Manyame River, 37 kilometres
southwest of Harare (Shekede
et al.
2008). Water
hyacinth appeared in the lake for the first time in
1953. Its proliferation was aided by the nutrient
enrichment of the lake from nearby farms and
from municipal and industrial waste from
Harare. By 1956, the first serious water hyacinth
outbreak had been successfully controlled using
chemical herbicides (UNEP 2008). The weed
reappeared, and by 1976 occupied
42 per cent of the lake, before declining to
36 per cent in 1989 and later to 22 per cent
in 2000. Chemical spraying and mechanical
weed control methods used during the 1960s
and 1970s also lowered levels of nutrient
enrichment are believed to have contributed
to the decline (Chikwenhere and Phiri 1999).
The introduction of biological control in 1990
using the water-hyacinth weevils,
Neochetina
eichhorniae
and
N. bruchi
, reduced most of the
water hyacinth in the lake in the late 1990s
(Chikwenhere and Phiri 1999). By 2005, the
invasive plants had returned, covering as much
as 40 per cent of the lake (UNEP 2008). Another
widespread weed in the lake is the spaghetti
weed (
Hydrocotyle ranunculoides
).
Invasive Alien Species
15 May 1989
23 Jun 1990
31 Aug 1986
6 May 1990