Temporal effects of biocontrol processes on nutrient dynamics in a shallow lake: Case study of Lake Xini, Okavango Delta in Botswana

Abstract

Little is known about the effects of weeds on nutrient concentration changes in aquatic environments. We examined alterations to nutrient dynamics from the biocontrol process of Salvinia molesta by the weevil Cyrtobagous salviniae in the Lake Xini of Okavango, Botswana. The abundance of weevils steadily increased from March to December 2009 in the range of 15 to 62 ind. kg⁻¹ fresh biomass with 93% bud damage at the time of mat sinking. Dissolved oxygen was 1.2 mg l⁻¹ with >90% salvinia coverage in March 2009 and increased to 4.56 mg l⁻¹ after 12 months of biocontrol, when salvinia cover was reduced to <10% cover. The study was conducted by placing plastic containers in the lake containing 1 kg weevil infested salvinia and water from the lake. The declines in weevil-damaged ‘floating salvinia’ were inversely proportional to the increases in ‘salvinia detritus’ in the study containers. Leaching of nutrients (N and
P) and major ions (Ca, Mg, Na and K) in floating salvinia and salvinia detritus, as a result of weevil damage, showed P and K consistently leached at higher rates. Nutrients in the lake were high during biocontrol process, compared with after control. Biocontrol resulted in an increase in dissolved oxygen and re-emergence of native vegetation.

Keywords: Cyrtobagous salviniae, decomposition, Kariba weed, leaching, salvinia, Salvinia molesta, sediments, water quality, weevil