Possible factors limiting the growth of an aquatic weed, Hydrilla verticillata (L.F.) Royle under tropical conditions

Abstract

The aim of this study was to help understand the possible factors limiting the growth 'of an aquatic weed Hydrilla verticillata in the littoral waters of Lake Victoria. The study was confined to investigation of inorganic carbon as source of nutrient and a possible biocontrol of the. weed by fresh w~ter snails, Lymnea natalensis and Bulinus spp found in the lake. A series of experiments were conducted for the purpose of evaluating these factors. From the studies it was observed that Hydrilla is able to photosynthesize in a medium where HCO-3 ions are predominant. The weed would thus be at an advantage where carbon dioxide i~ reduced or lacking as a result of local ecological effects. Hydrilla has a maximum growth in a medium of 220 mg per litre (predominantly HCO-3 ions) optimum total alkalinity. This would mean that Hydrilla can do wel1in calcareous waters, an observation well supported by the fact that the weed flourishes well in many hard water habitats. Consumption of Hydrilla by snails was 34.3070of the total available biomass. These results, show that while fresh water snails may be playing relatively little role in the control of Hydrilla, it is most likely that the low inorganic carbon source in Lake Victoria of total alkalinity 55.8 mg per litre {CQ2 + HCO-l) may be contributing to the low growth of the aquatic weed. Further studies on nutrient requirements by Hydrilla in relation to the nutrient status in Lake Victoria littoral zone may contribute profitably to the present conclusions.